Patent Publication Number: US-2021189521-A1

Title: Metal-binding protein and use thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit U.S. Provisional Application 62/727,114 filed on Sep. 5, 2018, the entire disclosure of which is hereby incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to metal-binding protein, and in particular to a protein with a selective affinity for rare earth elements. 
     BACKGROUND 
     Rare earth elements (REEs) comprise the lanthanides (Lns), yttrium, and scandium. These elements, particularly the Lns, are in high demand as key components in technologies such as permanent magnets, electric and hybrid car batteries, lasers, phosphors, and smartphones. However, the breadth of the applications of these elements is contrasted with the existence of relatively few economically viable mineable deposits and the difficulty of Ln extraction from ores and other feedstocks. The insolubility of REE salts and similarity of their chemical properties requires harsh, environmentally harmful, and laborious methods for accessing, recycling, and separating them. 
     The technological importance of REEs and the challenges associated with their acquisition have inspired bioengineering methods for more facile and environmentally friendly REE isolation. Whereas some plants and microbes have long been known to accumulate these metals, only recently has it been established that Lns play an essential biological role, in catalytic activity of certain pyrroloquinoline quinone (PQQ)-dependent alcohol dehydrogenase enzymes, especially methanol dehydrogenases (MDHs). These observations provide the possibility of learning from biology in designing new, effective methods for REE sensing, capture, and separations. 
     Thus, the identification and characterization of proteins that can selectively bind lanthanides may enable their uses in these capacities. 
     SUMMARY OF THE DISCLOSURE 
     An advantage of the present disclosure includes a metal-binding protein, which has a high selective affinity certain trivalent and/or tetravalent cations over non-trivalent and non-tetravalent cations. In some aspects, the metal binding protein has a high selective affinity (e.g., at least 1000 times, such as at least 10 4 , 10 5 , 10 6 , or greater) for trivalent cations and/or tetravalent cations from, e.g., a rare earth element, compounds or salt thereof, or an actinide element, compound or salt thereof, over non-trivalent and/or non-tetravalent cations such as bivalent cations from, e.g., a group II element compound or salt thereof. Such bivalent cations include for example, calcium cations. In an embodiment, the metal-binding protein has an affinity for a lanthanide ion that is at least 1000 times, such as at least 10 4 , 10 5 , 10 6 , or 10 7  times, an affinity of the metal-binding protein for divalent ions such as, for example, calcium ions. In an embodiment, the metal-binding protein has an affinity for an actinide ion that is at least 1000 times, such as at least 10 4 , 10 5 , 10 6 , or 10 7  times, an affinity of the metal-binding protein for divalent ions such as, for example, calcium ions. In an embodiment, the metal-binding protein does not occur in nature. In embodiment, the metal-binding protein comprises a non-natural tag such as, for example, a His6 tag. Advantageously, the metal-binding proteins of the present disclosure can be purified to at least 70%, such as at least 75%, 80%, 85%, 90%, 95% pure form. 
     Embodiments of the present disclosure include, for example, individually or in combination, wherein the metal-binding protein comprises a sequence having at least 80% identity, such as, for example, at least 85%, 90%, 92%, 95%, 97%, 98%, 99% or 100% identity, to the sequence of SEQ ID NO: 1. In other embodiments, the metal-binding proteins is not natural and/or comprises a non-natural tag such as, for example, a His6 tag. 
     Another aspect of the present disclosure includes an isolated metal-binding protein comprising at least 2 EF hand motifs comprising a sequence of SEQ ID NO: 3, wherein adjacent EF hand motifs are separated by 10-15 amino acids. Embodiments of the present disclosure include, for example, individually or in combination, wherein the adjacent EF hand motifs are separated by 11, 12, or 13 amino acids. In some embodiments, the isolated metal-binding protein can comprise 2, 3, or 4 EF hand motifs, at least one of the EF hand motifs comprises at least 3 carboxylate residues, and at least 2 adjacent EF hand motifs are separated by 10-15 amino acid residues, e.g., separated by 12-13 amino acid residues. In other embodiments, the metal-binding proteins is not natural and/or comprises a non-natural tag such as, for example, a His6 tag. 
     Another aspect of the present disclosure includes an isolated metal-binding protein encoded by a nucleotide comprising a sequence having at least 80% identity to the sequence of SEQ ID NO: 2. Embodiments of the present disclosure include, for example, individually or in combination, wherein the nucleotide comprises a sequence with at least 85%, 90%, 93%, 95%, 97%, 98%, 99% or 100% identity to the sequence of SEQ ID NO: 2. In other embodiments, the metal-binding proteins is not natural and/or comprises a non-natural tag such as, for example, a His6 tag. 
     Another aspect of the present disclosure includes a sensor for sensing presence of certain trivalent and/or tetravalent cations over non-trivalent and non-tetravalent cations such as one or more trivalent cations from, e.g., a rare earth element, compound or salt thereof, or an actinide element, compound or salt thereof, such as lanthanide ions. The sensor includes a metal-binding protein having a high selective affinity for tri- and/or tetravalent cations from, e.g., a rare earth element, compound or salt thereof, or an actinide element, compound or salt thereof, over non-trivalent and non-tetravalent cations such as bivalent cations from, e.g., a group II element (e.g., calcium compounds or its salt). In an embodiment, the sensor further includes a first fluorescent protein and a second fluorescent protein, the second fluorescent protein having an excitation wavelength substantially identical to an emission wavelength of the first fluorescent protein. 
     Another aspect of the present disclosure includes a method for separating trivalent cations or tetravalent cations over non-trivalent cations or non-tetravalent cations from a medium, e.g., a solution, a suspension, or a colloid. The method includes contacting a medium containing a tri or tetravalent cation with a metal-binding protein to bind the tri or tetravalent cation. In some embodiments, the tri- or tetravalent cations are selected from the group consisting of tri- or tetravalent cations from one or more rare earth elements, compounds or salt thereof (e.g. a lanthanide or a salt thereof) or from one or more actinides, compounds thereof, or salts thereof, or from a combination of rare earths and actinides. The metal-binding protein can have a high selective affinity for trivalent cations over non-trivalent cations. In addition, or as an alternative, the metal-binding protein can have a high selective affinity for tetravalent cations over non-tetravalent cations. The method can also include separating the metal-binding protein to which trivalent cations and/or tetravalent cations have bound (e.g., a lanthanide (3+) ion) from the medium; and optionally separating the trivalent and/or tetravalent cations from the metal-binding protein. 
     Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is made to the attached drawings, wherein elements having the same reference numeral designations represent similar elements throughout and wherein: 
         FIG. 1A  shows SDS-PAGE analysis of XoxF purification and copurifying band (lanmodulin). 
         FIG. 1B  shows sequences of EF hands of LanM and of human CaM (as a representative canonical EF hand-containing protein). 
         FIG. 1C  shows comparison of the amino acid sequences of M extorquens AM1 LanM (SEQ ID NO: 1) and  H. sapiens  calmodulin. 
         FIG. 2A  shows partial elution profile (67-83.5 mL, fractions 40-50, 1.5 mL/fraction) for Superdex S75 chromatography of XoxF following SP FF chromatography. 
         FIG. 2B  shows SDS-PAGE analysis of a purification of XoxF from  M. extorquens,  illustrating co-purification and separation of LanM. 
         FIG. 2C  shows a linear MALDI-TOF mass spectrum of fraction 44 (lane 10 in B), with peak at 11759.574 Da corresponding to LanM. 
         FIG. 3A  shows a linear MALDI-TOF mass spectrum of purified LanM-TEV-His after Superose 75 chromatography. 
         FIG. 3B  shows predicted amino acid sequence of full-length LanM with C-terminal TEV-cleavable His6 tag (GENLYFQRREIRHH) (SEQ ID NO: 8), and observed cleavage sites when expressed in  E. coli.    
         FIG. 4A  shows SDS-PAGE analysis of purified His-LanM and LanM-His. 
         FIG. 4B  shows determination of Kas of complexes of wt LanM, LanM-His, and His-LanM (15 μM each protein), with Sm III , monitored by CD spectroscopy. 
         FIG. 5A  shows CD spectra of LanM (20 μM) in the presence of 0-8 equiv. La III . 
         FIG. 5B  shows magnitude of molar ellipticity change for CD titrations of LanM (20 μM) with 0-8 equiv. Ln II  ions (La III -Lu III  and Y III ) and with Ca II . 
         FIG. 5C  shows stoichiometry of Ln III  binding to LanM as monitored by competitive titrations of LanM (5 μM) in the presence of xylenol orange (7 μM). 
         FIG. 6A  shows UV-visible spectra (normalized to extinction coefficient) of LanM in the presence of various concentrations of La III . 
         FIG. 6B  shows difference spectra from the titration, demonstrating two sharp absorption features at 280 and 287 nm. 
         FIG. 6C  shows that plots of A 280  nm and A 287  nm for the titration demonstrate an endpoint at 4 equiv. La III /LanM. 
         FIGS. 7A and 7B  shows complete far-UV CD spectra for titration of (A) wt LanM (20 μM) with Ca II  (0-160 μM and 0-5 mM) and (B) 20 μM LanM-His with 0-5 mM 
         FIG. 8  shows representative titration of xylenol orange with Ln III  (in this case, Sm III ) in the presence and absence of 6 μM LanM. 
         FIG. 9  shows K a,app  values for various Ln III -LanM complexes (Ln=La, Nd, Sm, Gd, Tb, Ho, Y) plotted against effective ionic radius. 
         FIGS. 10A, 10B, 10C and 10D  show CD titrations of LanM and LanM(4P-4A) with La III  and Ca II  for apparent K d  determination. Points have been fitted (red lines) to a cooperative binding model (Hill equation) with one or two (in the case of La III  binding to wt protein) set of sites, with parameters given in Table 3. 
         FIG. 10  shows design of a sensor (e.g., LaMP1) in an aspect of the present disclosure. 
         FIGS. 12A, 12B, 12C  provides data which show the selectivity of LaMP1 for Ln III s over selected common mono- di- and trivalent metal ions. 
         FIG. 13  shows determination of the limit of detection of LaMP1 in fluorescence plate reader measurements. 
         FIG. 14  shows Ka,ap p  determination for LaMP1(4P&gt;4A) using La III  and Ca II . 
         FIG. 15  shows the fold FRET change of LaMP1(4P→4A) in the absence of added metal ions (baseline) and in the presence of 5 μM Ln III s, 10 μM Y III , and Fe III ; 1 mM Al III , Mn II , and Ca II ; or 100 μM Cu II . 
         FIG. 16  shows characterization of fluorescence response, apparent K d  values, and metal selectivity of the LanM-based FRET sensor, LaMP1. 
         FIGS. 17A, 17B and 17C  show structures of Y(III)-bound LanM. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     The present disclosure relates to a metal-binding protein that has a high selective affinity for certain trivalent and/or tetravalent cations over non-trivalent and non-tetravalent cations. In particular, the metal-binding proteins of the present disclosure can have a high selective affinity for trivalent cations over non-trivalent cations. In addition, or as an alternative, the metal-binding protein can have a high selective affinity for tetravalent cations over non-tetravalent cations. Metal-binding proteins disclosed herein have an affinity for tri- and/or tetravalent cations from, e.g., a rare earth element, compound or salt thereof, or an actinide element, compound or salt thereof, that is at least 1000-fold higher than that for non-trivalent cations and/or tetravalent cations such as a bivalent cation from e.g., a group II element (such as calcium), compound or salt thereof. Metal-binding proteins disclosed herein can also have a high affinity for tetravalent cations from hafnium and zirconium elements, compounds and salts thereof. In an embodiment, such metal-binding proteins can have an affinity for a lanthanide ion or an actinide ion versus calcium ions, compounds or salts that is at least 10 3 , 10 4 , 10 5 , 10 6 , 10 7  or greater. In an embodiment, the metal-binding protein does not occur in nature. In embodiment, the metal-binding protein comprises a non-natural tag such as, for example, a His6 tag. Advantageously, such metal binding proteins can be included as a sensor for detecting a presence of a rare earth or an actinide element having high selectivity for rare-earth or actinide elements. 
     During efforts to characterize Ln-dependent methanol dehydrogenase (Ln-MDH) from  Methylorubrum extorquens  AM1, XoxF, a Ln III -binding protein that co-purified with the Ln-MDH was identified. This protein, which has been named lanmodulin (LanM), for lanthanide-modulated protein, contains metal coordination motifs (EF hands) traditionally associated with nanomolar- to millimolar-affinity Ca II  binding. However, LanM responds to Ln III s and rare earths with up to 100 million-fold conformational selectivity (or even more) over Ca II . Among other things, the present disclosure describes the identification, purification and isolation of LanM, its rare earth element binding characteristics, and the molecular basis of its binding and conformational selectivity for rare earth elements over other metal ions, such as Ca II .  FIG. 1C  shows the amino acid sequences of LanM and human ( H. sapiens ) calmodulin (CaM). Advantageously, an isolated metal-binding protein includes a sequence that is at least 80% identical to SEQ ID NO: 1 (LanM) such as at least 82%, 85%, 87%, 90%, 92%, 95%, 99% or 100% identical to SEQ ID NO: 1. For example, in an embodiment, the isolated metal-binding protein has a sequence of SEQ ID NO: 1 with the first 21 amino acid residues removed. In addition, or as an alternative, the isolated metal-binding protein comprises certain motifs with certain spacing between pairs of the motifs that can exhibit similar properties to LanM, as described in more detail elsewhere herein. For example, an isolated metal-binding protein comprising carboxylate residues at the 9 th  position of the EF hands and a similar number of amino acids between each predicted EF loop as in LanM will have properties similar to LanM. In an embodiment, such a metal-binding protein does not occur in nature. In another embodiment, the metal-binding protein comprises a non-natural tag such as, for example, a Hi S6 tag. Advantageously, the metal-binding proteins of the present disclosure can be purified to at least 70%, such as at least 75%, 80%, 85%, 90%, 95% pure form. 
     A polypeptide having an amino acid sequence at least, for example, about 95% “identical” to a reference an amino acid sequence, e.g., SEQ ID NO:1, etc. is understood to mean that the amino acid sequence of the polypeptide is identical to the reference sequence except that the amino acid sequence may include up to about five modifications per each 100 amino acids of the reference amino acid sequence. In other words, to obtain a peptide having an amino acid sequence at least about 95% identical to a reference amino acid sequence, up to about 5% of the amino acid residues of the reference sequence may be deleted or substituted with another amino acid or a number of amino acids up to about 5% of the total amino acids in the reference sequence may be inserted into the reference sequence. These modifications of the reference sequence may occur at the N-terminus or C-terminus positions of the reference amino acid sequence or anywhere between those terminal positions, interspersed either individually among amino acids in the reference sequence or in one or more contiguous groups within the reference sequence. 
     As used herein, “identity” is a measure of the identity of nucleotide sequences or amino acid sequences compared to a reference nucleotide or amino acid sequence. In general, the sequences are aligned so that the highest order match is obtained. “Identity” per se has an art-recognized meaning and can be calculated using well known techniques. While there are several methods to measure identity between two polynucleotide or polypeptide sequences, the term “identity” is well known to skilled artisans (Carillo (1988) J. Applied Math. 48, 1073). Examples of computer program methods to determine identity and similarity between two sequences include, but are not limited to, GCG program package (Devereux (1984) Nucleic Acids Research 12, 387), BLASTP, ExPASy, BLASTN, FASTA (Atschul (1990) J. Mol. Biol. 215, 403) and FASTDB. Examples of methods to determine identity and similarity are discussed in Michaels (2011) Current Protocols in Protein Science, Vol. 1, John Wiley &amp; Sons. 
     In one embodiment of the present invention, the algorithm used to determine identity between two or more polypeptides is BLASTP. In another embodiment of the present invention, the algorithm used to determine identity between two or more polypeptides is FASTDB, which is based upon the algorithm of Brutlag (1990) Comp. App. Biosci. 6, 237-245). In a FASTDB sequence alignment, the query and reference sequences are amino sequences. The result of sequence alignment is in percent identity. In one embodiment, parameters that may be used in a FASTDB alignment of amino acid sequences to calculate percent identity include, but are not limited to: Matrix=PAM, k-tuple=2, Mismatch Penalty=1, Joining Penalty=20, Randomization Group Length=0, Cutoff Score=1, Gap Penalty=5, Gap Size Penalty 0.05, Window Size=500 or the length of the subject amino sequence, whichever is shorter. 
     If the reference sequence is shorter or longer than the query sequence because of N-terminus or C-terminus additions or deletions, but not because of internal additions or deletions, a manual correction can be made, because the FASTDB program does not account for N-terminus and C-terminus truncations or additions of the reference sequence when calculating percent identity. For query sequences truncated at the N- or C-termini, relative to the reference sequence, the percent identity is corrected by calculating the number of residues of the query sequence that are N-and C-terminus to the reference sequence that are not matched/aligned, as a percent of the total bases of the query sequence. The results of the FASTDB sequence alignment determine matching/alignment. The alignment percentage is then subtracted from the percent identity, calculated by the above FASTDB program using the specified parameters, to arrive at a final percent identity score. This corrected score can be used for the purposes of determining how alignments “correspond” to each other, as well as percentage identity. Residues of the reference sequence that extend past the N- or C-termini of the query sequence may be considered for the purposes of manually adjusting the percent identity score. That is, residues that are not matched/aligned with the N- or C-termini of the comparison sequence may be counted when manually adjusting the percent identity score or alignment numbering. 
     For example, a 90 amino acid residue query sequence is aligned with a 100 residue reference sequence to determine percent identity. The deletion occurs at the N-terminus of the query sequence and therefore, the FASTDB alignment does not show a match/alignment of the first 10 residues at the N-terminus. The 10 unpaired residues represent 10% of the reference sequence (number of residues at the N- and C-termini not matched/total number of residues in the reference sequence) so 10% is subtracted from the percent identity score calculated by the FASTDB program. If the remaining 90 residues were perfectly matched (100% alignment) the final percent identity would be 90% (100% alignment 10% unmatched overhang). In another example, a 90 residue query sequence is compared with a 100 reference sequence, except that the deletions are internal deletions. In this case the percent identity calculated by FASTDB is not manually corrected, since there are no residues at the N- or C-termini of the subject sequence that are not matched/aligned with the query. In still another example, a 110 amino acid query sequence is aligned with a 100 residue reference sequence to determine percent identity. The addition in the query occurs at the N-terminus of the query sequence and therefore, the FASTDB alignment may not show a match/alignment of the first 10 residues at the N-terminus. If the remaining 100 amino acid residues of the query sequence have 95% identity to the entire length of the reference sequence, the N-terminal addition of the query would be ignored and the percent identity of the query to the reference sequence would be 95%. 
     In another aspect, the disclosure provides deletion variants wherein one or more amino acid residues in the metal-binding protein are removed. Deletions can be effected at one or both termini of the modified metal-binding protein, or with removal of one or more non-terminal amino acid residues of the modified metal-binding protein. Deletion variants, therefore, include all fragments of the modified metal-binding protein. 
     Within the confines of the disclosed percent identity, the invention also relates to substitution variants of disclosed polypeptides of the invention. Substitution variants include those polypeptides wherein one or more amino acid residues of truncated metal-binding protein are removed and replaced with alternative residues. In one aspect, the substitutions are conservative in nature; however, the invention embraces substitutions that are also non-conservative. Conservative substitutions for this purpose may be defined as set out in the tables below. Amino acids can be classified according to physical properties and contribution to secondary and tertiary protein structure. A conservative substitution is recognized in the art as a substitution of one amino acid for another amino acid that has similar properties. Exemplary conservative substitutions are set out in below. 
     
       
         
           
               
             
               
                 TABLE 18 
               
             
            
               
                   
               
               
                 Conservative Substitutions 
               
            
           
           
               
               
               
            
               
                   
                 Side Chain 
                   
               
               
                   
                 Characteristic 
                 Amino Acid 
               
               
                   
                   
               
               
                   
                 Aliphatic 
                   
               
               
                   
                 Non-polar 
                 Gly, Ala, Pro, Iso, Leu, Val 
               
               
                   
                 Polar-uncharged 
                 Cys, Ser, Thr, Met, Asn, Gln 
               
               
                   
                 Polar-charged 
                 Asp, Glu, Lys, Arg 
               
               
                   
                 Aromatic 
                 His, Phe, Trp, Tyr 
               
               
                   
                 Other 
                 Asn, Gln, Asp, Glu 
               
               
                   
                   
               
            
           
         
       
     
     Alternatively, conservative amino acids can be grouped as described in Lehninger (1975) Biochemistry, Second Edition; Worth Publishers, pp. 71-77, as set forth below. 
     
       
         
           
               
             
               
                 TABLE 19 
               
             
            
               
                   
               
               
                 Conservative Substitutions 
               
            
           
           
               
               
               
            
               
                   
                 Side Chain Characteristic 
                 Amino Acid 
               
               
                   
                   
               
               
                   
                 Non-polar (hydrophobic) 
                   
               
               
                   
                 Aliphatic: 
                 Ala, Leu, Iso, Val, Pro 
               
               
                   
                 Aromatic: 
                 Phe, Trp 
               
               
                   
                 Sulfur-containing: 
                 Met 
               
               
                   
                 Borderline: 
                 Gly 
               
               
                   
                 Uncharged-polar 
                   
               
               
                   
                 Hydroxyl: 
                 Ser, Thr, Tyr 
               
               
                   
                 Amides: 
                 Asn, Gln 
               
               
                   
                 Sulfhydryl: 
                 Cys 
               
               
                   
                 Borderline: 
                 Gly 
               
               
                   
                 Positively Charged (Basic): 
                 Lys, Arg, His 
               
               
                   
                 Negatively Charged (Acidic) 
                 Asp, Glu 
               
               
                   
                   
               
            
           
         
       
     
     And still other alternative, exemplary conservative substitutions are set out below. 
     
       
         
           
               
             
               
                 TABLE 20 
               
             
            
               
                   
               
               
                 Conservative Substitutions 
               
            
           
           
               
               
               
            
               
                   
                 Original 
                   
               
               
                   
                 Residue 
                 Exemplary Substitution 
               
               
                   
                   
               
               
                   
                 Ala (A) 
                 Val, Leu, Ile 
               
               
                   
                 Arg (R) 
                 Lys, Gln, Asn 
               
               
                   
                 Asn (N) 
                 Gln, His, Lys, Arg 
               
               
                   
                 Asp (D) 
                 Glu 
               
               
                   
                 Cys (C) 
                 Ser 
               
               
                   
                 Gln (Q) 
                 Asn 
               
               
                   
                 Glu (E) 
                 Asp 
               
               
                   
                 His (H) 
                 Asn, Gln, Lys, Arg 
               
               
                   
                 Ile (I) 
                 Leu, Val, Met, Ala, Phe 
               
               
                   
                 Leu (L) 
                 Ile, Val, Met, Ala, Phe 
               
               
                   
                 Lys (K) 
                 Arg, Gln, Asn 
               
               
                   
                 Met (M) 
                 Leu, Phe, Ile 
               
               
                   
                 Phe (F) 
                 Leu, Val, Ile, Ala 
               
               
                   
                 Pro (P) 
                 Gly 
               
               
                   
                 Ser (S) 
                 Thr 
               
               
                   
                 Thr (T) 
                 Ser 
               
               
                   
                 Trp (W) 
                 Tyr 
               
               
                   
                 Tyr (Y) 
                 Trp, Phe, Thr, Ser 
               
               
                   
                 Val (V) 
                 Ile, Leu, Met, Phe, Ala 
               
               
                   
                   
               
            
           
         
       
     
     Methanol dehydrogenases (MDHs), found in methylotrophic bacteria, are soluble, periplasmic enzymes that catalyze the oxidation of methanol to formaldehyde, a key metabolic step enabling methanol to serve as the sole carbon source for growth, thereby playing a critical role in the global carbon cycle. The most extensively characterized MDHs require a CO ion for activity, but recent work has demonstrated the presence of specifically Ln III -dependent MDHs (Ln-MDHs, XoxF) in several organisms, with the enzymes from the model methylotroph,  Methylorubrum extorquens,  and  Methylaciollphilum fumariolicum  SolV being best characterized. In cases in which genes encoding both systems are present, such as  M. extorquens,  expression of the Ca-MDH is repressed in the presence of Ln III s, with a strict requirement for early Les (La—Nd, and to a lesser extent, Sm). The specific incorporation of Ln III  instead of Ca II  into the Ln-MDH active site is of great interest from a chemical perspective, given the similar ionic radii of Ca II  to the biologically relevant early Ln III s, the nearly identical metal coordination environments in the two classes of MDHs, and the significantly greater environmental abundance of Ca versus Lns. 
     LanM (SEQ ID NO: 1), which was previously uncharacterized, has been identified as a protein that partially co-purified with XoxF when isolated from  M. extorquens  cells. Like XoxF, LanM is a periplasmic protein, and it is conserved in many methylotroph genomes. It exhibits a strong conformational response to all rare earth metals, e.g., La III  to Lu III  and Y III , at picomolar concentrations, whereas it only responds to Ca II  at near-millimolar concentrations. The protein also responds to the other rare earth metal, Sc III  (K d  not determined, but μM). This result is surprising, as LanM contains metal coordination motifs (EF hands) traditionally associated with high-affinity (nanomolar to micromolar) Ca&#39; binding. The analysis herein suggests that some of this 10 8 -fold selectivity for Ln III  over Ca II  is attributable to Pro residues found in each of LanM&#39;s EF hands. Other aspects of the protein, such as the presence of carboxylate residues at the 9 th  position of each EF hand, and the fusion of adjacent EF hands, also contribute to lanthanide or actinide trivalent cation recognition by the EF hand motif and thus by homologous proteins. For example, replacing the Pro residues with Ala increases conformational response for Ca&#39; by at least 100-fold. 
     An uncharacterized protein, LanM, co-purifies with XoxF. XoxF was purified from  M. extorquens  AM1 cells grown in the presence of 1 μM LaCl 3  and methanol as sole carbon source. The protein was purified by ammonium sulfate precipitation, cation exchange chromatography, and size exclusion chromatography (SEC) to high purity ( FIG. 1A ), and a specific activity of 22 μmol/min/mg at 23° C. (Table 7). After the cation exchange step, we were struck by the prominence of several co-eluting low-molecular weight bands that had not been noted previously. These bands were analyzed by in-gel trypsin digestion, liquid chromatography, and mass spectrometry; one of them, at 12 kDa ( FIG. 1A ), corresponded to a hypothetical protein, MexAM1_META1p1786 (LanM), based on 9 unique peptides representing 75% of the protein sequence (Table 1). LanM co-purified with XoxF in two independent purifications. Densitometric analysis of the LanM and XoxF gel bands indicates ˜1:1 LanM:XoxF (monomer) after normalization for protein molecular weight. On separation of LanM from XoxF in a final size exclusion chromatography (SEC) step ( FIG. 2 ), MALDI-MS analysis on the intact protein gave a molecular mass of 11759.574 Da, consistent with cleavage of the predicted signal peptide to yield Ala22 as the N-terminal residue (expected mass: 11757.0 Da). This result indicates that LanM is localized to the  M. extorquens  periplasm, where both XoxF and the ethanol dehydrogenase ExaF, the only Ln-containing proteins previously characterized in  M. extorquens,  also reside. 
       FIG. 1  shows purification and sequence analysis of lanmodulin (LanM), showing that it is a novel EF hand-containing protein. A) SDS-PAGE analysis of XoxF purification and copurifying band. Lane 1: Molecular weight marker. Lane 2:  M. extorquens  crude extract. Lane 3: XoxF after cation exchange chromatography (highlighted band was excised and analyzed by mass spectrometry, revealing LanM). Lane 4: Purified XoxF. Lane 5: Heterologously expressed and purified LanM. B) Sequences of the EF hands of LanM (SEQ ID NO: 1) and of human CaM (as a representative canonical EF hand-containing protein). In CaM, residues providing sidechain Ca u  ligation are shown in red, and residues involved in a hydrogen bonding network to a metal-coordinated water molecule are shown in green; in LanM, aligning residues are shown in blue, and the unique Pro residues are bolded in purple. C) Comparison of the amino acid sequences of M extorquens AM1 LanM and H. sapiens calmodulin. In both sequences, the EF hand metal coordination motifs are shown in red. In LanM, the signal peptide is in blue. Note that LanM&#39;s predicted EF hand loops possess shorter sequences between each EF hand (12 residues between EF1/2 and EF3/4, and 13 residues between EF2/3) compared with calmodulin (24 residues between EF1/2 and 25 residues between EF2/3 and EF3/4). In calmodulin (and canonical EF hands in general), each EF hand comprises a helix-loop-helix motif with a 9-residue entering helix, 9-residue loop, and 11-residue exiting helix. With only 12-13 residues between each EF hand coordination motif, the typical EF hand helix-loop-helix structure may only be possible with fusion of the entering and exiting helices, suggesting a novel fold for LanM as shown by the NMR solution structure of Y(III)-bound LanM. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Error, 
                 Observed 
                 SEQ ID 
               
               
                 Peptide (position) 
                 −10 IgP 
                 Mass 
                 ppm 
                 m/z 
                 NO: 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 K.EYLAAVEAQFK.A (95-105) 
                 101.66 
                 1267.6448 
                 0.3 
                 634.8298 
                 11 
               
               
                   
               
               
                 K.VDIAAFDPDKDGTIDLK.E (29-45) 
                 93.45 
                 1831.9203 
                 −1.8 
                 916.9658 
                 12 
               
               
                   
               
               
                 R. ELASPAGSALVNLIR (119-133) 
                 89.99 
                 1509.8514 
                 0.4 
                 755.9333 
                 13 
               
               
                   
               
               
                 K.EALAAGSAAFDK.L (46-57) 
                 87.81 
                 1149.5665 
                 0.7 
                 575.791 
                 14 
               
               
                   
               
               
                 K.AANPDNDGTIDAR.E (106-118) 
                 68.82 
                 1328.5956 
                 0.1 
                 665.3051 
                 15 
               
               
                   
               
               
                 K.KLDPDNDGTLDKK.E (82-94) 
                 61.78 
                 1457.7361 
                 −0.7 
                 486.919 
                 16 
               
               
                   
               
               
                 K.EALAAGSAAFDKLD 
                 61.25 
                 2418.1914 
                 −0.2 
                 807.0709 
                 17 
               
               
                 PDKDGTLDAK.E (46-69) 
                   
                   
                   
                   
                   
               
               
                   
               
               
                 R.VSEADLK.K (75-81) 
                 50.41 
                 760.3967 
                 1.3 
                 381.2061 
                 18 
               
               
                   
               
               
                 K.LDPDKDGTLDAK.E (58-69) 
                 41.56 
                 1286.6354 
                 1.1 
                 429.8862 
                 19 
               
               
                   
               
            
           
         
       
     
     Table 1 above provides peptide position and data. LanM peptides identified from in-gel digestion and nanoLC-MS 2  analysis of 12-kDa protein band co-purifying with XoxF following SP Fast Flow chromatography. Nine unique peptides representing 75% of the full-length protein sequence (100/133 residues) were identified by the PEAKS DB search algorithm with confidence scores −10 logP≥30.1 (protein confidence score—101gP≥20) which corresponds to 0.9% false discovery rate (FDR). 
       FIG. 2  shows a partial elution profile (67-83.5 mL, fractions 40-50, 1.5 mL/fraction) for Superdex S75 chromatography of XoxF following SP FF chromatography. LanM elutes in this region. Fractions 44-46 contain the most intense bands corresponding to LanM; the retention time of 73-77.5 mL correlates with that of purified La III -LanM-His on the same column, suggesting that LanM is isolated from  M. extorquens  in a La III -bound form. B) SDS-PAGE analysis of purifications of XoxF from  M. extorquens,  showed co-purification and separation of LanM. Lane 1: Molecular weight marker. Lanes 2-4: XoxF (purification #1) soluble lysate, after SP FF column, and after S75 column. Lanes 5-6: XoxF (purification #2) after SP FF column and S75. Lane 7: LanM (His 6  tag cleaved using TEV protease), 4 μg. Lanes 8-15: from purification #2, S75 fractions 42-49 (5 μL each).  FIG. 2C  shows a linear MALDI-TOF mass spectrum of fraction 44 (lane 10 in B), with peak at 11759.574 Da corresponding to LanM. Peaks at 7952.316 and 3974.678 appear to be m/z=+1 and +2 peaks for an unknown hemoprotein (but not XoxG or cytochrome cH) given the UV-visible features of these fractions. 
     The amino acid sequence of LanM was intriguing because of its four carboxylate-rich EF hand motifs ( FIG. 1B ). Although many classes of Ca II -binding EF hand-containing proteins have been characterized in the literature, there are no characterized homologs of LanM in terms of overall sequence or function. Therefore, here we discuss salient features of LanM in the context of EF-hand proteins in general. The ubiquitous eukaryotic Ca II -binding protein, calmodulin (CaM), is referred to here as a representative example of this diverse protein family, due to certain shared properties of these two proteins, as well as the extensive characterization of Ca II  and Ln III  binding and metal ion-linked conformational changes in CaM. However, it is noted that there are also clear differences between CaM and LanM in structure and function. 
     EF hands are structural motifs consisting of a metal-binding loop flanked by two a helices, and they are frequently found in pairs, conferring cooperativity of metal ion binding. For example, CaM comprises two pairs of EF hands, which cooperatively bind 4 Ca II  ions, inducing a conformational change to promote binding to target proteins. In most EF hands, the Ca II  ions are 7-coordinate: the 1 st , 3 rd , 5 th , and 12 th  positions of the motif provide side chain oxygen ligands, the 7 th  position provides a main-chain carbonyl oxygen, and the 9 th  position either directly coordinates the Ca II  or, more typically, hydrogen bonds to a coordinated solvent molecule ( FIG. 1B ). EF hands also bind Ln III s, often slightly more tightly than Ca II  ( e.g., ˜ 1 nM for Tb III -CaM vs. 1 μM for Ca II -CaM), although Ln III  binding is not physiologically relevant for these proteins. 
     LanM&#39;s EF hands possess several unique features relative to canonical EF hands. First, LanM retains all of the metal-binding Asp and Asn residues present in typical EF hands but also features an Asp residue in the 9 th  position in each of its EF hands, whereas Asp is encountered at this position in roughly one-third of EF hand sequences. An Asp residue at position 9 has been shown in a model EF hand to contribute 2 orders of magnitude selectivity for Ln III  over Ca II . Second, Asn is rarely if ever observed at the first position in functional EF hands, as it is in EF-hand 4 (EF4) in LanM. Third, all of LanM&#39;s EF hands also possess a Pro residue at the 2n d  position, a highly uncommon feature of EF hands in general, and encountered in only &lt;0.5% of EF hands of predicted CaMs and CaM-like proteins. Finally, LanM features unusually short sequences between each EF hand loop (12-13 residues) instead of the 24-25 residues present in canonical EF hands ( FIG. 1C ). Each of these unusual EF hand features is reflected in LanM&#39;s metal-binding properties, characterized below, and suggest principles for highly selective rare earth element recognition. 
     Purification and characterization of LanM. LanM was expressed in  E. coli  for biochemical characterization. We initially expressed the full-length LanM protein (residues 1-133), including the native signal peptide, heterologously in  E. coli  with a C-terminal tobacco etch virus (TEV) protease cleavage sequence followed by a His 6  tag. Whereas periplasmic extraction and affinity purification of this protein was successful ( FIG. 3 ), confirming LanM&#39;s periplasmic localization, yield was low (&lt;1 mg/L culture). As LanM contains no Cys residues in its sequence and therefore would be unlikely to require localization to the periplasm for proper folding, we sought to achieve higher protein yield via cytosolic expression. We removed the codons for the signal peptide and expressed residues 22-133, the native form in  M. extorquens,  following a protease-cleavable His 6  tag (wt-LanM). This construct enabled purification of significantly higher quantities of LanM (15 mg/L culture). The N-terminal His 6  purification tag that was removed prior to biochemical assays, yielding the wt protein. LanM was also purified as a cytosolic construct with an N-terminal or C-terminal His 6  tag without the cleavable linker (His-LanM and LanM-His, respectively). The biophysical properties of wt-LanM, His-LanM, and LanM-His were similar ( FIG. 4 ), and LanM-His was used for most experiments. 
       FIG. 3  shows characterization of LanM-TEV-His by MALDI-MS. A) A linear MALDI-TOF mass spectrum of fractions 9 and 10 (pooled) from Superose S75 chromatography. The expected masses of protein cleaved before L14, Y20, and A22 are 14245.70, 13723.07, and 13488.81 Da, respectively, as calculated by the ExPASy ProtParam tool (https://web.expasy.org/protparam/). The latter mass is that predicted by SignalP4.0. B) Predicted amino acid sequence of full-length LanM with C-terminal TEV-cleavable His 6  tag (GENLYFQHRHIRHH) (SEQ ID NO: 8), used for this analysis. The observed signal peptide cleavage sites, based on MS analysis, are indicated by dashes. 
       FIG. 4  shows comparison of His-LanM, LanM-His, and wt LanM.  FIG. 4A : Gel of purified His-LanM and LanM-His. Lane 1: molecular weight marker; lane 2: His-LanM (2 μg); lane 3: LanM-His (2 μg). The minor lower molecular weight bands on the gel may be different conformations of LanM resulting from adventitious metal binding, as LanM elutes as a single peak by Superdex 75 chromatography, and only minor impurities are observed by mass spectrometry.  FIG. 4B : Determination of Kits of complexes of wt LanM, LanM-His, and His-LanM (15 μM each protein), with Sm III , monitored by CD spectroscopy. Sm III  concentrations were buffered using EDDS as described in the Methods. The data were fitted to the Hill equation with a single set of sites, yielding K d  s of 1.9 pM (wt LanM), 4.3 pM (LanM-His), and 2.1 pM (His-LanM), all with Hill slopes ˜3. 
     During purification, we noted that apo-LanM migrates on an SEC column with an anomalous apparent molecular weight (46 kDa), which shifts to the expected 15 kDa upon incubation with La III  (Table 2). Whereas this result may suggest oligomerization of the apoprotein, further characterization instead suggests that the apoprotein exhibits little defined structure: the far-UV circular dichroism (CD) spectrum of apo-LanM, with a prominent band at 203 nm, is consistent with an irregular structure with little helical character ( FIG. 5A ). The low helical content of apo-LanM is surprising and rare for an EF hand-containing protein, as the two α helices of each EF hand are typically present in both apo and metallated states. Remarkably, all Ln III  ions (La—Lu, excepting Pm; and Y), resulted in a ˜200% increase in the molar ellipticity at 222 nm, [θ] 222 nm , up to ˜4 Ln III /LanM ( FIG. 5B ), characteristic of increased a helicity and concomitant with reduction in spectral features associated with a random coil. The stoichiometry of 4 Ln III /LanM is anticipated based on the presence of 4 EF hands. Based on a mean residue ellipticity of −30000 deg cm 2  dmol −1  for a fully α-helical protein, LanM&#39;s mean residue ellipticity with metal bound of −16000 suggests ˜50% helical character in that state. This degree of helicity of the holoprotein, but not the apoprotein, is consistent with that calculated by disorder and secondary structure prediction algorithms (e.g. Pondr and Jpred), suggesting that helical structure is being suppressed in the apo state. Addition of up to 8 equiv. (160 μM) Ca II  led to only minimal increase in helical character ( FIG. 7A ). Therefore, LanM displays a conformational change from a largely disordered state to an ordered one, in a highly Ln III -selective manner. 
     Table 2 below provides evidence that wt LanM and LanM(4P→4A) undergo a large conformational change upon metal binding as well as for differences in wt vs. mutant structure in the apo state. LanM or LanM(4P→4A) (100 μL 1.7 mM protein, apo or pre-incubated with 4 equiv. LaC13) was loaded onto a HiLoad 16/600 Superdex S75 column (120 mL volume) using a 100 μL loop. The column was pre-equilibrated and run in 20 mM MOPS, 20 mM KCl, 5 mM acetate, pH 7.0 (with 1 mM EDTA added for apoprotein experiments), at a flow rate of 0.75 mL/min. Protein was detected by Amonm. The elution times for the LanM or LanM(4P→4A) peaks are reported below. The column was calibrated using the GE Gel Filtration Calibration Kit LMW, from which apparent molecular weights were calculated. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                 Apparent 
               
               
                   
                   
                 Elution 
                 molecular 
               
               
                   
                   
                 volume  
                 weight 
               
               
                   
                 Protein 
                 (mL) 
                 (kDa) 
               
               
                   
                   
               
             
            
               
                   
                 LanM, apo 
                 59.8 ± 0.4 
                 46.1 
               
               
                   
                 LanM, 4 equiv. La III   
                 76.3 ± 0.2 
                 15.1 
               
               
                   
                 LanM(4P→4A), apo 
                 62.1 ± 0.7 
                 39.4 
               
               
                   
                 LanM(4P→4A), 4 equiv. La III   
                 76.0 ± 0.3 
                 15.4 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 5  shows LanM undergoes a conformational change selectively in the presence of Ln III .  FIG. 5A : CD spectra of LanM (20  i l.M) in the presence of 0-8 equiv. La III .  FIG. 5B : Magnitude of molar ellipticity change for CD titrations of LanM (20 μM) with 0-8 equiv. Ln III  ions (La III -Lu III , excepting Pm III , as well as Y III ) and with Ca II .  FIG. 5C  shows the stoichiometry of Ln III  binding to LanM as monitored by competitive titrations of LanM (5 μM) in the presence of xylenol orange (7 μM). 
       FIG. 7  shows complete far-UV CD spectra for titration of (A) 20 μM wt-LanM with 0-160 μM Ca II  and (B) 20 μM LanM-His with 0-5 mM Ca II . The spectra have been corrected for the volume change upon addition of Ca II  to the cuvette. Note in both titrations that the helical feature at 222 nm does not start to develop until &gt;100 μM (5 equiv.) Ca 11  have been added. The data in (B) are representative of the titration curve shown in  FIG. 10B . 
     The extinction coefficient of purified LanM was determined to be 1400 M −1  cm −1  at 275 nm. In early experiments, we noted that this extinction coefficient exhibited a dependence on the presence of Lns. Titration of LanM with La III  led to a shift in its UV-visible absorption spectrum, with sharp peaks growing in at 280 and 287 nm ( FIG. 6A, 6B ). Four equiv. La III  were required to yield the maximal absorbance change. On the basis of assignment of similar absorption bands in other proteins to a hydrogen bond involving the hydroxyl group of Tyr residues, we suggest that these bands reflect a similar transition of LanM&#39;s sole Tyr residue, Y96, directly following EF3. The sensitivity of the UV-vis absorption spectrum of Y96 to Lns, as well as the proximity of Y96 to EF3 in primary sequence, suggested that we might be able to monitor metal binding and/or conformational changes in LanM by Tyr fluorescence measurements. Indeed, the fluorescence emission of Y96 (λ ex =278 nm) was quenched by 70% by addition of ˜4 equiv. Ln III  ( e.g., La   III  and Gd III  in  FIG. 6C ). Using buffered metal solutions, the K d,app  for La III  was determined. Unlike the CD titration data (see below), the fluorescence titration data could be fitted well to a single phase, with K d,app =14±3 pM and n=3.2±0.2. This K d,app  value is remarkably similar to that of the minor, and slight lower-affinity, binding event visible in CD titrations, suggesting that Tyr fluorescence may be reporting specifically on the second of two metal-induced conformational changes in LanM. 
       FIG. 6  shows that the UV-vis absorption spectrum of LanM is altered in titrations with La III . ApoLanM-His (15 μM) in chelexed 20 mM MOPS, 100 mM KCl, 10 mM acetate, pH 7.0, was titrated with a solution of 5 mM LaCl 3  in the same buffer.  FIG. 6A : Spectra, normalized to extinction coefficient. The calculated extinction coefficient for holo-LanM is ε 280 nm =2000 M −1  cm 31 1 .  FIG. 6B : Difference spectra from the titration, demonstrating two sharp absorption features at 280 and 287 nm. These difference spectra are nearly identical to difference spectra of native insulin vs. a tryptic digest releasing a peptide containing Tyr26, ribonuclease at pH 6.94 vs. at pH 1.91, and unfolding of  E. coli  L-asparaginase. In the former two cases, experiments suggested differences in hydrogen bonding to the Tyr hydroxyl group were responsible for the observation of the bands.  FIG. 6C : Plots of A 280 nm  and A 287 nm  for the titration demonstrate an endpoint at 4 equiv. La III /LanM. 
     In order to probe approximate metal affinities of these four sites in LanM, competition assays were carried out using a colorimetric probe, xylenol orange (XO). When LanM was included in titrations of XO with Le III , the increase in absorbance at ˜575 nm associated with Ln III -XO complex formation was completely inhibited until 2.9±0.3 equiv. Ln III  were added ( FIG. 5C ,  FIG. 8 ; average for all Ln III s). It is proposed that this stoichiometry can be reconciled with the 4 sites observed in our CD titrations and 4 EF hands in LanM by noting the Asn at the 1s t  position in EF4 (Asn108). Asp is the consensus 1st residue in functional EF hands, and substitution with Asn would be expected to destabilize EF hand structure and impair metal binding, suggesting that EF4 is the lowest affinity site for Ln III  binding (on the order of XO&#39;s K d  for Ln III  ions, ˜1-10 μM) and therefore not observed in the XO titrations. This proposal is also supported by the NMR solution structure of Y III -bound LanM ( FIG. 17A ). 
       FIG. 8  shows representative titration of xylenol orange with Ln III  (in this case, SmCl 3 ) in the presence and absence of LanM. The concentration of LanM in the experiment was 6.0 μM. The endpoint was taken to be when A 575 nm  had risen by 10% of the total increase observed (here, at 19.4 μM Sm III , or 3.2 equiv. metal), suggesting saturation of tight binding sites on LanM. For example, NMR solution structure of Y III -LanM indicates that metal coordination by EF4 is weak. 
       FIG. 17  shows the NMR solution structure of Y III -LanM (PDB code 6MI5).  FIG. 17A  shows the overall structure. Y III  ions are in cyan and EF-loops are shown in gray. Intermediate exchange on the NMR timescale suggests that metal coordination by EF4 is weak.  FIG. 17B : Detail of Y III  coordination in LanM (EF3), with coordinating residues and the N i+1 −H . . . N i  hydrogen bond involving the Pro shown.  FIG. 17C : The hydrogen bonding connectivity of the EF2/3 pair illustrates the importance of cooperativity in rare earth recognition. 
     LanM exhibits 100 million-fold selectivity for Le III  over Ca II . In order to investigate Ln III  binding to the 3 tight binding sites, we used metal chelators, ethylene glycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid (EGTA) and ethylenediamine NN′-disuccinic acid (EDDS), to buffer Le III  concentrations in the low picomolar range and monitored the metal-dependent conformational change by CD spectroscopy (Table 3). EDDS was found to be a suitable chelator for all Le III s except for for which EGTA was used. The full increase in [θ] 222 nm  observed in stoichiometric titrations ( FIG. 5B ) was also seen in these buffered titrations, suggesting that the binding events monitored at picomolar free metal concentrations accounted for LanM&#39;s full conformational change. The [θ] 222 nm  data were fitted to a minimal binding model, the Hill equation, to provide basic information on approximate apparent K d (K d,app ) values and cooperativity (Table 3). In all cases, titrations yielded K d,appS  in the low picomolar range, with Hill coefficients (n) of 3.2±1.3 (average for all Lns tested), indicating cooperativity in the metal-induced conformational change. We observed a slight asymmetry in the titration curves for all Ln III s ( FIG. 10A ), suggestive of the presence of two sets of sites (also supported by titrations monitoring the intrinsic fluorescence of LanM&#39;s sole Tyr residue, Y96). However, only in the case of La III  were the two sets of sites sufficiently resolved to give acceptable errors in fitting; therefore, we report fits to a binding model with one set of sites for all Ln III  other than La III . Interestingly, the Ln III s that induce LanM&#39;s conformational change at lowest concentration, La III -Sm III  ( FIG. 9 ), are also the only Lns that induce expression from the xoxF1 promoter and activate  M. extorquens  XoxF in vivo, perhaps suggesting that LanM may be involved in discrimination between early and late Lns in cells. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Apparent K dS , Hill coefficients (n), and changes in molar ellipticity at 222 nm (Δ[θ])  
               
               
                 from fitted CD titrations of LanM and LanM(4P→4A) with Ln III s and Ca II . Uncertainties  
               
               
                 were determined by combining standard deviations from at least 3 independent titrations 
               
               
                 [2 for Nd III -LanM(4P-4A)] and errors calculated as described. To the right of Er III  in the 
               
               
                 periodic table, uncertainties in determining the endpoints of Ln III -EDDS CD titrations were 
               
               
                 too great to reliably determine K dS . Data for La III  were fitted to two phases but the second 
               
               
                 phase was not well resolved for other metal ions. 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Metal 
                   
                   
                   
               
               
                 Protein 
                 ion 
                 K d  (M) 
                 n 
                 Δ[θ] (deg cm 2  dmol −1  × 10 −3 ) 
               
               
                   
               
               
                 LanM 
                 La III   
                 (5.3 ± 0.6) × 10 −12   
                 4.3 ± 0.2 
                 −1000 ± 50 
               
               
                   
                   
                 (1.7 ± 0.4) × 10 −11   
                 1.5 ± 0.2 
                  −280 ± 50 
               
               
                   
                 Nd III   
                 (5.3 ± 0.6) × 10 −12   
                 2.8 ± 0.4 
                 −1150 ± 70 
               
               
                   
                 Sm III   
                 (6.6 ± 2.8) × 10 −12   
                 3.0 ± 0.5 
                 −1090 ± 60 
               
               
                   
                 Gd III   
                 (1.0 ± 0.4) × 10 −11   
                 2.8 ± 0.4 
                 −1150 ± 80 
               
               
                   
                 Tb III   
                 (2.1 ± 0.2) × 10 −11   
                 3.9 ± 0.6 
                 −1100 ± 60 
               
               
                   
                 Ho III   
                 (2.5 ± 0.4) × 10 −11   
                 2.4 ± 0.5 
                 −1150 ± 90 
               
               
                   
                 Y III   
                 (1.7 ± 0.2) × 10 −11   
                 3.4 ± 0.5 
                 −1110 ± 60 
               
               
                   
                 Ca II   
                 (7.1 ± 0.3) × 10 −4   
                 3.0 ± 0.2 
                  −900 ± 10 
               
               
                 LanM 
                 La III   
                 (1.0 ± 0.1) × 10 −12   
                 2.6 ± 0.2 
                 −1150 ± 40 
               
               
                 4P→4A 
                 Nd III   
                 (4.4 ± 0.5) × 10 −13   
                 2.6 ± 0.2 
                 −1120 ± 30 
               
               
                   
                 Ca II   
                 (2.6 ± 0.3) × 10 −6   
                 2.9 ± 0.2 
                 −1190 ± 10 
               
               
                   
               
            
           
         
       
     
       FIG. 9  shows K d,app  values for Ln III -LanM complexes (Ln=La, Nd, Sm, Gd, Tb, Ho, Y) plotted versus effective ionic radius. 
       FIG. 10  shows that LanM exhibits 10 8 -fold selectivity for early Ln III s over Ca II , which is altered by substitution of LanM&#39;s EF-hand Pro residues. [θ] 222 nm  was monitored at various chelator-buffered free metal ion concentrations using CD spectroscopy, and data were fitted to the Hill equation to determine K a,app  (Table 3).  FIG. 15A : La III -LanM.  FIG. 15B : Ca II -LanM (unbuffered Ca II ).  FIG. 15C : La III -LanM(4P→4A).  FIG. 15D : Ca II -LanM(4P→4A). 
     By contrast with the Les, LanM&#39;s response to Ca II  was so weak that Ca II  titrations were carried out with unbuffered Ca II . Ca II  induces a similar change in CD spectrum as the Les, although somewhat smaller in magnitude and with a K a,app  of 710±30 μM and n=3.0±0.2 ( FIG. 10B , Table 3). Therefore, LanM exhibits a remarkable 10 8 -fold selectivity in its conformational response to Ln III  over Ca II . This selectivity is substantially greater than in other characterized EF hand-containing proteins; for example, CaM only exhibits ˜10 3 -fold selectivity for Tb III  over Ca II , and the EF hand of the wt  E. coli  galactose binding protein (Gln at position 9) exhibits &lt;10-fold selectivity over Ln III s and only 100- to 200-fold selectivity when an Asp or Glu is substituted at position 9. 
     Mutation of LanM&#39;s conserved Pro residues restores Ca II  response at lower concentrations. Because our CD-derived K a,app  values reflect a conformational change and not necessarily an intrinsic K d  for metal binding, we assessed Ca II  binding by isothermal titration calorimetry (ITC) (Table 4). Both binding and conformational changes potentially contribute to ITC isotherms, but the experiment was carried out at 40 μM LanM, far below the K d,app  for Ca II , to minimize the contribution from the conformational change. The best fit to our data was obtained using a model with 2 sets of non-interacting sites, with an endothermic phase with K d =2.5 μM and 2.8±0.4 sites, and an exothermic phase with K d =40 μM and 7.2±0.9 sites. While further analysis is necessary to develop a complete model, our interpretation of these results is that Ca II  initially binds to 3 EF hands (perhaps EF1-3) without inducing a conformational change, followed by multiple further binding events that trigger the conformational response. Therefore, initial Ca II  binding appears to be decoupled from the structural transition in wildtype (wt) LanM. In order to test whether this decoupling, to our knowledge not observed in other EF hand-containing proteins, might be associated with the unusual conserved Pro residues at the 2n d  position of LanM&#39;s EF hands (“Pro2”), we mutated all four Pro2 residues to Ala, a common residue at this position in canonical EF hands. SEC of this “4P→4A” variant suggested that the Pro→Ala mutations significantly alter the protein conformation in the apo, but not holo, state (Table 3). LanM(4P→4A) retained wt LanM&#39;s full molar ellipticity change to Ln III s and Ca II , but it displayed altered metal binding properties. Whereas LanM(4P→4A) responded to La III  and Nd III  at 5- to 10- fold lower concentrations than wt LanM in CD titrations ( FIG. 10C , Table 3), the mutant responded to Ca II  at 300-fold lower concentrations, with K d,app =2.6±0.3 μM ( FIG. 10D , Table 3). Interestingly, this value is similar to the K d  measured by ITC for the first phase of Ca II  binding to wt LanM. Unfortunately, our attempts to obtain an intrinsic K d  for Ca II -LanM(4P→4A) by ITC have been unsuccessful due to complex binding isotherms and contributions from conformational changes, and specialized methods such as flow dialysis will have to be pursued. Nevertheless, substitution of the Pro residues in LanM&#39;s EF hands clearly makes the protein significantly more conformationally sensitive to Ca II . 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Thermodynamic parameters for Ca II  binding to wt LanM, assessed by ITC.  
               
               
                 Conditions: 40 μM wt LanM in cell, 4.0 mM Ca II  in syringe,  
               
               
                 buffer B, 25° C. Data were fitted to a binding model with two 
               
               
                 sets of non-interacting sites. K d  ranges calculated from these K aS :  
               
               
                 1.6-5.7 μM (site 1), 28-60 μM (site 2). 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 ΔS 
               
               
                   
                   
                   
                 ΔH 
                 ΔG 
                 (J K −1   
               
               
                   
                 K a  (M −1 ) 
                 n 
                 (kJ mol −1 ) 
                 (kJ mol −1 ) 
                 mol −1 ) 
               
               
                   
               
               
                 Site 1 
                 (4.1 ± 2.3) × 10 5   
                 2.8 ± 0.4 
                 1.4 ± 0.5 
                 −31.7 ± 11.4 
                 111 ± 5  
               
               
                 Site 2 
                 (2.6 ± 0.9) × 10 4   
                 7.2 ± 0.9 
                 −11.9 ± 3.2  
                 −25.0 ± 10.4 
                 47 ± 14 
               
               
                   
               
            
           
         
       
     
     LanM, a highly selective Ln-binding protein. Our characterization of LanM reveals that nature has adapted the ubiquitous Ca II -binding framework of EF hands to bind Ln III  with high affinity and selectivity over Ca II . While metal ion properties such as ionic radius and charge likely contribute, our data suggest that LanM does not so much suppress Ca II  binding (compared with affinities of dedicated, Ca II -binding EF hand-containing proteins) as it does suppress the conformational response resulting from that binding. It is presently unknown whether the same is true of Ln II  binding to LanM, but because only picomolar Les are needed to trigger the conformational change, this issue may be of lesser importance to the cell than whether Ca II  responsiveness is minimized, given the abundance of the latter ion. The detailed mechanism by which LanM&#39; s conserved Pro residues, as well as other features of its unusual EF hands, accomplish this feat is further discussed elsewhere herein. 
     In addition to the importance of LanM&#39;s Pro2 residues to the high selectivity of the protein&#39;s conformational response to Ln II s over Ca II , two other potential contributing factors are ionic radius and charge. LanM exhibits a slight preference for the larger early Ln III  over smaller later ones ( FIG. 9 ), with apparent K d  s ranging from 5 pM for La III  (effective ionic radius of 1.172 Å for coordination number 6, Table 6) to 25 pM for Ho III  (1.041 Å), the smallest Ln III  investigated in detail. The effective ionic radius of Ca II  is well within this range (1.14 Å), similar to that of Pr III . Therefore, whereas ionic radius may account for preferential response for one Ln over another, it seems to contribute little to LanM&#39;s selectivity for Le III s over Ca II . 
     Similarly, a conformational selectivity for Ln III  over Ca II  of 10 8  (and a selectivity in terms of affinity of at least 10 6 ) is much greater than that of CaM (˜10 3 ) or certain engineered EF-hand peptides (˜50), indicating that the higher charge of Le III s versus Ca II  is also not sufficient to explain LanM&#39;s Ln selectivity. Studies of Falke and co-workers have shown that the presence of an Asp or Glu residue at position 9 of the EF hand (as occurs in all of LanM&#39;s EF hands) of an  E. coli  protein containing a single EF-hand increases Tb III  binding affinity by 10- to 100-fold versus other amino acids at this position. Similarly, perhaps the closest precedent to Ln III  coordination in LanM is the lanthanide binding tag (LBT), a Ca II -binding EF hand engineered to coordinate luminescent Ln III  s more selectively, which also possess a Glu residue at the 9 th  position of the EF hand, coordinated to the Ln III  ion. However, LanM&#39;s K d,app  values for Le III s are 10 3 -10 6 -fold tighter than the K d  s of the LBT (or any other coordination motif for Lns using biological ligands, to our knowledge) depending on the Ln III  ion. Together, these literature precedents suggest that LanM&#39;s Asp at position 9 may contribute to the protein&#39;s high affinity for Les. Characterization of the 4D9N-LaMP1 variant (see below), which exhibits lower apparent K d  for Ca&#39; than wt but a higher apparent K d  for La m , supports this argument. Finally, the unusual structure of LanM ( FIG. 17 ), with the fusion of adjacent EF hands allowing for substantial hydrophobic packing upon metal binding to the EF hands, likely also contributes to the high affinity of the protein for rare earths. These principles derived from our results have been used, as discussed elsewhere herein, to design LanM variants with increased or decreased selectivity for different lanthanides or actinides, and they can also be applied to yield other variants for binding of actinide ions such as, for example, uranium by suitably mutating appropriate amino acids. In other words, homologues of LanM can potentially be tailored to provide selectivity between different lanthanides or actinides. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Primers used for cloning and 
               
               
                 sequencing 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 SEQ 
               
               
                   
                   
                   
                 ID 
               
               
                   
                   
                 Sequence 
                 NO: 
               
               
                   
                   
               
               
                   
                 Name 
                   
                   
               
               
                   
                 LanM-Ndel- 
                 5′-ATAACATATGGCGCCAA 
                 21 
               
               
                   
                 for 
                 CTACGACTACCAAAG-3′ 
                   
               
               
                   
                   
               
               
                   
                 LanM-EcoRl- 
                 5′-AATAGAATTCTTAACGA 
                 22 
               
               
                   
                 rev 
                 ATTAAGTTGACCAGGGC-3′ 
                   
               
               
                   
                   
               
               
                   
                   
                 5′-ATAACATATGCATCATC 
                 23 
               
               
                   
                   
                 ACCATCATCAC 
                   
               
               
                   
                   
               
               
                   
                 His-LanM- 
                 GCGCCAACTACGACTACCAA 
                 24 
               
               
                   
                 Ndel-for 
                 AG-3′ 
                   
               
               
                   
                   
               
               
                   
                 LanM-His- 
                 5′-ATAAGAATTCTTAGTGA 
                 25 
               
               
                   
                 EcoRl-rev 
                 TGATGGTGATGATGACG 
                   
               
               
                   
                   
               
               
                   
                   
                 AATTAAGTTGACCAGGGCCG 
                 26 
               
               
                   
                   
                 AC-3′ 
                   
               
               
                   
                   
               
               
                   
                 Sequencing 
                   
                   
               
               
                   
                 primers 
                   
                   
               
               
                   
                 T7P 
                 5′-TAATACGACTCACTATA 
                 27 
               
               
                   
                   
                 GGG-3′ 
                   
               
               
                   
                   
               
               
                   
                 T7T 
                 5′-GCTAGTTATTGCTCAGC 
                 28 
               
               
                   
                   
                 GG-3′ 
               
               
                   
                   
               
            
           
         
       
     
     Conservation of LanM in methylotrophs and physiological role of LanM. To date, Ln-containing enzymes have only been characterized in several methylotrophs and in  Pseudomonas putida.  BLAST analysis of the  P. putida  KT2440 genome found no LanM homologs but a putative EF-hand with a Pro2 residue (DPDEDGFTICGD) was identified in a predicted hybrid sensor hi stidine kinase/response regulator (locus WP_010954492); these observations motivate exploration of whether EF hands with Pro2 residues constitute Ln-selective coordination motifs in biology more generally, which may aid identification of new organisms that can utilize Lns for growth. More importantly, however, BLAST analysis using the LanM sequence as a query indicates that LanM-like genes with at least one EF hand containing the Pro2 signature are widespread in  Methylobacterium  and  Bradyrhizobium  genomes. One of those genes, c02050 from  Methylobacterium aquaticum  strain 22A, which has 59% sequence identity with LanM and contains two Pro2 motifs among its four EF hands, was very recently shown to be induced 5-fold in response to La III  (consistent with the growth conditions from which LanM was identified. The encoded protein was not characterized and no evidence that it would bind Lns with such selectivity over Ca and other metals was presented. 
     In the  M. extorquens  genome, lanM is adjacent to META1p1785, encoding a putative TonB-dependent transporter. TonB-dependent systems often function in metal acquisition (e.g., Fe III -citrate and Fe III -siderophore uptake), and include an outer membrane transporter and a periplasmic binding protein that receives the cargo from the transporter. Indeed, the LanM-derived sensor LaMP1 has been used to show that these bacteria secrete a Ln-binding molecule (lanthanophore) to facilitate selective uptake of early Lns (La—Nd, and Sm to a lesser extent. Further work is necessary to define the physiological function of LanM, although its highly Le-selective disorder-to-order transition points to a function in which it is critical to the cell that formation of an “active” structure occurs only in the presence of Le. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Effective ionic radii (crystal radii, Å) of Ln(III), Ca(II), and 
               
               
                 Y(III), for coordination numbers (CN) = 6-12. 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Element 
                 CN = 6 
                 CN = 7 
                 CN = 8 
                 CN = 9 
                 CN = 10 
                 CN = 12 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Ca(II) 
                 1.14 
                 1.20 
                 1.26 
                 1.32 
                 1.37 
                 1.48 
               
               
                 Y(III) 
                 1.040 
                 1.10 
                 1.159 
                 1.215 
                   
                   
               
               
                 La(III) 
                 1.172 
                 1.24 
                 1.300 
                 1.356 
                 1.41 
                 1.50 
               
               
                 Ce(III) 
                 1.15 
                 1.21 
                 1.283 
                 1.336 
                 1.39 
                 1.48 
               
               
                 Pr(III) 
                 1.13 
                   
                 1.266 
                 1.319 
                   
                   
               
               
                 Nd(III) 
                 1.123 
                   
                 1.249 
                 1.303 
                   
                 1.41 
               
               
                 Sm(III) 
                 1.098 
                 1.16 
                 1.219 
                 1.272 
                   
                 1.38 
               
               
                 Eu(III) 
                 1.087 
                 1.15 
                 1.206 
                 1.260 
                   
                   
               
               
                 Gd(III) 
                 1.078 
                 1.14 
                 1.193 
                 1.247 
                   
                   
               
               
                 Tb(III) 
                 1.063 
                 1.12 
                 1.18 
                 1.235 
                   
                   
               
               
                 Dy(III) 
                 1.052 
                 1.11 
                 1.167 
                 1.223 
                   
                   
               
               
                 Ho(III) 
                 1.041 
                   
                 1.155 
                 1.212 
                 1.26 
                   
               
               
                 Er(III) 
                 1.030 
                 1.085 
                 1.144 
                 1.202 
                   
                   
               
               
                 Tm(III) 
                 1.020 
                   
                 1.134 
                 1.192 
                   
                   
               
               
                 Yb(III) 
                 1.008 
                 1.065 
                 1.125 
                 1.182 
                   
                   
               
               
                 Lu(III) 
                 1.001 
                   
                 1.117 
                 1.172 
               
               
                   
               
            
           
         
       
     
     The highly Ln-selective conformational response of LanM strongly suggests that this protein is a member of the emerging “lanthanome” in methylotrophs, consisting of proteins and small molecules involved in regulation, uptake, trafficking, storage, and utilization of Lns in certain bacteria and possibly other organisms. Characterization of this network will not only enrich our understanding of the fundamental coordination chemistry of Ln ions; it will also inspire model complexes to probe the chemistry of biology&#39;s important Ln-catalyzed reactions, enable electrocatalytic applications of these enzymes, and inform efforts to engineer methylotrophs or proteins/small molecules produced by them (such as LanM itself) for more sustainable extraction and separation of these relatively inaccessible yet increasingly technologically useful elements, in addition to medical applications (e.g., Gd-bound LanM or derivatives as an Mill contrast agent, or radioactive Lu-bound LanM or derivatives as a radiopharmaceutical). 
     The following examples are intended to further illustrate certain preferred embodiments of the invention and are not limiting in nature. Those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific substances and procedures described herein. 
     The disclosure that follows describes the experiments conducted for characterizing and measuring various properties of LanM. In these experiments, lanthanide salts (chloride or acetate) were at a minimum purity of 99.9% rare earth metal content. SmCl 3  was obtained from Alfa Aesar, TmCl 3  was from Strem, YCl 3  was from Acros, and all other Le III s were from Sigma-Aldrich.  Methylobacterium  (now  Methylorubrum ) extorquens (ATCC 14718, NUB 9133) was obtained from the American Type Culture Collection.  M. extorquens  AMI gene sequences and genome maps were obtained from BioCyc (https://biocyc.org). Primers and gBlocks were ordered from Integrated DNA Technologies (IDT).  E. coli  strains [5alpha and BL21(DE3)] for cloning and recombinant protein expression, respectively, as well as cloning reagents (restriction enzymes, Q5 DNA polymerase, OneTaq DNA polymerase, T4 DNA ligase, and the HiFi Assembly Kit, KLD Enzyme Mix) were purchased from New England Biolabs. PCR cleanup and miniprep kits were from Qiagen and Omega Bio-tek, and gel extraction utilized the Zymoclean gel DNA recovery kit from Zymo Research. Ni-NTA resin was purchased from Qiagen and Thermo Scientific. Protein gel electrophoresis was carried out using either homemade or BioRad 4-20% gradient gels and a Biorad Mini-Protean apparatus, or Life Tech 16% Tris-glycine gels and a mini gel apparatus. Automated protein chromatography used a GE Healthcare Biosciences Akta Pure fast protein liquid chromatography (FPLC) system. UV-visible absorption spectra were obtained on an Agilent Cary 60 UV-visible spectrophotometer using a quartz cuvette (Starna Cells). Well plate analyses were carried out using a Tecan Infinite M1000 Pro plate reader. Vector DNA sequences were verified by sequencing at the Huck Genomics Facility, and CD and Direct Detect measurements were carried out in the X-ray Crystallography and Automated Biological calorimetry Facility at Penn State. To minimize metal contamination, all buffers used for LanM experiments were treated with 10 g/L Chelex 100 (BioRad, 100-200 mesh) for &gt;1 h, followed by pH re-adjustment, according to the manufacturer&#39;s protocol. All protein and metal solutions were made in 1.5 mL microcentrifuge tubes or 15 mL or 50 mL centrifuge tubes obtained from Sarstedt. 
     EXPERIMENTAL TECHNIQUES 
     Growth of  M. extorquens.  MP medium was prepared as described by Marx and coworkers, except that iron (as ferrous ammonium sulfate) was withheld from the C7-metals solution and added as a solid prior to autoclaving. The medium consisted of: 30 mM PIPES (pH 6.75), 1.45 mM K 2 HPO 4,  1.88 mM NaH 2 PO 4 , 8 mM (NH 4 ) 2 SO 4 , 0.5 mM MgCl 2 , 20 μM CaCl 2 , 45.6 μM sodium citrate, 18 μM (NH 4 ) 2 Fe(SO 4 ) 2 , 1.2 μM ZnSO 4 , 1 μM MnCl 2 , 14 μM Na 2 MoO 4 , 1 μM CuSO 4 , 2 μM CoCl 2 , and 0.33 μM Na 2 WO 4 , supplemented with 0.5% v/v methanol. All growths were carried out at 30° C. M extorquens was streaked out from a glycerol stock onto MP medium/methanol-1.5% agar plates. A single colony was used to inoculate 3 mL MP media/0.5% methanol culture and grown for ˜2 d with 200 rpm shaking in a 14-mL polypropylene culture tube (BD Falcon). The starter culture (2 mL) was used to inoculate 200 mL MP-methanol media supplemented with 1μM LaC13, and cells were grown to an OD 600 ˜0.8, at which point the culture was used to inoculate 4×2 L MP-methanol media containing 1 μM LaCl 3  in 6 L flasks to an OD 600 ˜0.02. The cultures were grown to an OD 600 ˜0.8-1.0. Cells were harvested by centrifugation at 7000 g, 4° C., 7 min and frozen at −80° C. Typical yield is 1 g cell paste per L culture. 
     Purification of XoxF. All operations were performed at 4° C. The cell paste from an 8 L  M. extorquens  growth was resuspended in 5 mL/g 20 mM MES, pH 5.5 (Buffer A) and 1 Roche Complete mini protease inhibitor tablet per 10 mL. The suspension was passed twice through a French pressure cell at 14000 psi, and debris was pelleted by centrifugation at 30000×g for 20 min at 4° C. The supernatant was decanted and ammonium sulfate was added to 40% saturation (226 g/L at 4° C.) over 15 min, with stirring. After 15 min further stirring, the suspension was centrifuged at 20000×g for 10 min at 4° C. The supernatant was decanted and ammonium sulfate was added to 80% saturation (an additional 258 g/L at 4° C.), followed by further stirring and centrifugation as above. The pellet after centrifugation was resuspended in a minimal volume (˜4 mL) Buffer A, supplemented with 0.25 mM PMSF, and applied to a 2.5×16 cm (80 mL) Sephadex G50 column, pre-equilibrated in Buffer A with 0.25 mM PMSF. Fractions were collected by following the pale red protein band and protein-containing fractions were pooled based on A28onm using a plate reader. The pooled fractions were concentrated to 5 mL using an Amicon Ultra 10 kDa MWCO centrifugal filtration device and insoluble material was removed by centrifugation (14000×g, 2 min). The supernatant was loaded onto a HiPrep SP FF 16/10 column (20 mL column volume, CV), preequilibrated in Buffer A, on a GE Healthcare Akta Pure fast protein liquid chromatography (FPLC) system. The column was washed with 2 CV Buffer A and eluted with a gradient of 0-250 mM NaCl in Buffer A over 10 CV. Fractions (4 mL) were collected in peak fractionation mode, and chromatograms were collected at 280 nm, 346 nm (PQQ absorption), and 413 nm (cytochrome Soret band). The flow rate was 2 mL/min. Fractions were pooled based on activity using the dye-linked assay described in the Supporting Information (2-5 μL each fraction). Activity eluted with the 346 nm peak at 110-150 mM NaCl. The pooled fractions were concentrated to 2 mL using an Amicon Ultra 30 kDa MWCO centrifugal filtration device and loaded to a HiLoad 16/600 Superdex 75 pg column (120 mL) in Buffer B (20 mM MOPS, 20 mM acetate, 100 mM KCl, pH 7.0). The column was run at 0.75 mL/min, collecting 1.5 mL fractions, and following absorbance at 280, 346, and 413 nm. XoxF eluted at 48-53 mL and LanM eluted at 73-77 mL. Based on subsequent studies of the heterologously expressed protein, LanM&#39;s elution time suggests that it is bound to La m , but the protein&#39;s purity was insufficient to assess this observation further. Fractions were pooled based on purity assessed by SDS-PAGE. XoxF was concentrated using a 30 kDa MWCO filtration device. The concentration of purified XoxF was determined using 628onm=137 mM −1  cm −1  (per monomer), obtained via the ExPASy ProtParam tool. The A 280 nm /A 350 nm  ratio was 10.2. For generation of the purification table (Table 7), the protein concentration in samples prior to the final chromatographic step was determined using the Pierce BCA Protein Assay Kit (Thermo) with bovine serum albumin as standard. The yield of XoxF was 2-3 mg from 8 L culture. 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 Purification of XoxF from 8 L culture of  M .  extorquens  cells,  
               
               
                 ~10 g cell paste 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                 Total 
                 Specific 
                   
                   
               
               
                   
                 Protein 
                 activity 
                 activity 
                 Percent 
                 Purification 
               
               
                 Purification step 
                 (mg) 
                 (U) 
                 (U/mg) 
                 recovery 
                 factor 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Crude extract 
                 576 
                 430 
                 0.75 
                 100 
                 1 
               
               
                 40% (NH 4 ) 2 SO 4   
                 316 
                 389 
                 1.2 
                 91 
                 1.6 
               
               
                 supernatant 
                   
                   
                   
                   
                   
               
               
                 40:80% 
                 266 
                 323 
                 1.2 
                 75 
                 1.6 
               
               
                 (NH 4 ) 2 SO 4 * 
                   
                   
                   
                   
                   
               
               
                 G50 
                 189 
                 323 
                 1.7 
                 75 
                 2.3 
               
               
                 SP FF 
                 3.9 
                 85 
                 22 
                 20 
                 29 
               
               
                 Superdex 
                 3.1 
                 67 
                 22 
                 16 
                 29 
               
               
                   
               
               
                 *Resuspension of pellet following addition of ammonium sulfate to 80% saturation and centrifugation 
               
            
           
         
       
     
     Dye-linked activity assays. Activity of chromatography fractions and of purified XoxF was determined by reduction of 2,6-dichlorophenolindophenol (DCPIP) using phenazine thiosulfate (PES) as an electron acceptor, according to the method of Day and Anthony. Each assay mixture (485 μL) contained: 100 mM Tris, pH 9.0, 15 mM NH 4 Cl, 1 mM phenazine ethosulfate (PES), and 100 μM DCPIP. Assays were performed at room temperature (23° C.) on a Cary 60 UV-visible spectrophotometer in Kinetics mode, monitoring at 600 nm. The assay sample (1-10 μL) was added to the mixture. A large amount of background activity was present, especially in crude samples; after several minutes (1-5 min), the A 600 nm  reading stabilized and 10 μL 1 M (4.0% v/v) methanol was added to measure methanol-dependent activity. The slope measured (ΔAU/min) was used to calculate enzyme activity, which is reported in units (U), where 1 U is defined as 1 μmol O 2  reduced per min between 15 and 30 s after starting reaction, corresponding to 1 μmol DCPIP reduced/min. The extinction coefficient for DCPIP in assay buffer was determined by dilution of an aqueous stock solution of ˜3 mM DCPIP into 20 mM MOPS pH 7.0 (ε 600 nm =20600 M −1  cm −1  at pH 7.0) and into 100 mM Tris pH 9.0, followed by comparison of the absorbance values, yielding ε 600 nm =22500 M −1  cm −1 . Protein content in crude samples was determined by BCA assay. 
     Mass spectrometric analysis on gel sections to identify LanM. 
     a) Sample preparation, gel sections. Gel sections were diced using a clean razor blade into 1 mm 3  pieces and washed 3 times with approximately 5 gel volumes of 50% acetonitrile (MeCN) containing 50 mM triethylammonium bicarbonate (TEAB) for 20 min at 37° C. Reduction of disulfides and alkylation of Cys residues was performed by incubating the gel sections in 200 mM TEAB containing 5 mM tris(2-carboxyethyl)phosphine hydrochloride (TCEP) for 30 min at 37° C. followed by a 15-min incubation in 50 mM TEAB, 100 mM iodoacetamide at 37° C. with mixing in an Eppendorf Thermomixer R. The reducing and alkylating reagents were removed; the gels were washed 3 times in 50% MeCN, 50 mM TEAB as described above and dehydrated in neat MeCN. Residual MeCN was removed in a Speedvac (Thermo). 
     b) Proteolysis. A 1 μg/μL stock solution of sequencing-grade trypsin (Thermo) in 1 mM aqueous HCl was diluted 100-fold with chilled 50 mM TEAB, and the dried gel pieces were allowed to completely re-hydrate in the protease solution. Excess protease solution was removed and a sufficient volume of 50 mM TEAB was added to cover the gel. Proteolysis was carried out overnight at 37° C., after which the TEAB solution was transferred to a new microcentrifuge tube. The peptides were extracted with three 50-4, volumes of 50% MeCN, 0.1% formic acid and one 50-4, volume of neat MeCN; and these solutions were combined with the TEAB solution and dried in the Speedvac. The dried peptides were reconstituted in 10 μL 2% MeCN, 0.1% formic acid for LC MS 2  analysis. 
     c) Nano-LC MS 2 . The peptides were loaded onto an Acclaim PepMap100 trapping column (100 μm×2 cm, C18, 5 μm, 100 Å, Thermo) at a flow rate of 5 μL/min using 2% aqueous MeCN, 0.1% formic acid as a mobile phase. The peptides were separated on an Acclaim PepMap RSLC column (75 μm×15 cm, C18, 2 μm, 100 Å, Thermo) with a 30-min linear gradient of 2-35% MeCN in water containing 0.1% formic acid. The gradient was delivered by a Dionex Ultimate 3000 nano-LC system (Thermo) at 300 nL/min. Mass spectra were acquired on an LTQ Orbitrap Velos (Thermo) using an ‘Nth order double play’ method: full FT MS scan at R 60,000 followed by 10 ion-trap MS 2  scans on the most intense precursors with CID activation. Only the precursors with charge states 2 or higher were selected for MS 2 ; monoisotopic precursor selection was enabled, and the isolation window was 2 m/z. Polysiloxane signal at 445.120030 m/z was used as the lock mass; ion injection times and automatic gain control target values were set to default. 
     d) Data analysis. Mass spectra were analyzed using two different applications: Proteome Discoverer 1.3 (P.D. 1.3, Thermo) with SEQUEST search engine, and PEAKS Studio (Bioinformatics Solutions, bioinfor.com) with PEAKS DB and Spider search algorithms. In both applications, cysteine carbamidomethylation (+57.021) was selected as the static modification, and methionine oxidation (+15.995 m.u.) was selected as the dynamic modification; two missed cleavages were allowed; and the mass tolerances for precursors and fragments were set to 15 ppm and 0.5 Da respectively. The protein sequence database was compiled from the Uniprot reference proteome UP000009081, Methylobacterium extorquens (strain ATCC 14718/DSM 1338/JCM 2805/NCIMB 9133/AM1) containing 6233 sequences and a common contaminants database (299 sequences). 
     Construction, expression, and purification of C-terminally TEV/His 6 -tagged LanM from the  E. coli  periplasm. 
     a) Cloning.  M. extorquens  AM1 META1p1786 was obtained as a 461-bp gBlock gene fragment containing a TEV protease cleavage site followed by a His 6  tag (GENLYFQGHEIHHHH) (SEQ ID NO: 8) at the C-terminus, codon optimized for expression in E. coli using IDT&#39;s online codon optimization tool (Table 9). To prepare the insert, the gBlock (250 ng, 10 ng/μL) was digested with NdeI and EcoRI-HF for 1 h, and the enzymes were inactivated by incubation at 65° C. for 20 min. pET-24a (2 μg, Table 8 for all plasmids used in this study) was digested with NdeI and EcoRI-HF (20 U each) for 1 h, and following gel electrophoresis (1% agarose), the vector fragment was excised and purified using the Zymoclean Gel DNA Recovery Kit. The insert was ligated into the digested vector (5:1 insert:vector) using T4 DNA ligase according to the manufacturer&#39;s protocol. Transformants were screened for insert by colony PCR (OneTaq Quick-Load) and the correct insert was confirmed by DNA sequencing using primer T7P (Table 5 for all primers used in this study) at the Penn State Genomics Core Facility, yielding pET24a-LanM-TEV/His. 
     
       
         
           
               
             
               
                 TABLE 9 
               
               
                   
               
               
                 DNA sequence for  M. extorquens  AM1 LanM 
               
               
                 (META1p1786) (SEQ ID NO: 2), and gBlocks 
               
               
                 for expression of LanM. The region encoding 
               
               
                 the signal peptide, Met1-Ala21, is shown in 
               
               
                 double underline, and the TEV protease 
               
               
                 recognition sequence and the His 6  tag are 
               
               
                 underlined. 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
            
               
                 &gt;META1p1786 (SEQ ID NO: 2) 
               
               
                 atgGCGTTCC GCCTCTCCTC TGCCGTTCTG CTCGCTGCGC 
               
               
                 TCGTCGCCGC TCCGGCCTAC GCCGCCCCGA CGACGACCAC 
               
               
                 GAAGGTCGAT ATCGCCGCCT TCGATCCGGA CAAGGACGGC 
               
               
                 ACGATCGATC TGAAGGAGGC CCTGGCCGCC GGCTCCGCTG 
               
               
                 CCTTCGACAA GCTCGATCCG GATAAGGACG GCACGCTCGA 
               
               
                 CGCTAAGGAG CTGAAGGGCC GCGTCAGCGA GGCCGACCTC 
               
               
                 AAGAAGCTCG ACCCGGACAA CGACGGCACC CTCGACAAGA 
               
               
                 AGGAATACCT TGCCGCCGTC GAGGCGCAGT TCAAGGCCGC 
               
               
                 TAACCCGGAC AACGACGGCA CCATCGACGC TAGGGAATTG 
               
               
                 GCCAGCCCGG CCGGTTCGGC CCTGGTCAAT CTGATCCGCt 
               
               
                 aa 
               
               
                   
               
               
                 &gt;Codon-optimized, C-terminal TEV and 
               
               
                 His 6  sites (SEQ ID NO: 11)  
               
               
                 aatacat ATGGCGTTTCGTTTGAGCAGTGCCGTGTTATTAGCTGCGCTTG   
               
               
                   TAGCAGCTCCGGCGTATGCC GCGCCAACTACGACTACCAAAGTTGATATC 
               
               
                 GCGGCGTTTGACCCGGACAAAGATGGGACCATCGATCTGAAAGAGGCTTT 
               
               
                 GGCGGCAGGTTCCGCGGCCTTCGACAAGTTGGACCCGGATAAAGATGGTA 
               
               
                 CTCTGGACGCCAAAGAGCTGAAGGGCCGCGTGTCTGAGGCAGACCTTAAG 
               
               
                 AAGCTGGACCCTGACAATGACGGAACCCTGGAGAAGAAAGAGTACTTAGG 
               
               
                 AGCGGTAGAGGCACAGTTTAAGGCCGCTAACCCTGACAACGATGGCACTA 
               
               
                 TTGACGCCCGTGAACTTGCAAGCCCAGCGGGGTCGGCCCTGGTCAACTTA 
               
               
                 ATTCGT GGTGAAAACCTGTATTTTCAGGGCCATCATCACCATCATCAC TA 
               
               
                 Agaattctatt 
               
               
                   
               
               
                 &gt;Codon-optimized, N-terminal TEV site, 
               
               
                 His 6  tag from pET28a (SEQ ID NO: 12)  
               
               
                 aatacatATG GAAAACCTGTATTTTCAGGGC GCGCCAACTACGACTACCA 
               
               
                 AAGTTGATATCGCGGCGTTTGACCCGGACAAAGATGGGACCATCGATCTG 
               
               
                 AAAGAGGCTTTGGCGGCAGGTTCCGCGGCCTTCGACAAGTTGGAC CCG GA 
               
               
                 TAAAGATGGTACTCTGGACGCCAAAGAGCTGAAGGGCCGCGTGTCTGAGG 
               
               
                 CAGACCTTAAGAAGCTGGACCCTGACAATGACGGAACCCTGGACAAGAAA 
               
               
                 GAGTACTTAGCA GCG GTAGAGGCACAGTTTAAGGCCGCTAACCCTGACGA 
               
               
                 CGATGGCACTATTGACGCCCGTGAACTTGCAAGCCCAGCGGGGTCGGCCC 
               
               
                 TGGTCAACTTAATTCGTTAAgaattctatt 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 Plasmids used in this disclosure 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 SEQ ID 
               
               
                 Name 
                 Notes 
                 Source 
                 NO: 
               
               
                   
               
               
                 pET-24a 
                 Km R   
                 Novagen 
                 71 
               
               
                 pET-28a 
                 Km R   
                 Novagen 
                 72 
               
               
                 pET-24a-LanM-TEV/His 
                 Expression of a TEV-cleavable, 
                 Present 
                 73 
               
               
                   
                 C-terminally His 6 -tagged LanM, 
                 Disclosure 
                   
               
               
                   
                 for isolation from periplasm 
                   
                   
               
               
                 pET-28a-His/TEV-LanM 
                 Cytosolic expression of LanM 
                 Present 
                 74 
               
               
                   
                 with an N-terminal, TEV- 
                 Disclosure 
                   
               
               
                   
                 cleavable His 6  tag, to yield wt 
                   
                   
               
               
                   
                 LanM 
                   
                   
               
               
                 pET-24a-His-LanM 
                 N-terminally His 6 -tagged LanM, 
                 Present 
                 75 
               
               
                   
                 cytosolic expression 
                 Disclosure 
                   
               
               
                 pET-24a-LanM-His 
                 C-terminally His 6 -tagged LanM, 
                 Present 
                 76 
               
               
                   
                 cytosolic expression 
                 Disclosure 
                   
               
               
                 pET28a-His/TEV-LanM(4P→4A) 
                 His/TEV-LanM with 
                 Present 
                 77 
               
               
                   
                 P36A/P60A/P85A/P109A 
                 Disclosure 
                   
               
               
                 pET24a-LanM(4P→4A)-His 
                 LanM-His with 
                 Present 
                 78 
               
               
                   
                 P36A/P60A/P85A/P109A 
                 Disclosure 
                   
               
               
                   
               
            
           
         
       
     
     b) Expression. Chemically competent  E. coli  BL21 (DE3) cells were transformed with pET24a-LanM-TEV/His and plated on LB-agar plates containing 50 μg/mL kanamycin (Km) and grown at 37° C. A single colony was used to inoculate 7 mL of LB (50 μg/mL Km in all growth media), which was grown for ˜16 h at 37° C. with shaking at 200 rpm. This culture was used to inoculate one 2 L culture (in a 6 L flask) of LB media supplemented with 0.5 mM MgCl 2  and 0.1 mM CaCl 2 , and the culture was grown at 37° C. with shaking at 200 rpm. At OD 600 nm ˜0.6, isopropyl β-D-1-thiogalactopyranoside (IPTG, Amresco) was added to a final concentration of 0.2 mM; after 3 h further incubation, the cells were pelleted by centrifugation for 7 min at 7000×g, at 4° C., yielding ˜2 g cell paste per L culture. 
     c) Purification. The cell paste was re-suspended in 40 mL/g of 30 mM Tris, 1 mM EDTA, 20% sucrose, pH 7.4 (Buffer C) and stirred for 20 min at room temperature. The suspension was centrifuged at 9000×g for 10 minutes at 4° C. The supernatant was decanted and the cell pellet was re-suspended in 20 mL 5 mM MgSO 4  per g of the original cell paste, with stirring for 20 min at 4° C., and the suspension was centrifuged at 9000×g for 10 min at 4° C. The supernatant was decanted and 0.05 volumes of 1.0 M Tris, pH 7.4 was added, along with solid NaCl to 100 mM. This solution was applied to a 0.7×1.0 cm (1.0 mL) Ni-NTA agarose column, pre-equilibrated in 50 mM sodium phosphate, 10 mM imidazole, 5% glycerol, pH 7.0 (Buffer D). The column was washed with 30 CV of Buffer D containing 100 mM NaCl and 0.25 mM phenylmethylsulfonyl fluoride (PMSF), followed by elution with 15 CV of 50 mM sodium phosphate, 250 mM imidazole, 5% glycerol, pH 7.0 (Buffer E). The eluted protein was concentrated to 1.5 mL using an Amicon Ultra 15 3-kDa MWCO centrifugal filtration device. 
     Higher molecular weight contaminants were removed and buffer exchanged into 20 mM MOPS, 100 mM KCl, 1 mM EDTA, pH 7.0 (Buffer F), by size-exclusion chromatography on a HiLoad 16/600 Superdex 75 pg column (GE Healthcare, 120 mL). After column equilibration at 1.0 mL/min, the protein was loaded onto the column using a 2-mL capillary loop, and the column was eluted with 1.2 CV Buffer F at 0.75 mL/min, with 1 mL fractions collected in peak fractionation mode (1 mAU threshold). LanM-TEV/His eluted at 54-63 mL. Protein-containing fractions were collected and analyzed by SDS-PAGE. Fractions were concentrated using an Amicon Ultra 3-KDa MWCO centrifugal filtration device and protein concentration was estimated using 6280nm=1490 M −1  cm −1  (ExPASy ProtParam). The purification yielded 1.4 mg LanM-TEV/His from 2 L cultured) Mass spectrometry on LanM-TEV/His. LanM-TEV/His (˜100 μM, 100  1 μL) was exchanged into water using a 0.5 mL Zeba spin column (Thermo Fisher), according to the manufacturer&#39;s protocol. The intact protein was analyzed by MALDI-TOF MS in linear positive-ion mode on an Ultraflextreme mass spectrometer (Bruker Daltonics) at the Penn State Proteomics and Mass Spectrometry Facility. 
     Cloning of LanM for cytosolic expression. The soluble domain of M extorquens AM1 META1p1786, lacking the N-terminal signal sequence and containing a TEV protease cleavage site (GENLYFQG) (SEQ ID NO: 61) was obtained as a 380-bp gBlock gene fragment, codon optimized for expression in E. coli using IDT&#39;s online codon optimization tool (Table 9). Following digestion with NdeI and EcoRI, the gBlock was inserted into similarly digested pET-28a, and the correct insert confirmed by DNA sequencing using primer T7P at the Penn State Genomics Core Facility, yielding pET28a-His/TEV-LanM. 
     Expression and purification of His/TEV-LanM. Chemically competent  E. coli  BL21(DE3) cells were transformed with pET28a-His/TEV-LanM and plated on LB-agar plates containing 50 μg/mL Km and grown at 37° C. A single colony was used to inoculate 100 mL of LB (50 μg/mL Km in all growth media), which was grown for 16 h at 37° C. with shaking at 220 rpm. This culture (40 mL) was used to inoculate one 2 L culture (in a 6 L flask) of LB supplemented with 0.5 mM MgCl 2  and 0.1 mM CaCl2, and the cultures were grown at 37° C. with shaking at 200 rpm. At OD 600 nm ˜0.5, IPTG was added to a final concentration of 0.2 mM; after 3 h further incubation, the cells were pelleted by centrifugation for 7 min at 7000×g, 4° C., yielding 2.2 g cell paste per L culture. The cell pellet was frozen at −80° C. 
     All operations were performed at 4° C. The cell paste was resuspended in 5 mL/g of Buffer D containing 2 Roche Complete mini protease inhibitor tablets, 2 U/mL DNase, and 0.25 mM phenylmethanesulfonyl fluoride (PMSF). The suspension was passed twice through a French pressure cell at 14000 psi, and debris was pelleted by centrifugation at 30000×g for 20 min at 4° C. The supernatant was decanted and applied to a 1.5×2 cm (3.5 mL) Ni-NTA agarose column, pre-equilibrated in Buffer D. The column was washed with 20 CV Buffer D containing 100 mM NaCl, followed by elution with 10 CV Buffer E. The eluted protein was concentrated to 10 mL using an Amicon Ultra 3-kDa MWCO centrifugal filter device. The His6 tag and TEV recognition sequence were cleaved from 5 mL of the protein by 16 h incubation in 50 mM Tris, 5 mM dithiothreitol, 1 mM EDTA, pH 7.4, with 100 μL 25 μM TEV protease (a gift from X. Zhang, Pennsylvania State University), in a total volume of 25 mL. The approximate protein concentration during digestion was 150 μM, based on ε 275 nm =2800 M −1 cm −1  (one Tyr residue is in the tag and one in the native protein sequence). The solution was then diluted to 125 mL with 50 mM Tris, pH 7.4 and passed through the same pre-equilibrated Ni-NTA column as above, and the column was washed with 1.5 CV of 50 mM Tris, pH 7.4. The pooled flowthrough and wash was concentrated to 1.5 mL by centrifugal filtration (Amicon Ultra 3 kDa MWCO). 
     Higher molecular weight contaminants were removed and buffer was exchanged into Buffer B by size-exclusion chromatography on a HiLoad 16/600 Superdex 75 pg column. After column equilibration, the protein sample was loaded using a 2 mL capillary loop, washed with 3 mL Buffer B, and eluted with 1 CV Buffer B at 0.75 mL/min, with 1.5 mL fractions collected (1 mAU threshold in peak fractionation mode). LanM-containing fractions (55-63 mL) were pooled based on A28onm and concentrated by centrifugal filtration to &lt;3 mL. The protein was dialyzed overnight (-16 h) at room temperature against 500 mL Buffer B containing 5 g Chelex-100 (following stirring for 1 h and correction of the pH to 7.0) using a Slide-a-Lyzer dialysis cassette (MWCO 3500). Protein concentration was determined using ε 275 nm =1400 M −1 cm −1 , determined using an EMD Millipore Direct Detect instrument. The purification yielded 15 mg/L culture. The resulting Chelex-treated buffer was used for all subsequent experiments except for XO titrations and K d  determination using buffered metal solutions. 
     Thus, an aspect of the present disclosure includes an isolated metal-binding protein encoded by a nucleotide comprising a sequence having at least 80% identity to the sequence of SEQ ID NO: 2 (see Table 9). In an embodiment, the nucleotide includes a sequence with at least 85%, 90%, 93%, 95%, 97%, 98%, 99% or 100% identity to the sequence of SEQ ID NO: 2. For example, in an embodiment, the nucleotide comprises a sequence of SEQ ID NO: 2 with the first 21 nucleotides removed. 
     Construction and purification of His 6 -tagged LanMs. Constructs with an N-terminal or C-terminal His 6  tag were generated by PCR amplification of the LanM gene in pET28a-His/TEV-LanM using primers His-LanM-NdeI-for, LanM-EcoRI-rev, LanM-NdeI-for, and LanM-His-EcoRI-rev. Following digestion with Ndel and EcoRI-HF, the inserts were ligated into a similarly digested pET24a, screened, and verified by sequencing. These proteins were expressed and purified analogously to His/TEV-LanM, except that the column used was 8 mL (1.5×4.5 cm), no TEV digestion was necessary, and the buffer used for Superdex 75 chromatography was 20 mM MOPS, 20 mM KCl, 5 mM acetate, 1 mM EGTA, pH 7.0 (Buffer G). The purifications yielded 15 mg and 23 mg protein/L culture for His-LanM and LanM-His, respectively. 
     Competition assays using xylenol orange (XO). A cuvette contained 500 μL 20 mM MES-KOH, 20 mM acetate, 100 mM KCl, pH 6.1 (Buffer H, stirred with 10 g/L Chelex 100 for 1 h followed by pH adjustment), 7 μM xylenol orange, and 0 or ˜5 μM LanM, into which 0.5 or 1 μL aliquots of a 625-1250 μM solution of LnCl 3  in the same buffer were added. The cuvette was washed with 6 N HCl prior to each experiment to minimize the potential for metal contamination across experiments. For experiments utilizing protein, buffer and protein were pre-mixed and a spectrum acquired prior to addition of XO, in order to calculate the protein concentration in the cuvette. A spectrum between 240 and 800 nm was acquired after each addition of LnCl 3  stock until there was no further change in the Ln III -XO absorption feature at 575 nm (Xmax was dependent on Ln in the experiment). The absorption at this wavelength, corrected for volume change of the solution, was plotted against metal ion concentration. The metal ion concentration at which A=A initial +0.1(A final −A initial ), the point of 10% saturation of XO, was taken as an estimate of the point at which the tight binding sites on LanM were saturated. 
     Circular dichroism spectroscopy. Circular dichroism (CD) spectra of LanM were collected using a Jasco J-1500 CD spectrometer, thermostatted at 25° C., and a 1-mm pathlength quartz CD cuvette (Jasco J/0556). Samples were scanned from 260 to 200 nm, with the following instrument settings: 1 nm bandwidth, 0.5 nm data pitch, 50 nm/min scan rate, 4 s average time. Three scans were acquired and averaged for each condition. For stoichiometric titrations of LanM with all Ln III  ions and Ca II , the cuvette contained 20 μM LanM in 200 μL Chelex-treated Buffer B, into which 1-8 equiv. each metal ion (from a 1 mM solution in Buffer B) were titrated, and each spectrum was acquired. A buffer blank spectrum was subtracted from the sample spectra, and the spectra were corrected for volume change before plotting. 
     Preparation of buffered Ca II  solutions. 
     a. Preparation of buffer stock. The buffer used for all buffered metal titrations was prepared as a 2× stock: 60 mM MOPS, 200 mM KCl, pH 7.2 (Buffer I). Solids were dissolved in 400 mL Milli-Q water and stirred at room temperature with 25 g Chelex 100 resin overnight. The pH of the resulting solution was adjusted to 7.2 and the bottle was filled to a final volume of 500 mL. 
     b. Preparation of Ca II -EGTA “high-Ca” buffer. A concentrated stock of Ca II -EGTA was prepared using a pH titration to monitor the relative amounts of Ca II  and EGTA, adapted from the method of Tsien and Pozzan. In a 50 mL Sarstedt conical tube, EGTA (99%, 3.8415 g, 10.1 mmol) was dissolved in 15 mL Milli-Q water with 19 mmol (1.0661 g) KOH. CaCl 2  (1.397 g, 9.5 mmol) was dissolved in this solution, and the pH was adjusted to 7.3. CaCl 2  (20 μL of 1 M, 0.02 mmol) was added to this solution, and the resulting change in pH was noted, and KOH (40 μL of 1 M, 0.04 mmol) was added to bring the pH back to the initial value. This process was repeated until the change in pH per addition of CaCl 2  was half of the initial value (9 additions or 180 μL of 1 M CaCl 2 ). The pH of the solution was adjusted to 7.2 and the solution was brought to a final volume of 20.0 mL, resulting in 0.5 M Ca II -EGTA stock (50×). The final Ca II -EGTA buffer (20 mL) contained: 10 mL 2× Buffer I and 400 μL 50× Ca II -EGTA stock (10 mM Ca II -EGTA), brought to 20.0 mL with Milli-Q water. 
     c. Preparation of the “low-Ca” buffer (30 mM MOPS, 100 mM KCl, 10 mM EGTA, pH 7.2). This buffer was prepared by dissolving 0.07683 g of EGTA (99%, 0.2 mmol) with 2 equivalents of KOH in 8 mL of Milli-Q water. Buffer I (10 mL) was added, and the solution was brought to pH 7.2 and adjusted to a final volume of 20 mL. 
     Preparation of buffered Ln III -EDDS solutions. Ethylenediamine N,N′-disuccinic acid (EDDS) was obtained as a 35% (˜1 M) trisodium salt solution in water (Sigma). For the purpose of metal-protein K d  determination, 1:1 metal:EDDS stock solutions were prepared by taking advantage of the change in the CD signal of the free ligand upon metal binding. Using Buffer H, water, and EDDS, a solution of 30 mM MOPS, 100 mM KCl, and (nominally) 10 mM EDDS, pH 7.2 (Buffer J) was prepared in a Sarstedt 50 mL conical tube. This solution (300 μL) was placed in a 1-mm pathlength CD cuvette and titrated with solutions of ˜100 mM YCl 3 , LaCl 3 , NdCl 3 , SmCl 3 , GdCl 3 , TbCl 3 , or HoCl 3  derived by 10× dilution from 1 M stock solutions, in 30 mM MOPS, 100 mM KCl, pH 7.2. CD spectra were acquired at 260-215 nm under the same conditions as above. Metal solutions were added in 1.5-4 μL increments, until the CD signal at 220 nm, adjusted for volume change during the titration, was no longer altered by metal addition (1:1 metal:ligand complex). The volume of the metal stock solution required to reach the equivalence point was calculated from the intersection of the linear regression line fitted to points in the first half of the titration and the average ellipticity from the first 3 points after signal was constant. Assuming an EDDS concentration in the commercial stock of 1 M, we determined the molar ellipticities at 220 nm ([0]220, all in deg cm 2  Cline &#39; ) to be: +2490 (EDDS), −9500 −4300 (La III -EDDS), −5630 (Nd III -EDDS), −7750 (Sm III -EDDS), −6420 (Gd III -EDDS), −6980 (Tb III -EDDS), and −9640 (Ho III -EDDS). To make the 1:1 metal:EDDS buffers, 10 mL of Buffer H was mixed with 200 μL˜1 M EDDS solution and the amount of metal 1 M stock determined from the titration to give 1:1 metal:ligand. The pH of the solution was adjusted to 7.2 using 25 μL of 5 M KOH and brought to a final volume of 20 mL with water, to make “high-Ln” buffer. Solutions were stable for 1 day and were used immediately after mixing metal into a pre-prepared Buffer H-EDDS-KOH solution. 
     Preparation of La III -EGTA. Because LanM&#39;s K d  for La III  was too low to be measured accurately using EDDS solutions (except for fluorescence titrations), a solution of La III -EGTA was prepared. A pH titration method analogous to that used to prepare Ca II -EGTA could not be used due to the strong Lewis acidity of the Ln salts, which overwhelmed the pH changes from proton release from EGTA. Another approach was developed using the UV-vis signature of XO to standardize a metal stock solution to a ˜1.0 M EGTA solution. A 10 mL stock of ˜1.0 M EGTA was prepared in Milli-Q water with 2 equiv. (˜20 mmol) KOH. This solution was brought to pH 7.2 and diluted with water to 10 mM. A˜1 M stock solution of LaCl 3  was made in Buffer H, with addition of a minimal volume of 6.0 N HCl, and the solution was sterile filtered to remove any precipitate. This solution was diluted to 10 mM. A cuvette contained 470 μL Buffer H, 25 μL 10 mM EGTA, and 5μL˜7 mM XO. The ˜10 mM LaCl 3  solution was titrated into the cuvette in 1 μL increments, and A 579 nm  was monitored. Because of the much higher K d  for La III -XO vs. La III -EGTA, La:EGTA equivalency was taken to be the point at which the increase in A 579 nm  exceeded 2% of the total change at the end of the titration. The final La III -EGTA buffer (20 mL) contained: 10 mL 2× Buffer H, 200 μL˜1 M EGTA, and the appropriate amount of 1 M La III  stock determined from the titration to give equimolar amounts of metal and EGTA, and after pH adjustment to 7.2, it was brought to 20 mL with Milli-Q water. This protocol was also followed to make Nd m -EGTA solutions used for CD titration of LanM(4P→4A). 
     K d  determinations by CD spectroscopy. A concentrated solution of LanM (-2 mM) was diluted separately in high-Ln buffer and in Buffer J to 15 μM. These two protein solutions were mixed in different ratios to give various free metal concentrations (calculated as described below), with each sample being 200 μL, in Sarstedt tubes. The same ratios of high-Ln buffer and Buffer J (without protein) were mixed together to yield the blank samples for CD experiments. The samples were incubated at room temperature for ˜1 h to allow for equilibration prior to collection of CD spectra at 25° C. The blank spectra at each high-Ln:Buffer J ratio were subtracted from the Ln III -LanM spectra, and the [θ] 222 nm  was plotted vs. free metal concentration. For determination of the K d  for La III -LanM(4P→4A), the solutions did not equilibrate in 1 h, so solutions were incubated overnight prior to data collection. For determination of the K d  for Ca II -LanM, unbuffered Ca II  was added in aliquots to 200 μL of 15 μM LanM. For titrations involving EGTA-buffered metals, a single blank spectrum was sufficient, as EGTA does not have a CD signal in this spectral region. All curve fitting was performed in Origin 2018 (OriginLab Corporation), to the Hill equation with one or two independent sets of sites. 
     Calculation of free metal concentrations for K d  determinations. The concentration of free metal ion (Ca II  or Ln III ), in each mixture of “low metal” and “high metal” buffers was determined using the following equation: 
       [ M   free ]=(K d,M [ ML ])/[ L ]  (1)
 
     Where [L] is the concentration of all EDDS or EGTA species not bound to M, [ML] is the concentration of metal ion-bound EDDS or EGTA, and K d,M  is the effective K d  of the ligand for M, defined as: 
       K d,M =[1+10 (pKa1−pH) +10 (pKa2−pKa1− 2pH) ]/K   M    (2)
 
     In eq. 2, pK a1  and pK a2  are the first and second pK a s of L, which were corrected for an ionic strength of 0.1 M by addition of 0.11 units to each published value. pH is the buffer pH, and K M  is defined as: 
         K   M =[ ML ]/([ M ][ L ])   (3)
 
     Because the concentration of ligand (10 mM) was much greater than the protein concentration (15 μM), eq. 1 was approximated by eq. 4 for each sample, where V high  and V low  are the volumes of the high and low metal solutions mixed together: 
       [ M   free ]= K   d,M   ·V   high   /V   low    (4)
 
     It must be noted that the intrinsic Kds may be lower than the apparent K d  s reported here. Furthermore, because rare earth ion-catalyzed hydrolysis (which would decrease free metal concentrations) was not considered in these calculations, it is likely that the true free metal concentrations in the titration experiments and therefore the true Kds are smaller than those calculated here, especially for the more Lewis-acidic later Ln III s and Y III  (i.e. the metal affinities of LanM and selectivity vs. other metal ions like Ca II  are even higher than stated). 
     Error analysis for K d  determinations. For most titrations, the errors between replicates using different protein samples or buffered high metal solutions were typically small (&lt;1 pM). Therefore, we reasoned that the largest source of uncertainties in our Kd measurements was likely to be judgment of the endpoint in the titration of EDDS with metal ion. For example, titration of 300 μL of 10 mM EDDS with a 100 mM GdCl 3  stock solution yielded an endpoint at 30.8-33.1 μL metal stock added, depending on the range of points included in the linear regression lines used to determine endpoint (see “Preparation of buffered Ln III -EDDS solutions”). Similar uncertainties were exhibited in titrations with other metal ions. To determine the impact of these endpoint ranges on K d  determination, we made “high-Gd” solutions based on each of these endpoints (30.8 and 33.1 μL) and performed titrations of LanM-His as described. The fits to these data yielded apparent K d s of 16 and 14 pM, respectively. Therefore, we concluded that the uncertainty in metal-EDDS titration endpoint determination was approximately ±10%. This value taken to be similar for all Lns studied given the similar uncertainties in endpoint determination. Although La-EGTA and Nd-EGTA titrations did not use EDDS for standardization (as described above), we estimated that there was a similar uncertainty in judging the titration endpoint in this case as well. These calculated errors were combined with the errors between experimental replicates, and these combined values have been reported as the uncertainties. 
     Construction, expression, and purification of LanM(4P→4A). The quadruple mutant LanM P36A/P60A/P85A/P109A was obtained as a 380-bp gBlock gene fragment identical to the wt protein except that the CCG codons for P36 and P60 were mutated to GCG and the CCT codons for P85A and P109A were mutated to GCT. The gBlock was ligated into pET-28a using NdeI and EcoRI sites, as for wt LanM, yielding pET28a-His/TEV-LanM(4P→4A). This plasmid was used as template to generate LanM(4P→4A)-His, with a C-terminal His 6  tag, using primers LanM-for and LanM-His-rev. This protein was expressed and purified analogously to LanM-His, with similar yield. 
     Spectrofluorometry. Fluorescence experiments were carried out using a Varian Cary Eclipse spectrofluorometer at 23° C. The excitation wavelength was 278 nm, and emission data were collected at 300-400 nm, with 5 nm excitation and emission slits, 1 nm steps, and 0.5 s averaging time. All samples were 600 μL and were prepared in a semi-micro quartz fluorimeter cell with 10 mm path length (Starna Cells). For stoichiometric titrations of LanM-His with La III  and Gd III , the cuvette contained 15 μM LanM-His in Buffer B, into which 0-8 equiv. metal ion was titrated from a 15 mM stock solution prepared from the 1 M solution standardized against EDDS as described above. For titrations to determine Kaa pp  values, samples were prepared at various free La III  concentrations as described for CD experiments, except that each sample was 600 μL, 10 μM LanM-His, and La III -EDDS was used. Two scans were averaged for each titration point. All samples were analyzed following subtraction of a blank spectrum of Buffer B. 
     Isothermal titration calorimetry. Binding of Ca II  to wt-LanM was analyzed by ITC using a MicroCal Auto-iTC200 instrument. All experiments were performed at 25° C., in Chelex-treated Buffer B from the dialysis procedure. All solutions were degassed prior to the experiments. The ITC cell contained 40 μM wt-LanM. The titrant syringe contained 4.0 mM CaCl 2  in the same buffer. Titration settings were the following: 600 rpm stirring speed, 60 s initial delay, 5 μcal/s reference power, and 180 s spacing between each injection. Titrations consisted of a first 0.2 μL injection followed by 48×0.8 μL injections. The heat of dilution was determined by titrating the identical metal solutions into cell containing buffer without protein. These data were subtracted from the heats of reaction prior to analysis. The resulting data were fitted using MicroCal Origin to a model, provided by the manufacturer, with two sets of non-interacting sites, to obtain for each set of sites, i, the number of sites (n i ), association constants (K i ), binding enthalpies (ΔH i ), and entropy change (ΔS i ), from which the free energy change (ΔG i ) and could be calculated. Other models provided with the software package, with either sequential or non-sequential binding, did not yield fits that were judged to be acceptable. 
     General considerations for LaMP1 studies. Chemical reagents were obtained from Sigma-Aldrich, unless noted otherwise, at the highest purity available. All rare earth element salts (chloride or acetate) were at a minimum purity of 99.9% rare earth metal content. All were obtained from Sigma-Aldrich, except for SmCl 3  and ScCl 3  from Alfa Aesar, TmCl 3  was from Strem, and YCl 3  was from Acros. Primers were ordered from Integrated DNA Technologies (IDT). E. coli strains [5alpha and 10beta] for cloning and recombinant protein expression, respectively, as well as cloning reagents (restriction enzymes, Q5 DNA polymerase, OneTaq DNA polymerase, T4 DNA ligase) were obtained from New England Biolabs. PCR cleanup and miniprep kits were from Qiagen and Omega Bio-tek, and gel extraction utilized the Zymoclean gel DNA recovery kit from Zymo Research. Ni-NTA resin was purchased from Thermo Scientific. Protein gel electrophoresis was carried out using Life Tech 16% Tris-glycine gels and a mini gel apparatus. Automated protein chromatography used a GE Healthcare Biosciences Akta Pure fast protein liquid chromatography (FPLC) system. UV-visible absorption spectra were obtained on an Agilent Cary 60 UV-visible spectrophotometer using a quartz cuvette (Starna Cells). Well plate analyses were carried out using a Tecan Infinite M1000 Pro plate reader. 
     Construction of LaMP1 and LaMP1(4P→4A). LanM and LanM(4P→4A) inserts were amplified from pET24a-LanM-TEV/His and pET28a-His/TEV-LanM(4P→4A), respectively, using primers ECFP-LanM-SphI-for and Citrine-LanM-SacI-rev. The PCR products were digested using SphI and Sad (10 U/μg) and purified. pBAD-D2 (Addgene #37470) was similarly digested and, and following agarose gel electrophoresis, the vector fragment was excised and purified. The inserts were ligated into the digested vector (5:1, insert:vector) using T4 DNA ligase according to the manufacturer&#39;s protocol. Transformants were screened for insert by colony PCR and the correct insert was confirmed by DNA sequencing at Penn State Genomics Core Facility using primers pBAD-F, pBAD-R, and ECFP-mid, yielding pBAD-LaMP1 and pBAD-LaMP1(4P→4A). 
     Expression and purification of LaMP1 and LaMP1(4P-4A). Chemically competent  E. coli  10beta cells were transformed with pBAD-LaMP1 or pBAD-LaMP1(4P→4A) and plated on LB-agar plates containing 100 μg/mL ampicillin (Amp) and grown at 37° C. A single colony was used to inoculate 100 mL LB (100 μg/mL Amp in all growth media) and grown for 16 h at 37° C. with shaking at 200 rpm. Of this culture, 40 mL were used to inoculate one 1.5-L culture in a 6 L flask of LB media. The cultures were grown at 37° C. with shaking at 200 rpm. At OD 600 nm ˜0.5, the culture was placed at 4° C. for 30 min, after which L-arabinose (Oakwood Chemical) was added to a final concentration of 500 μM. After further incubation at 23° C. with shaking at 200 rpm for 16 h, the cells were pelleted by centrifugation at 7000 ×g for 7 min at 4° C., yielding 6-7 g cell paste. 
     For sensor purification, all operations were performed at 4° C. Cell paste was resuspended in 5 mL/g of 50 mM sodium phosphate, 10 mM imidazole, 5% glycerol, pH 7.0 (Buffer A) containing 0.4 mM phenylmethanesulfonyl fluoride (PMSF). The suspension was passed through a French pressure cell twice at 14000 psi. Debris was pelleted by centrifugation at 40000 ×g for 30 min at 4° C. The supernatant was decanted and applied to a 1.5×4.5 cm (8.0 mL) Ni-NTA agarose column, pre-equilibrated in Buffer A. The column was washed with 15 CV of Buffer A, followed by elution with 3 CV 50 mM sodium phosphate, 250 mM imidazole, 5% glycerol, pH 7.0 (Buffer B). The eluted protein was concentrated to 1.5 mL using an Amicon Ultra 30-kDa MWCO centrifugal filter device. The protein was applied to a HiLoad 16/600 Superdex 75 pg column (GE Healthcare) using a 2 mL capillary loop, and the column was run with 1.2 CV 30 mM MOPS, 100 mM KCl, pH 7.2 (Buffer C) at 0.75 mL/min, with 1.5 mL fractions collected (1 mAU threshold in peak fractionation mode). Fractions containing LaMP1 or LaMP1(4P→4A) were judged by A 280 nm , A 434 nm , A 515 nm , and the fractions eluting at 45-58 mL [LaMP1] or 45-52 mL [LaMP1(4P→4A)] were pooled and concentrated by centrifugal filtration to 1.0 mL. Protein concentration was determined using ε 515 nm =77000 M −1 cm −1 . The purification yielded 38 mg/L culture. 
     In vitro characterization of LaMP1 sensors. Ethylenediamine N,N′-disuccinic acid (EDDS)-buffered solutions of Ln III s (La III −Er III ) were prepared in Buffer C as described. For buffering of La III  in the range of 10 −10  to 10 −6  M free metal ion concentration, trimethylenedinitrilotetraacetic acid (TMDTA) was used. LaMP1 was diluted to 500 nM in metalated and chelator-only buffers and these solutions were combined in various ratios to give 100 μL final volume for each titration point. Following 15-min incubation, fluorescence measurements were carried out in Greiner Cellstar 96-well half-area μClear plates, using 433 nm excitation, 450-550 nm emission, and a gain of 130. Unbuffered titrations were performed in 30 mM MOPS, 100 mM KCl, 20 mM acetate, pH 7.2 (Buffer D). FRET ratios were calculated from the intensities of the fluorescence emission peaks of ECFP (475 nm) and citrine (529 nm), as F 529 nm /F 475 nm . 
     Preparation of buffered La III -TMDTA solutions. A 20 mL stock solution of 20 mM TMDTA was prepared by dissolving 0.404 mmol TMDTA (99%) (the extra 0.011% mass of TMDTA was included to compensate for 99% purity) and 1.01 mmol KOH in Buffer C to give a 10 mL total volume. The 1:1 La III -TMDTA solution was prepared immediately prior to use by adding an equimolar quantity of La III  from a 0.9 M LaCl 3  stock solution in Buffer C (calibrated by titration against EGTA as described) to 2.5 mL of the 20 mM TMDTA stock. The pH of the resulting solution was adjusted to 7.2 using KOH, and brought to a volume of 5 mL using Buffer C. The final solution was 30 mM MOPS, 100 mM KCl, 10 mM TMDTA, 10 mM LaCl 3 , pH 7.2. This buffer was combined in various ratios with Buffer C containing 10 mM TMDTA prepared from the same TMDTA stock in order to produce a range of free La III  concentrations. Free La III  concentrations were calculated as described using 25° C., I=0.11, and pH 7.2. 
     The limit of detection assay was performed using 5 nM LaMP1 in Buffer C. The plate reader was set to a gain of 250 and flash frequency of 200 Hz. The buffer contribution to the signal was subtracted using a buffer blank. 
     
       
         
           
               
             
               
                 TABLE 10 
               
             
            
               
                   
               
               
                 Plasmids used for Sensor studies. 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 SEQ 
               
               
                   
                   
                   
                 ID 
               
               
                 Name 
                 Notes 
                 Source 
                 NO: 
               
               
                   
               
               
                 pBAD-D2 
                 Amp R   
                 Addgene #37470 
                 111 
               
               
                 pET-24a-LanM-TEV/His 
                 Km R   
                 Present disclosure 
                 112 
               
               
                 gET-28a-His/TEV-LanM(4P→4A) 
                 Km R   
                 Present disclosure 
                 113 
               
               
                 pBAD-LaMP1 
                 LanM 
                 Present disclosure 
                 114 
               
               
                   
                 inserte 
                   
                   
               
               
                 pBAD-LaMP1(4P-4A) 
                 LanM 
                 Present disclosure 
                 115 
               
               
                   
                 (4P→4A) 
                   
                   
               
               
                   
                 inserted  
                   
                   
               
               
                   
                 into 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 11 
               
             
            
               
                   
               
               
                 Primers used for cloning and sequencing 
               
               
                 for sensor experiments* 
               
            
           
           
               
               
               
            
               
                   
                   
                 SEQ 
               
               
                   
                   
                 ID 
               
               
                   
                 Sequence 
                 NO 
               
               
                   
               
               
                 Name 
                   
                   
               
               
                 ECFP-LanM- 
                 5′-ATAA GCATGC CAACTACGACTA 
                 31 
               
               
                 Sphl-F 
                 CCAAAGTTGATATCG-3′ 
                   
               
               
                   
               
               
                 Citrine- 
                 5′-AATA GAGCTC ACGAATTAAGTT 
                 32 
               
               
                 LanM- 
                 GACCAGGGC-3′ 
                   
               
               
                 Sacl-R 
                   
                   
               
               
                   
               
               
                 Sequencing 
                   
                   
               
               
                 primers 
                   
                   
               
               
                 pBAD-F 
                 5′-ATGCCATAGCATTTTTATCC-3′ 
                 33 
               
               
                   
               
               
                 pBAD-R 
                 5′-GATTTAATCTGTATCAGG-3′ 
                 34 
               
               
                   
               
               
                 ECFP-mid 
                 5′-CAACCACTACCTGAGCAC-3′ 
                 35 
               
               
                   
               
               
                 *Restriction sites are underlined 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 12 
               
               
                   
               
               
                 DNA SEQUENCES OF SENSORS 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
            
               
                 &gt;LaMP1 (SEQ ID NO: 4) 
               
               
                 
                   atggtgagcaagggcgaggagctgttcaccggggtggtgcccatcctggtcgagctggacggcgacgtaaacggccac 
                 
               
               
                 
                   aggttcagcgtgtccggcgagggcgagggcgatgccacctacggcaagctgaccctgaagttcatctgcaccaccggc 
                 
               
               
                 
                   aagctgcccgtgccctggcccaccctcgtgaccaccctgacctggggcgtgcagtgcttcagccgctaccccgaccac 
                 
               
               
                 
                   atgaagcagcacgacttcttcaagtccgccatgcccgaaggctacgtccaggagcgtaccatcttcttcaaggacgac 
                 
               
               
                 
                   ggcaactacaagacccgcgccgaggtgaagttcgagggcgacaccctggtgaaccgcatcgagctgaagggcatcgac 
                 
               
               
                 
                   ttcaaggaggacggcaacatcctggggcacaagctggagtacaactacatcagccacaacgtctatatcaccgccgac 
                 
               
               
                 
                   aagcagaagaacggcatcaaggcccacttcaagatccgccacaacatcgaggacggcagcgtgcagctcgccgaccac 
                 
               
               
                 
                   taccagcagaacacccccatcggcgacggccccgtgctgctgcccgacaaccactacctgagcacccagtccgccctg 
                 
               
               
                   agcaaagaccccaacgagaagcgcgatcacatggtcctgctggagttcgtgaccgccgcc CGCATGCCAACTACGACT 
               
               
                 ACCAAAGTTGATATCGCGGCGTTTGACCCGGACAAAGATGGGACCATCGATCTGAAAGAGGCTTTGGCGGCAGGTTCC 
               
               
                 GCGGCCTTCGACAAGTTGGACCCGGATAAAGATGGTACTCTGGACGCCAAAGAGCTGAAGGGCCGCGTGTCTGAGGCA 
               
               
                 GACCTTAAGAAGCTGGACCCTGACAATGACGGAACCCTGGACAAGAAAGAGTACTTAGCAGCGGTAGAGGCACAGTTT 
               
               
                 AAGGCCGCTAACCCTGACAACGATGGCACTATTGACGCCCGTGAACTTGCAAGCCCAGCGGGGTCGGCCCTGGTCAAC 
               
               
                 TTAATTCGTGAGCTC Atggtgagcaagggcgaggagctgttcaccggggtggtgcccatcctggtcgagctggacggc   
               
               
                 
                   gacgtaaacggccacaagttcagcgtgtccggcgagggcgagggcgatgccacctacggcaagctgaccctgaagttc 
                 
               
               
                 
                   atctgcaccaccggcaagctgcccgtgccctggcccaccctcgtgaccaccttcggctacggcctgatgtgcttcgcc 
                 
               
               
                 
                   cgctaccccgaccacatgaagcagcacgacttcttcaagtccgccatgcccgaaggctacgtccaggagcgcaccatc 
                 
               
               
                 
                   ttcttcaaggacgacggcaactacaagacccgcgccgaggtgaagttcgagggcgacaccctggtgaaccgcatcgag 
                 
               
               
                 
                   ctgaagggcatcgacttcaaggaggacggcaacatcctggggcacaagctggagtacaactacaacagccacaacgtc 
                 
               
               
                 
                   tatatcatggccgacaagcagaagaacggcatcaaggtgaacttcaagatccgccacaacatcgaggacggcagcgtg 
                 
               
               
                 
                   cagctcgccgaccactaccagcagaacacccccatcggcgacggccccgtgctgctgcccgacaaccactacctgagc 
                 
               
               
                 
                   taccagtccgccctgagcaaagaccccaacgagaagcgcgatcacatggtcctgctggagttcgtgaccgccgccggg 
                 
               
               
                 
                   atcactctcggcatggacgagctatacaagtaa 
                 
               
               
                   
               
               
                 4p4a Sensor (SEQ ID NO: 5) 
               
               
                 
                   atggtgagcaagggcgaggagctgttcaccggggtggtgcccatcctggtcgagctggacggcgacgtaaacggccac 
                 
               
               
                 
                   aggttcagcgtgtccggcgagggcgagggcgatgccacctacggcaagctgaccctgaagttcatctgcaccaccggc 
                 
               
               
                 
                   aagctgcccgtgccctggcccaccctcgtgaccaccctgacctggggcgtgcagtgcttcagccgctaccccgaccac 
                 
               
               
                 
                   atgaagcagcacgacttcttcaagtccgccatgcccgaaggctacgtccaggagcgtaccatcttcttcaaggacgac 
                 
               
               
                 
                   ggcaactacaagacccgcgccgaggtgaagttcgagggcgacaccctggtgaaccgcatcgagctgaagggcatcgac 
                 
               
               
                 
                   ttcaaggaggacggcaacatcctggggcacaagctggagtacaactacatcagccacaacgtctatatcaccgccgac 
                 
               
               
                 
                   aagcagaagaacggcatcaaggcccacttcaagatccgccacaacatcgaggacggcagcgtgcagctcgccgaccac 
                 
               
               
                 
                   taccagcagaacacccccatcggcgacggccccgtgctgctgcccgacaaccactacctgagcacccagtccgccctg 
                 
               
               
                   agcaaagaccccaacgagaagcgcgatcacatggtcctgctggagttcgtgaccgccgcc CGCATGCCAACTACGACT 
               
               
                 ACCAAAGTTGATATCGCGGCGTTTGACGCGGACAAAGATGGGACCATCGATCTGAAAGAGGCTTTGGCGGCAGGTTCC 
               
               
                 GCGGCCTTCGACAAGTTGGACGCGGATAAAGATGGTACTCTGGACGCCAAAGAGCTGAAGGGCCGCGTGTCTGAGGCA 
               
               
                 GACCTTAAGAAGCTGGACGCTGACAATGACGGAACCCTGGACAAGAAAGAGTACTTAGCAGCGGTAGAGGCACAGTTT 
               
               
                 AAGGCCGCTAACGCTGACAACGATGGCACTATTGACGCCCGTGAACTTGCAAGCCCAGCGGGGTCGGCCCTGGTCAAC 
               
               
                 TTAATTCGTGAGCTC Atggtgagcaagggcgaggagctgttcaccggggtggtgcccatcctggtcgagctggacggc   
               
               
                 
                   gacgtaaacggccacaagttcagcgtgtccggcgagggcgagggcgatgccacctacggcaagctgaccctgaagttc 
                 
               
               
                 
                   atctgcaccaccggcaagctgcccgtgccctggcccaccctcgtgaccaccttcggctacggcctgatgtgcttcgcc 
                 
               
               
                 
                   cgctaccccgaccacatgaagcagcacgacttcttcaagtccgccatgcccgaaggctacgtccaggagcgcaccatc 
                 
               
               
                 
                   ttcttcaaggacgacggcaactacaagacccgcgccgaggtgaagttcgagggcgacaccctggtgaaccgcatcgag 
                 
               
               
                 
                   ctgaagggcatcgacttcaaggaggacggcaacatcctggggcacaagctggagtacaactacaacagccacaacgtc 
                 
               
               
                 
                   tatatcatggccgacaagcagaagaacggcatcaaggtgaacttcaagatccgccacaacatcgaggacggcagcgtg 
                 
               
               
                 
                   cagctcgccgaccactaccagcagaacacccccatcggcgacggccccgtgctgctgcccgacaaccactacctgagc 
                 
               
               
                 
                   taccagtccgccctgagcaaagaccccaacgagaagcgcgatcacatggtcctgctggagttcgtgaccgccgccggg 
                 
               
               
                 
                   atcactctcggcatggacgagctatacaagtaa 
                 
               
               
                   
               
               
                 ECFP underlined, 
               
               
                 citrine double-underlined 
               
            
           
         
       
     
     Sensor 
     Another aspect of the present disclosure includes a sensor for detecting a rare earth element. The sensor concept takes advantage of the high affinity and selectivity of LanM&#39;s metal binding sites for rare earth elements, and/or the large metal-dependent conformational change exhibited by the protein. In the particular embodiment described here, the sensor can be constructed to include a LanM as a linker between two fluorescent proteins such that the Frster Resonance Energy Transfer (FRET) efficiency between two fluorescent proteins is modulated when conformational changes occur in LanM upon binding to a rare earth element. The sensor protein is referred to herein as LaMP 1. The two fluorescent proteins are selected such that the first fluorescent protein emits at a wavelength at which the second fluorescent protein is excited, and emits at a higher wavelength. For example, the first protein may be a cyan fluorescent protein and the second protein may be a yellow fluorescent protein which is excited by cyan. Thus, when the two proteins are sufficiently close and in a suitable orientation, the cyan emitted by the first protein is absorbed by the second protein which then emits yellow. On the other hand, when the two proteins are spaced apart from each other or in a less suitable orientation, the first protein emits cyan, but does not excite the second protein as efficiently, and a lesser yellow signal is obtained. 
     Taking advantage of our discovery and characterization of LanM&#39;s rare earth element binding properties, as well as our NMR solution structure of the Y(III)-bound form of LanM, we used LanM as the basis for a ratiometric protein-based (genetically encoded) fluorescent sensor for rare earths, which we call LaMP1 (lanmodulin-based protein sensor 1). LaMP1 is the first of its kind for rare earths, and it suggests that this and related constructs may be broadly useful biotechnologies for rare earth detection in industrial, environmental, and biological samples. To construct LaMP1, we started with the calmodulin-based sensor D2, developed by Tsien and coworkers, consisting of a C-terminally truncated enhanced cyan fluorescent protein (ECFP) and citrine, a yellow fluorescent protein, as a FRET pair. We replaced the calmodulin and M13 peptide components of D2 with LanM(Ala22-Arg133), the full-length protein after cleavage of the signal peptide, yielding LaMP1 ( FIG. 11 ). In in vitro assays, LaMP1 exhibited a robust, 7-fold ratiometric FRET response to all rare earth elements, with apparent dissociation constants (K d,app ) for Ln III s and Y III  in the picomolar range, consistently only 2-fold weaker than those of native LanM (Table 13,  FIG. 16 ). The K d,app  of La“-LaMPl measured by circular dichroism spectroscopy agrees with the FRET-derived value, suggesting that LaMP1 undergoes a similar conformational change as LanM itself. Our method for titrating the chelator used for these determinations, ethylenediamine N,N′-disuccinic acid (EDDS), could not be used to calibrate solutions of Tm, Yb, and Lu; however, we anticipate K d,app  values of 40-50 pM range based on the relationship between K d,app  and ionic radius. The sensor also responds to Sc III , but the K d,app  was not determined ( FIG. 16D ). In the case of La”, we tested fluorescence response over the full range of free metal concentrations from 10 −13  M to 10 −2  M, buffered using EDDS or trimethylenedinitrilotetraacetic acid (TMDTA) or without chelator. No significant additional fluorescence response was observed above 10 −10  M until the unbuffered regime (&gt;1 μM), which may correspond to metal binding to EF4 based on our previous analyses of LanM ( FIG. 16B ). 
       FIG. 11  shows design of LaMP 1. The sensor comprises ECFP(1-228), LanM(22-133), and citrine. Fluorescent protein images were derived from PDB code 1CV7 (ECFP) using PyMOL. Table 13 shows titration data for Ln 111 -LaMP1 and Ca II -LaMP1 K d,app  titrations (0.5 μM sensor, pH 7.2). Free metal concentrations were buffered using EDDS for Lns (La-Er) and Y. Data were fitted to the Hill equation for one set of sites. 
     
       
         
           
               
             
               
                 TABLE 13 
               
             
            
               
                   
               
               
                 Apparent Kds, Hill coefficients (n), and fold changes in  
               
               
                 FRET ratio from fitted fluorescence titrations of LaMP1 
               
               
                 with buffered LnIIIs and CaII (25° C, pH 7.2). Uncertainties  
               
               
                 were determined by combining standard deviations from three  
               
               
                 titrations and uncertainties in calibration of metal-EDDS solutions,  
               
               
                 as described in ref. 1. To the right of ErIII in the periodic table,  
               
               
                 uncertainties in determining the endpoints of LnIII-EDDS  
               
               
                 CD titrations were too great to reliably determine Kds. 
               
               
                 Note that these data do not include the additional increase in FRET  
               
               
                 ratio occurring in the micromolar range of metal ion concentration. 
               
            
           
           
               
               
               
               
            
               
                 Metal 
                   
                   
                 Fold change 
               
               
                 ion 
                 K d,app  (pM) 
                 n 
                 (R/R 0 ) 
               
               
                   
               
               
                 La III   
                 9.4 ± 1.5 
                 3.5 ± 0.2 
                 6.0 
               
               
                 Ce III   
                 16 ± 3  
                 3.4 ± 0.1 
                 6.7 
               
               
                 Pr III   
                 13 ± 2  
                 4.0 ± 0.2 
                 5.7 
               
               
                 Nd III   
                 12 ± 2  
                 3.4 ± 0.2 
                 6.0 
               
               
                 Sm III   
                 12 ± 3  
                 3.4 ± 0.1 
                 5.7 
               
               
                 Eu III   
                 16 ± 3  
                 3.5 ± 0.1 
                 6.3 
               
               
                 Gd III   
                 10 ± 2  
                 3.3 ± 0.1 
                 6.3 
               
               
                 Tb III   
                 18 ± 3  
                 4.4 ± 0.1 
                 5.7 
               
               
                 Dy III   
                 26 ± 4  
                 3.9 ± 0.1 
                 6.5 
               
               
                 Ho III   
                 29 ± 5  
                 4.0 ± 0.1 
                 5.6 
               
               
                 Er III   
                 36 ± 7  
                 4.0 ± 0.2 
                 5.4 
               
               
                 Y III   
                 44 ± 8  
                 4.4 ± 0.2 
                 5.7 
               
               
                 Ca II   
                 (1.2 ± 0.1) x 10 9   
                 1.1 ± 0.1 
                 2.9 
               
               
                   
               
            
           
         
       
     
       FIG. 16  shows response of 500 nM LaMP1 to 0-4 (black red) equiv. La III  (λ ex =433 nm). (B) FRET ratio (F 529 nm /F 475 nm ) of LaMP1 as a function of free La III  concentrations either EDDS- or TMDTA-buffered or unbuffered. (C) Plot of apparent K d  of Ln III -LaMP1 versus ionic radius for La III -Er III  and Y III . (D) Metal selectivity of LaMP 1. Fold response of 500 nM LaMP1 to 5 μm La III  ; 10 μM Sc III , Y III  and Fe III ; 100 μM Cu II ; or 1 mM Al III , Mn II , and Ca II . 
     LaMP1 also exhibited fluorescence response to Ca II , but with a K d,app  of 1.2 mM, far above likely physiological levels, and a FRET response of just 3-fold (Table 13). We note, however, that this determination was limited by fluorescence quenching at Ca II  concentrations &gt;10 mM, which may have obscured a further FRET increase. We also tested LaMP1 against common metal ions —Fe III , Al III , and Cu II —at concentrations that are potentially relevant to aqueous Al III , Mn III , environmental samples such as mine leachates, and found little or no response ( FIG. 16D ). At higher concentrations, Fe III  quenched fluorescence and the sensor showed modest response to Al III  and Mn II , but all were outcompeted by 5 μM La III  ( FIG. 12 ). Biologically relevant ions Mg II , Na + , and K + also minimally interfere ( FIG. 12 ). Therefore, like LanM itself, LaMP1 exhibits high selectivity for Ln III  and Y III  over Ca II  and other common metal ions. 
       FIG. 12  shows the selectivity of LaMP1 for L III s over selected common mono- di- and trivalent metal ions. (A) LaMP1 (0.5 μM) was incubated with the indicated metal ions, which would be potential interferences in environmental samples, in the absence (white bars) or presence (shaded bars) of 5 μM La III . (B) LaMP1 (0.5 μM) was incubated with various concentrations of MgCl 2  in Buffer C, in the absence (white bars) or presence (shaded bars) of 4 equiv. La III  (Apo=no Mg II ). While LaMP1 exhibited a slight response to Mg II  in the absence of La III  and a slight interference in the presence of La III , bacterial Mg II  levels are generally held to be tightly regulated around 1-2 mM and therefore little interference is likely in vivo. C) Effect of Na +  and K +  on LaMP1 response (30 mM MOPS, pH 7.0, with buffers supplemented by the indicated concentrations of NaCl or KCl. Chloride is known to quench YFP fluorescence by 25% at 100 mM Cl − , pH 7.0. Therefore, the observed interference is likely due to Cl −  binding, rather than Na +  or K +  binding. 
     LaMP1&#39;s limit of detection in plate reader fluorescence assays (5 nM LaMP1) was 10 nM Le, within 1 min ( FIG. 13 ). LaMP 1&#39;s performance compares favorably with previously reported approaches to selectively detect Lns. He and co-workers engineered a bacterial two-component system to detect Lns via incorporation of a lanthanide binding tag peptide sequence. After 6 h of growth, this system exhibits 3-fold fluorescence response to 1 μM Tb III  and a smaller but significant response to 200 nM Tb III , but also responds 2-fold to 50 μM Ca II . Skovran, Martinez-Gomez, and co-workers have recently reported an  M. extorquens  strain with a fluorescent protein gene under control of the xoxl promoter, which responds to 2.5 nM La III  but requires several hours of incubation; furthermore, this promoter only responds to LaSm. A colorimetric method based on the arsenazo-III dye has a detection limit of ˜500 nM but only responds reliably to LaEu. Therefore, LaMP1 is a uniquely rapid, selective, and highly sensitive tool for detecting all REEs. 
       FIG. 13  shows determination of the limit of detection of LaMP1 in fluorescence plate reader measurements (**p&lt;0.01). 
     The pH dependence of LanM&#39;s metal binding affinity was investigated using EGTA-buffered PO&#39; solutions at pH 6.0 and LaMP1. The apparent K d  value for this titration was 26 pM, with a FRET change (R/Ro) of 2.7. The small effect of pH on apparent K d . (compare to  FIG. 16C ) suggests that LanM will retain protein stability and high affinity for REEs at lower pHs, until protonation of the carboxylate ligands responsible for metal coordination, which likely occurs at pH 3. This functionality at acidic pHs is important for potential REE sensing, extraction, and separation applications. The smaller FRET change at pH 6.0 than at pH 7.2 may be related to partial protonation of the citrine chromophore (pK a ˜5.7), which would reduce citrine fluorescence while ECFP fluorescence would be unaffected at this pH, or a slightly different conformational change. 
     We also generated a 4P→4A analog of LaMP1. Mirroring the properties of LanM(4P→4A) versus LanM, the fluorescence response of LaMP1(4P→4A) was similar to that of LaMP1 and exhibited a slightly lower K d,app  for La III  ( FIG. 14 ), but its response to Ca II  was distinct, in two phases with K d1,app =1.6±0.1μM and K d2,app =63±9μM ( FIG. 14 ). LaMP1(4P→4A) also exhibited somewhat larger responses to Mn II  and Cu II  than did LaMP1 ( FIG. 15 ). The minimum FRET ratios and overall FRET response for the two sensors are nearly identical. Due to its responsiveness to lower concentrations of La III  and likely also all other rare earth elements, LaMP1(4P→4A) may also be a useful tool for detection of rare earth elements in aqueous samples. 
       FIG. 14  shows K d,app  determination for LaMP1(4P→4A) using La III  and Ca II .  FIG. 15  shows the fold FRET change of LaMP1(4P→4A) in the absence of added metal ions (baseline) and in the presence of 5 μM La III -Lu III ; 10 μM Y III  and Fe III ; 1 mm; Mn III  and Ca II ; or 100 μM Cu II . 
     Site-directed mutagenesis to demonstrate the versatility of the LanM scaffold for re-engineering of metal selectivity 
     Nomenclature scheme examples: 4P2A=the P residue at position 2 is mutated to A in all four EF hands (in some of the FIG.s herein, this construct is also called “4P→4A,” as it was in the original LanM characterization manuscript; 3D9H =the D residue at position 9 is mutated to H in the first three EF hands. The 4P2A version of the LaMP1 sensor was generated to test responsiveness of this construct with a range of lanthanides. (If two values are listed, the response is biphasic.) 
     
       
         
           
               
             
               
                 TABLE 14 
               
             
            
               
                   
               
               
                 Apparent K dS  for LaMP1(4P→4A)  
               
               
                 (also called LaMP1-4P2A) 
               
               
                 for selected Ln   III   s, at pH 7.2 
               
            
           
           
               
               
               
            
               
                   
                 Ln(III) 
                 K d,app   
               
               
                   
                   
               
               
                   
                 La(III) 
                 8 pM 
               
               
                   
                 Nd(III) 
                 2 pM 
               
               
                   
                 Gd(III) 
                 ~100 fM, ~1 pM 
               
               
                   
                 Ho(III) 
                  ~20 fM, ~1 pM 
               
               
                   
                   
               
            
           
         
       
     
     The sensor response splits into 2 phases to the right of Nd. Our biochemical characterization of the protein and sensor support the hypothesis that this 2-phase response may be a result of EF4 pairing less well with EF1 for the smaller, later lanthanides, resulting in a first response (FRET ratio 1.2 to ˜3.5, ˜3-fold change overall) due to metal binding and conformational change in EF2/3, followed by a second response up to a FRET ratio of 9 due to metal binding to EF1. This suggests that removal of the EF1/4 pair would result in a single-phase response (see below). 
     The results with this construct also show a decrease in apparent K d  for LaMP1-4P2A from 8 pM for La to 20 fM for Ho. The ≥3 order of magnitude change in apparent K d  across the lanthanide series (La-Lu) suggests that this construct, and derivatives thereof, may be better suited than wild-type LanM itself for protein-based separations applications. 
     We also carried out a more extensive mutagenesis study using LaMP1 as the starting point to demonstrate the feasibility of altering metal selectivity of this protein, with metal titrations carried out under the same conditions as those described elsewhere in this disclosure: 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 15 
               
               
                   
               
               
                   
                 SEQ ID 
                 Ca(II) 
                   
                   
                 Mn(II) 
                   
                   
               
               
                 Construct 
                 NO: 
                 K d, app   
                 n 
                 R/R 0   
                 K d, app   
                 n 
                 R/R 0   
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 WT 
                 81 
                 1.4 mM 
                 1.1 
                 3.2 
                 745 μM 
                 1.9 
                 4.8 
               
               
                 4P2A 
                 82 
                 1.5 μM, 100 μM 
                 2.2 
                 3.3, 2.8 
                  73 μM 
                 1.2 
                 4.2 
               
               
                 4P2A/D9H 
                 83 
                  91 μM 
                 1.7 
                 4 
                 200 μM 
                 1.4 
                 3.5 
               
               
                 4P2A/D9Q 
                 84 
                  36 μM 
                 2 
                 3.2 
                 255 μM 
                 1.9 
                 2.8 
               
               
                 4D9N 
                 85 
                 238 μM 
                 1.5 
                 3.5 
                 298 μM 
                 1.3 
                 3.3 
               
               
                 3D9Q 
                 86 
                 459 μM 
                 2.5 
                 3.5 
                 510 μM 
                 1.6 
                 3 
               
               
                 3D9H 
                 87 
                 430 μM 
                 1.9 
                 3.3 
                 279 μM 
                 1.5 
                 3.1 
               
               
                 3E12D 
                 88 
                 308 μM 
                 1.9 
                 1.6 
                 199 μM 
                 1.1 
                 1.6 
               
               
                 3D9Q/3E12D 
                 89 
                 122 μM 
                 1.8 
                 1.5 
                  48 μM 
                 1.8 
                 1.5 
               
               
                 
                   3D9H/3E12D 
                 
                 90 
                 
                   151 μM 
                 
                 
                   1.2 
                 
                 
                   3.5 
                 
                   21 μM   
                 
                   1.2 
                 
                 2.9 
               
               
                 3D9H/3E12Q 
                 91 
                 713 μM 
                 1.8 
                 2.9 
                 427 μM 
                 1.3 
                 2.9 
               
               
                 3D9H/N87G 
                 92 
                 215 μM 
                 1.5 
                 2.2 
                 152 μM 
                 1.2 
                 2.4 
               
               
                 3D9H/3E12D/N87G 
                 93 
                  64 μM 
                 1.5 
                 2 
                  17 μM 
                 0.9 
                 2 
               
               
                 
                   3D9H/3E12D/3T7S 
                 
                 94 
                 
                   514 μM 
                 
                 
                   1.9 
                 
                 3 
                 
                   115 μM 
                 
                 
                   1.9 
                 
                 2.5 
               
               
                   
               
            
           
         
       
     
     Italicized constructs are likely to be the most useful in in-cell imaging applications. Selected constructs were assayed with other potentially interfering metals (Co(II) was used as a surrogate for Fe(II)). NS=no response detected at 2 mM. ND=not determined. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 16 
               
               
                   
               
               
                   
                 SEQ  
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 ID 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 NO: 
                 Co(II) 
                   
                   
                 Zn(II) 
                   
                   
                 Mg(II) 
                   
                   
               
               
                 Construct 
                 95 
                 K d, app   
                 n 
                 R/R 0   
                 K d, app   
                 n 
                 R/R 0   
                 K d, app   
                 n 
                 R/R 0   
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 3E12D 
                 96 
                 1.4 mM 
                   
                 2.5 
                 ~1.3 mM 
                   
                 ~2 
                 NS 
                 NS 
                 NS 
               
               
                 3D9Q/3E12D 
                 97 
                  74 μM 
                 1.4 
                 1.1 
                 NS 
                 NS 
                 NS 
                 NS 
                 NS 
                 NS 
               
               
                 
                   3D9H/3E12D 
                 
                 
                   98 
                 
                 
                   393 μM 
                 
                 
                   1.6 
                 
                 
                   2.7 
                 
                 
                   10 μM, 
                 
                 
                   2.5, 
                 
                 
                   2, 1.6 
                 
                 
                    1.8 mM 
                 
                 
                   1.6 
                 
                 
                   2.3 
                 
               
               
                   
                   
                   
                   
                   
                 
                   325 μM 
                 
                 
                   2.5 
                 
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 3D9H/3E12D/N87G 
                 99 
                 383 μM 
                 1.1 
                 1.5 
                 10 μM, &gt;2 mM 
                 1.2, ~2 
                 821 μM 
                 1.6 
                 1.3 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 
                   3D9H/3E12D/3T7S 
                 
                 
                   100 
                 
                 
                   1.2 mM 
                 
                 
                   2 
                 
                 
                   3 
                 
                 
                   25 μM 
                 
                 
                   0.8 
                 
                 
                   2 
                 
                 
                    &gt;2 mM 
                 
                 
                   ND 
                 
                 ND 
               
               
                   
               
            
           
         
       
     
     The properties of the 3D9H/3E12D and 3D9H/3E12D/3T7S constructs suggest that they can be used as selective sensors for Mn(II) within cells (the main interference, Zn, is generally present in cells at free concentrations far lower than μM, so they are unlikely to respond to Zn in cells). The responsiveness of 3D9H/3E12D to La(III) is ˜30 μM, far above a free concentration that would be physiologically relevant, and far above the K d  of the wild-type sensor for Lam. 
     Therefore, most pertinent to this application, in demonstrating the ability to rationally, substantially alter metal binding selectivity of LanM, these data describe a method for re-engineering LaMP1&#39;s and thus also LanM&#39;s metal selectivity for potentially any desired inorganic ion. 
     Minimal LanM constructs: As described above, the propensity for mutations to lead to biphasic responses motivates the development of constructs that exhibit a single response for sensing, capture, and separations applications. This minimal construct may also potentially be able to bind rare earths at a higher mg metal per g protein ratio than the wildtype protein. Based on our NMR structure of Y(III)-bound LanM, we determined that removal of EF1 and EF4 might yield a sensor that retains similar to wt apparent K d , but with a monophasic response and more amenable to protein engineering. For our initial experiments, we hypothesized that the residues of LanM/LaMP1 following EF4 are likely important for hydrophobic packing, and so they should be retained even if EF4 is removed. We note that other constructs could be tested lacking these C-terminal residues, or an analogous construct lacking EF2 and EF3 could also be designed to yield a construct with a single metal-binding site in EF1, or derivatives with other segments of the protein removed could be made. 
     In order to test the effects of N-terminal truncations, LanM was initially truncated at the 30 th  and 48 th  residues. These and other LanM truncations were placed into the LaMP1 sensor scaffold to quickly determine effects on metal binding. The 430 mutant truncates the protein immediately prior to EF1, whereas the 448 mutant truncates after this metal binding site. Both mutants were expected to have a functional EF2/3 pair. The sequences below represent the LanM portion inserted between the ECFP/citrine FRET pair. 
     
       
         
           
               
            
               
                 &gt;wild-type LanM 
               
               
                 (SEQ ID NO: 41) 
               
               
                 PTTTTKVDIAAFDPDKDGTIDLKEALAAGSAAFDKLDPDKDGTLDAKELK 
               
               
                   
               
               
                 GRVSEADLKKLDPDNDGTLDKKEYLAAVEAQFKAANPDNDGTIDARELAS 
               
               
                   
               
               
                 PAGSALVNLIREL 
               
               
                   
               
               
                 &gt;LanM-Δ30 
               
               
                 (SEQ ID NO: 42) 
               
               
                 DIAAFDPDKDGTIDLKEALAAGSAAFDKLDPDKDGTLDAKELKGRVSEAD 
               
               
                   
               
               
                 LKKLDPDNDGTLDKKEYLAAVEAQFKAANPDNDGTIDARELASPAGSALV 
               
               
                   
               
               
                 NLIREL 
               
               
                   
               
               
                 &gt;LanM-Δ48 
               
               
                 (SEQ ID NO: 43) 
               
               
                 LAAGSAAFDKLDPDKDGTLDAKELKGRVSEADLKKLDPDNDGTLDKKEYL 
               
               
                   
               
               
                 AAVEAQFKAANPDNDGTIDARELASPAGSALVNLIREL 
               
            
           
         
       
     
     Characterization of these constructs with EDDS-buffered Pr(III) solutions showed that the Δ30 variant displayed an apparent K d  almost identical to wt (−10 μM), but a FRET change of 3. For the Δ48 variant, the apparent K d  was ˜500 pM with the same FRET change. Ca(II) response was less than 2-fold, with an apparent K d  of ˜8 mM. The lower FRET changes than in wt LaMP1 suggest different relative orientations of the fluorescent proteins and/or destabilization of the EF1/4 pair. We are currently investigating truncations made in between these positions to minimize protein size while retaining low K da,pp . 
     The effects of EF4 removal were tested using the constructs below. All of these constructs exhibited the same apparent K d , ˜50 pM, in an initial response with a 3-fold FRET change, using EDDS-buffered Pr(III) solutions. 
     
       
         
           
               
            
               
                 &gt;LanM-ΔN108-E119_GSG 
               
               
                 (SEQ ID NO: 44) 
               
               
                 PTTTTKVDIAAFDPDKDGTIDLKEALAAGSAAFDKLDPDKDGTLDAKELK 
               
               
                   
               
               
                 GRVSEADLKKLDPDNDGTLDKKEYLAAVEAQFKAAGSGLASPAGSALVNL 
               
               
                   
               
               
                 IREL 
               
               
                   
               
               
                 &gt;LanM-ΔN108-E119_GGSGG 
               
               
                 (SEQ ID NO: 44) 
               
               
                 PTTTTKVDIAAFDPDKDGTIDLKEALAAGSAAFDKLDPDKDGTLDAKELK 
               
               
                   
               
               
                 GRVSEADLKKLDPDNDGTLDKKEYLAAVEAQFKAAGGSGGLASPAGSALV 
               
               
                   
               
               
                 NLIREL 
               
               
                   
               
               
                 &gt;LanM-ΔN108-E125_GSG 
               
               
                 (SEQ ID NO: 45) 
               
               
                 PTTTTKVDIAAFDPDKDGTIDLKEALAAGSAAFDKLDPDKDGTLDAKELK 
               
               
                   
               
               
                 GRVSEADLKKLDPDNDGTLDKKEYLAAVEAQFKAAGSGSALVNLIREL 
               
               
                   
               
               
                 &gt;LanM-ΔN108-E125_GGSGG 
               
               
                 (SEQ ID NO: 46) 
               
               
                 PTTTTKVDIAAFDPDKDGTIDLKEALAAGSAAFDKLDPDKDGTLDAKELK 
               
               
                   
               
               
                 GRVSEADLKKLDPDNDGTLDKKEYLAAVEAQFKAAGGSGGSALVNLIREL 
               
            
           
         
       
     
     Constructs in which the truncations of EF1 and EF4 are combined are in progress. 
     However, the following example has been made and preliminary tests have been carried out: 
     
       
         
           
               
            
               
                 &gt;LanM-Δ48_ΔN108-E119_GGSGG 
               
               
                 (SEQ ID NO: 47) 
               
               
                 LAAGSAAFDKLDPDKDGTLDAKELKGRVSEADLKKLDPDNDGTLDKKEYL 
               
               
                   
               
               
                 AAVEAQFKAAGGSGGLASPAGSALVNLIREL 
               
            
           
         
       
     
     This construct behaves essentially identically to the LanM-Δ48 construct. These results show that our mechanistic understanding of the function of this protein allow us to design significant mutations and deletions that retain metal-binding function, at least in EF2/3. These and related constructs may be useful for REE detection, capture, and separation applications. These results also suggest that similar experiments can be used to create other derivatives with a range of metal binding functions (different metal stoichiometries, metal affinities, and metal selectivities). 
     Other sensing modalities: Some of the limitations of fluorescence-based detection platform for rapid analysis of rare earths in arbitrary environmental samples are pH (at least for the ECFP/citrine FRET pair) and strongly absorbing/scattering samples (e.g. acid mine drainage, samples containing particle suspensions). Furthermore, while portable fluorescence spectrometers are available, detection using more widely available instrumentation like digital cameras or phones would be ideal. Bioluminescence-based sensing offers a solution to these challenges. We are pursuing approaches to turn LanM&#39; s selective conformational response to rare earths into a bioluminescent output using bioluminescence resonance energy transfer (BRET)), an analogous concept as FRET but using a luminescent rather than fluorescent donor. One possible approach uses bioluminescent donor such as NanoLuc and either sulfo-Cy3 (conjugated to a cysteine residue) or a fluorescent protein such as tdTomato as a BRET acceptor (see constructs below). Approaches with other BRET pairs can also be designed. 
     
       
         
           
               
            
               
                 &gt;Bioluminescent LaMP2: NanoLuc + LanM 
               
               
                 (underlined) + cysteine for conjugation 
               
               
                 to sulfoCy3-maleimide 
               
               
                 (SEQ ID NO: 48) 
               
               
                 mvftledfvgdwrqtagynldqvlegggvsslfqnlgvsvtpigrivlsg 
               
               
                   
               
               
                 englkidihviipyeglsgdgmgqiekifkvvypvddhhfkvilhygtiv 
               
               
                   
               
               
                 idgvtpnmidyfgrpyegiavfdgkkitvtgtlwngnkiiderlinpdgs 
               
               
                   
               
               
                 llfrvtingvtgwrlaerila pttttkvdiaafdpdkdgtidlkealaag   
               
               
                   
               
               
                 
                   saafdkldpdkdgtldakelkgrvseadlkkldpdndgtldkkeylaave 
                 
               
               
                   
               
               
                   aqfkaanpdndgtidarelaspagsalvnlir c 
               
               
                   
               
               
                 &gt;Bioluminescent LaMP3: NanoLuc + LanM 
               
               
                 (underlined) + tdTomato (uppercase) 
               
               
                 (SEQ ID NO: 49) 
               
               
                 mvftledfvgdwrqtagynldqvlegggvsslfqnlgvsvtpigrivlsg 
               
               
                   
               
               
                 englkidihviipyeglsgdgmgqiekifkvvypvddhhfkvilhygtiv 
               
               
                   
               
               
                 idgvtpnmidyfgrpyegiavfdgkkitvtgtlwngnkiiderlinpdgs 
               
               
                   
               
               
                 llfrvtingvtgwrlaerila pttttkvdiaafdpdkdgtidlkealaag   
               
               
                   
               
               
                 
                   saafdkldpdkdgtldakelkgrvseadlkkldpdndgtldkkeylaave 
                 
               
               
                   
               
               
                   aqfkaanpdndgtidarelaspagsalvnlirel MVSKGEEVIKEFMRFK 
               
               
                   
               
               
                 VRMEGSMNGHEFEIEGEGEGRPYEGTQTAKLKVTKGGPLPFAWDILSPQF 
               
               
                   
               
               
                 MYGSKAYVKHPADIPDYKKLSFPEGFKWERVMNFEDGGLVTVTQDSSLQD 
               
               
                   
               
               
                 GTLIYKVKMRGTNFPPDGPVMQKKTMGWEASTERLYPRDGVLKGETHQAL 
               
               
                   
               
               
                 KLKDGGHYLVEFKTIYMAKKPVQLPGYYYVDTKLDITSHNEDYTIVEQYE 
               
               
                   
               
               
                 RSEGRHHLFLGHGTGSTGSGSSGTASSEDNNMAVIKEFMRFKVRMEGSMN 
               
               
                   
               
               
                 GHEFEIEGEGEGRPYEGTQTAKLKVTKGGPLPFAWDILSPQFMYGSKAYV 
               
               
                   
               
               
                 KHPADIPDYKKLSFPEGFKWERVMNFEDGGLVTVTQDSSLQDGTLIYKVK 
               
               
                   
               
               
                 MRGTNFPPDGPVMQKKTMGWEASTERLYPRDGVLKGETHQALKLKDGGHY 
               
               
                   
               
               
                 LVEFKTIYMAKKPVQLPGYYYVDTKLDITSHNEDYTIVEQYERSEGRHHL 
               
               
                   
               
               
                 FLYGMDELYK 
               
            
           
         
       
     
     These results show that LanM and its derivatives can be engineered to have altered metal selectivity via rational approaches. Higher throughput methods such as directed evolution could also be used using LaMP1 or a derivative as the readout for the purposes of engineering. These studies have focused on mutations at the metal-binding sites, but residues outside the metal-binding sites can also be targeted. In theory, this protein could be used to develop sensors/binding proteins for any metal ion (or even possibly any inorganic anion) of interest. Of particular interest is the development of proteins for selective binding of actinides, such as uranyl (U02 2+ ), or tetravalent metals like Th(IV). Our data show that the second, ninth, and 12 th  positions in the EF hands seem to be particularly useful at altering metal binding selectivities without disrupting conformational response, although other positions within the EF hands and between EF hands can also be changed to alter affinities and cooperativities. For example, in order to selectively bind and sense uranyl, one method involves taking advantage of the EF hand carboxylates in the equatorial plane and modifying the axial carboxylates (residues at positions 1 and 9) to hydrogen bonding donors (e.g. Ser, Thr, Asn, or main chain amides) for interaction with the unique oxo groups of uranyl. 
     Based on our analysis, we found that a highly selective metal-binding has certain characteristics. In particular, an isolated metal-binding protein having a certain number of EF hand motifs and certain residues with a certain spacer leads to a protein with high tri- and/or tetravalent cation selectivity as disclosed herein. In an embodiment, an isolated metal-binding protein can include 2, 3 or 4 EF hand motifs, at least one of the EF hand motifs having at least 2 carboxylate residues, wherein at least 2 adjacent of the EF hand motifs are separated by 10-15 amino acid residues such as by 11, 12, 13, 14 and 15 amino acid residues, e.g., between and including 12-13 amino acid residues. For example, the protein can include at least 1, and preferably at least two 2, EF hand motifs of the form: 
                    [SEQ ID NO: 3]       (D/N)-X1-(D/N)-X2-(D/N)-X3-X4-X5-X6-X7-X8-(E/D)            
where each X1 through X8 are independently any residue (i.e., not necessarily the same residue in each position). At least two adjacent EF hands are separated by 10-15 residues such as, for example, 12-13 amino acid residues. In an embodiment, X1 is P (Proline). In certain aspects, X6 and/or X8 can be D or E. In certain aspects, glycine (G) is preferred, but not required at X3. Advantageously, metal-binding proteins of the present disclosure can be purified to at least about 70%, such as at least about 75%, 80%, 85%, 90%, 95%, pure form.
 
     For example, in an embodiment, the protein can include 2, 3 or 4 EF hand motifs comprising the above form (SEQ ID NO:3) and each adjacent pair of EF hand motif separated by 10-15, e.g., 12-13, amino acids. Other permutations are contemplated. Various examples of amino acid sequences for EF hand motifs are given in Table 17 below: 
     
       
         
           
               
             
               
                 TABLE 17 
               
             
            
               
                   
               
               
                 Various sequences of EF hand: 
               
            
           
           
               
               
            
               
                 SEQ ID NO: 
                 Sequence 
               
               
                   
               
               
                 51 
                 DPDKDGTIDLKE 
               
               
                   
               
               
                 52 
                 DPDKDGTIDAKE 
               
               
                   
               
               
                 53 
                 DPDNDGTIDKKE 
               
               
                   
               
               
                 54 
                 NPDKDGTIDARE 
               
               
                   
               
               
                 55 
                 NKDNDDSLEIAE 
               
               
                   
               
               
                 56 
                 DIDKDGRVSVEE 
               
               
                   
               
               
                 57 
                 DKDNDGTLDREE 
               
               
                   
               
               
                 58 
                 DKNGDGFIDKEE 
               
               
                   
               
            
           
         
       
     
     For example, in an embodiment, the protein can include 2 EF hand motifs comprising SEQ ID NO:3 and separated by 12-13 amino acids, or the protein can include 3 EF hand motifs comprising SEQ ID NO:3 with each adjacent motifs being separated by 12-13 amino acids, or the protein can include 4 EF hand motifs comprising SEQ ID NO:3 with each adjacent motifs being separated by 12-13 amino acids. Other permutations are contemplated. 
     Another aspect of the present disclosure includes a method for separating rare earth or actinide elements, compounds or salts thereof (e.g., lanthanides) from a medium, e.g., a solution, a suspension, or a colloid. The method includes contacting a medium containing a trivalent cation with a metal-binding protein to bind the trivalent cation; and isolating the metal-binding protein having the trivalent cation bound thereto. The metal-binding protein can advantageously have a higher selective affinity for trivalent cations over non-trivalent cations, wherein the trivalent cations are selected from the group consisting of trivalent cations from a rare earth element, a compound thereof, a salt thereof, an actinide, a compound thereof, a salt thereof, and combinations thereof. In some aspect of the present disclosure, the method includes contacting a medium containing one or more rare earth elements, compounds or salt thereof (e.g. a lanthanide or a salt thereof) and/or one or more actinide elements, compounds or salt thereof with a metal-binding protein having a high selective affinity for the rare earth or the actinide over a non-rare earth or non-actinide. The method can also include isolating the metal-binding protein to which the rare earth or actinide element, compound or salt thereof have bound (e.g., a lanthanide (3+) ion); and optionally separating the one or more rare earths or actinides from the metal-binding protein. 
     In an embodiment, the metal-binding protein may be immobilized to a substrate by, for example, biotinylating the protein and surface treating the substrate with avidin. In some embodiments, the protein is immobilized using click chemistry, e.g., azide-alkyne cycloaddition, such as by labeling the protein with an azide or alkyne and reacting it with an alkyne or azide, respectively, on the immobilization surface (e.g., a bead of a resin). In some embodiments, the medium including trivalent cations from rare earth or actinide element, compound or salt thereof is contacted with the substrate on which the metal-binding protein is immobilized. The trivalent cations from the rare earth or actinide element, compound or salt thereof are then allowed to bind with the metal-binding protein (e.g., but allowing certain time to be passed, or by applying an electrical field to promote movement of the cations toward the substrate). The medium is then removed, and the substrate to which the trivalent cations are bound is treated to separate the trivalent cations. The surface treatment may include, in various embodiments, treatment with a low pH solution, or treatment with chelators such as, for example, EGTA, EDTA, citrate, EDDS, etc. 
     Only the preferred embodiment of the present invention and examples of its versatility are shown and described in the present disclosure. It is to be understood that the present invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein. Thus, for example, those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific substances, procedures and arrangements described herein. Such equivalents are considered to be within the scope of this invention, and are covered by the following claims.