# Patent

## Title
Nucleic acids and polypeptides involved in the production of cryptophycin

## Abstract

The present invention provides polypeptides involved in cryptophycin biosynthesis and the nucleic acid molecules that encode such polypeptides. The nucleic acid molecules and polypeptides of the invention or variants thereof can be used in the methods of the invention to produce cryptophycins.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority under 35 U.S.C. §119(e) of U.S. Application No. 60/820,715, filed Jul. 28, 2006.


FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
The U.S. Government has certain rights in this invention pursuant to Grant Nos. CA083155 and CA009676 awarded by the National Institutes of Health.


TECHNICAL FIELD
This invention relates to production of cryptophycin, and more particularly to the polypeptides involved in the biosynthesis of cryptophycin and the nucleic acids encoding such polypeptides.
BACKGROUND
Cryptophycins are novel macrolides first isolated from blue-green algae (Nostoc sp. GSV22 and Nostoc sp. ATCC 53789) and are potent tumor selective cytotoxins in vivo. Many syntheses of the major natural products, cryptophycins 1-4, and a wide range of analogs have been published. For example, cryptophycins have been synthesized by a convergent method in which four components, Unit A, Unit B, Unit C, and Unit D (Golakati et al., 1995, J. Am. Chem. Soc., 117(49):12031), are coupled together to form the final product (see, for example, U.S. Pat. No. 6,013,626). In other methods, novel semi-synthetic compounds are generated, for example, by converting the epoxide of a natural cryptophycin to a carbon-carbon double bond (see, for example, U.S. Pat. Nos. 4,845,085 and 4,845,086). Stereo-selective addition of functional groups is often problematic during chemical synthesis of cryptophycins, however. Therefore, few of the methodologies for cryptophycin syntheses are considered viable or practical on a commercial scale.
SUMMARY
The present invention provides polypeptides involved in cryptophycin biosynthesis and the nucleic acid molecules that encode such polypeptides. The nucleic acid molecules and polypeptides of the invention or variants thereof can be used in the methods of the invention to produce cryptophycins.
In one aspect, the invention provides an isolated nucleic acid molecule that includes a nucleic acid sequence having at least 85% (e.g., 85%, 90%, 95%, 99%, or 100%) sequence identity to the sequence shown in SEQ ID NO:1 or to a fragment thereof. Such a sequence encodes at least one enzyme involved in biosynthesizing cryptophycin.
The invention further provides for a vector containing such a nucleic acid molecule, and host cells containing such vectors. The invention also provides for cryptophycin or cryptophycin analogues made by such host cells.
In another aspect, the invention provides methods of producing cryptophycin. Such a method generally includes the step of culturing the above-described host cells in the presence of an appropriate substrate and under conditions appropriate for the production of cryptophycin. Such a method can further include the step of purifying the cryptophycin.
In another aspect, the invention provides an isolated nucleic acid molecule comprising a nucleic acid sequence having at least 85% (e.g., 85%, 90%, 95%, 99%, or 100%) sequence identity to SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 42 or 44, or to a fragment thereof, wherein the nucleic acid sequence encodes a polypeptide that exhibits functional activity.
The invention further provides for vectors containing such nucleic acid molecules, and host cells containing such vectors. The invention also provides for intermediates in cryptophycin biosynthesis made by such host cells.
The invention further provides a polypeptide encoded by the nucleic acid sequence having at least 85% (e.g., 85%, 90%, 95%, 99%, or 100%) sequence identity to SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 42 or 44, or to a fragment thereof. Such polypeptides can have the sequence shown in 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 43 or 45, respectively.
In still another aspect, the invention provides an isolated nucleic acid molecule comprising a nucleic acid sequence having at least 85% (e.g., 85%, 90%, 95%, 99%, or 100%) sequence identity to nucleotides 9,199 to 10,032 of SEQ ID NO:6, or to a fragment thereof, wherein the nucleic acid sequence encodes a polypeptide that exhibits thioesterase activity under appropriate conditions.
In yet another aspect, the invention provides an isolated nucleic acid molecule comprising a nucleic acid sequence having at least 85% (e.g., 85%, 90%, 95%, 99%, or 100%) sequence identity to the sequence shown in SEQ ID NO:8, or to a fragment thereof, wherein the nucleic acid sequence encodes a polypeptide that exhibits epoxidase activity under appropriate conditions.
In another aspect, the invention provides an isolated nucleic acid molecule comprising a nucleic acid sequence having at least 85% (e.g., 85%, 90%, 95%, 99%, or 100%) sequence identity to the sequence shown in SEQ ID NO:14, or to a fragment thereof, wherein the nucleic acid sequence encodes a polypeptide that exhibits halogenase activity under appropriate conditions.
In another aspect, the invention provides an isolated nucleic acid molecule comprising a nucleic acid sequence having at least 85% (e.g., 85%, 90%, 95%, 99%, or 100%) sequence identity to SEQ ID NO:42 or 44, or to a fragment thereof, wherein the nucleic acid sequence encodes a polypeptide that exhibits polyketide synthetase activity under appropriate conditions.
In another aspect, the invention provides for methods of producing an intermediate in cryptophycin biosynthesis. Such a method includes culturing one or more host cells that contain one or more vectors comprising one or more of the nucleic acid sequences shown in SEQ ID NOs:2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 42 or 44 in the presence of one or more appropriate substrates under conditions appropriate for production of an intermediate in cryptophycin biosynthesis.
Representative appropriate conditions include pH, media, temperature, and/or the presence or absence of co-factors. Representative substrates and intermediates in cryptophycin biosynthesis include Cryptophycin 2, 3, 4, 5, 16, and 17 (see FIG. 1B).
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the drawings and detailed description, and from the claims.



DESCRIPTION OF DRAWINGS
FIG. 1 shows cryptophycin structures. FIG. 1A is a natural cryptophycin, Cryptophycin 1, and a synthetic cryptophycin, Cryptophycin 52. FIG. 1B illustrates the diversity of natural cryptophycins isolated from Nostoc spp.
FIG. 2 is a schematic of the lineage of biologically active cryptophycins.
FIG. 3 is a schematic of the modular structure of the cryptophycins and retro-biosynthesis assembly.
FIG. 4 is a schematic of cosmid pDAM163 and genes identified with relationships to cryptophycin biosynthesis.
FIG. 5 is the nucleotide sequence of the cloned insert of pDAM163 (SEQ ID NO:1).
FIG. 6 is the nucleotide and amino acid sequences of the genes and polypeptides involved in cryptophycin biosynthesis (SEQ ID NOs:2-31).
FIG. 7 shows SEQ ID NOs: 32-37, which have 75%, 80%, 85%, 90%, 95%, and 99% sequence identity, respectively, to SEQ ID NO:2.
FIG. 8 is a schematic depicting the predicted cryptophycin assembly line.
FIG. 9 is a schematic demonstrating that the cryptophycin epoxidase (CrpE) has substrate flexibility but a high degree of stereoselectivity.
FIG. 10 is a schematic of the synthesis of SNAC substrates.
FIG. 11 is a schematic of cryptophycin thioesterase-catalyzed conversion of substrate 1 to products 2, 3, and 4 in 0.1 M NaPi buffer (pH 8.0) containing 4% DMSO.
FIG. 12 are graphs of cryptophycin thioesterase-catalyzed hydrolysis of substrate 1 using 1.4 μM thioesterase in 50 μL reactions containing 0.1 M NaH2PO4 and 4% DMSO.
FIG. 13 is a schematic of cryptophycin thioesterase-catalyzed cyclization and hydrolysis of the seco-SNAC-ester of arenastatin and the seco-SNAC-ester of the vinyl derivative of arenastatin.
FIG. 14 is a nucleotide (SEQ ID NO:42) and encoded amino acid sequence (SEQ ID NO:43) of crpA.
FIG. 15 is a nucleotide (SEQ ID NO:44) and encoded amino acid sequence (SEQ ID NO:45) of crpB.



DESCRIPTION OF SEQUENCES
SEQ ID NO:1 is the nucleotide sequence of the cloned insert of pDAM163.
SEQ ID NO:2 is a nucleotide sequence having homology to crpA.
SEQ ID NO:3 is an amino acid sequence having homology to CrpA.
SEQ ID NO:4 is the nucleotide sequence of crpC.
SEQ ID NO:5 is the amino acid sequence of CrpC.
SEQ ID NO:6 is the nucleotide sequence of crpD.
SEQ ID NO:7 is the amino acid sequence of CrpD.
SEQ ID NO:8 is the nucleotide sequence of crpE.
SEQ ID NO:9 is the amino acid sequence of CrpE.
SEQ ID NO:10 is the nucleotide sequence of crpF.
SEQ ID NO:11 is the amino acid sequence of CrpF.
SEQ ID NO:12 is the nucleotide sequence of crpG.
SEQ ID NO:13 is the amino acid sequence of CrpG.
SEQ ID NO:14 is the nucleotide sequence of crpH.
SEQ ID NO:15 is the amino acid sequence of CrpH.
SEQ ID NO:16 is the nucleotide sequence of crpM.
SEQ ID NO:17 is the amino acid sequence of CrpM.
SEQ ID NO:18 is the nucleotide sequence of crpN.
SEQ ID NO:19 is the amino acid sequence of CrpN.
SEQ ID NO:20 is the nucleotide sequence of crpP.
SEQ ID NO:21 is the amino acid sequence of CrpP.
SEQ ID NO:22 is the nucleotide sequence of crpU.
SEQ ID NO:23 is the amino acid sequence of CrpU.
SEQ ID NO:24 is the nucleotide sequence of crpV.
SEQ ID NO:25 is the amino acid sequence of CrpV.
SEQ ID NO:26 is the nucleotide sequence of crpX.
SEQ ID NO:27 is the amino acid sequence of CrpX.
SEQ ID NO:28 is the nucleotide sequence of crpY.
SEQ ID NO:29 is the amino acid sequence of CrpY.
SEQ ID NO:30 is the nucleotide sequence of crpZ.
SEQ ID NO:31 is the amino acid sequence of CrpZ.
SEQ ID NO:32 is a nucleotide sequence having 75% sequence identity to SEQ ID NO:2.
SEQ ID NO:33 is a nucleotide sequence having 80% sequence identity to SEQ ID NO:2.
SEQ ID NO:34 is a nucleotide sequence having 85% sequence identity to SEQ ID NO:2.
SEQ ID NO:35 is a nucleotide sequence having 90% sequence identity to SEQ ID NO:2.
SEQ ID NO:36 is a nucleotide sequence having 95% sequence identity to SEQ ID NO:2.
SEQ ID NO:37 is a nucleotide sequence having 99% sequence identity to SEQ ID NO:2.
SEQ ID NO:38 is the sequence of an oligonucleotide.
SEQ ID NO:39 is the sequence of an oligonucleotide.
SEQ ID NO:40 is the sequence of an oligonucleotide.
SEQ ID NO:41 is the sequence of an oligonucleotide.
SEQ ID NO:42 is the nucleotide sequence of crpA.
SEQ ID NO:43 is the amino acid sequence of CrpA.
SEQ ID NO:44 is the nucleotide sequence of crpB.
SEQ ID NO:45 is the amino acid sequence of CrpB.
DETAILED DESCRIPTION
Cryptophycin biosynthesis is accomplished via a mixed Type I PKS/NRPS system. Manipulation of polyketide synthetases (PKSs) and non-ribosomal peptide synthetases (NRPSs) through mutasynthesis, combinatorial biosynthesis, and directed biosynthesis feeding (chemoenzymatic synthesis) has been described for many PKS and NRPS polypeptides. The identification of the corresponding genes allows for these types of approaches with the cryptophycin system. It is possible that altering the PKS enzyme for Unit A formation or the NRPS for Unit B, C, and D formation could generate a wide variety of new cryptophycins. With this invention, it is also possible to incorporate these enzymes in “total synthesis” of cryptophycins to lower the cost and increase the overall yields. For example, the ability of biosynthetic enzymes to exhibit high levels of stereo-chemical control and relaxed substrate specificity, and the sensitivity of the biological and chemical assays for identifying cryptophycins, allow for production of rational “biologically” derived cryptophycins that have superior properties.
Cryptophycins
FIG. 1A shows Cryptophycin 1 and Cryptophycin 52. Cryptophycin 52 is nearly identical to Cryptophycin 1, the most active natural compound, except for the presence of gem-dimethyl on the β-alanine unit of Cryptophycin 52 instead of the methyl group on Cryptophycin 1. FIG. 1B shows numerous other natural cryptophycins that have been isolated from Nostoc spp. A chlorohydrin analog (Cryptophycin 309; see U.S. Publication No. 20020065261 and FIG. 2 of the instant application) has been identified and has been shown to be much more active than the current clinical candidate, Cryptophycin 52.
Purification
Routine chromatographic techniques such as high-performance liquid chromatography (HPLC) or thin-layer chromatography (TLC) can be used to purify cryptophycins. See, for example, U.S. Pat. No. 5,952,298, which describes specific HPLC conditions for purifying different cryptophycins.
Structure Identification
The structures of cryptophycins can be determined using methodology that is well known to those of skill in the art. Mass spectral analysis can be used, for example. Proton and carbon NMR data obtained from COSY, HMQC, HMBC, and NOESY spectra allows determination of the gross structures of the depsipeptide-type compounds. The presence of the various hydroxy and amino acid units in each compound can be detected by gas chromatographic mass spectral analysis. Total structures, including absolute stereochemistries, can be determined using a combination of chemical degradative and analytical techniques on cryptophycin compounds.
Anti-Fungal Activity
Cryptophycin compounds can be tested against fungal organisms known to be sensitive to such compounds using, for example, a disk-diffusion assay such as a Corbett assay (see, for example, Kemp, 1980, Organic Chemistry, Worth Publishers Inc.). The anti-fungal activity of a cryptophycin is usually correlated with the size of the zone of inhibition (i.e., an area of no microbial growth around an antimicrobial agent in a disk-diffusion test). An organism that can be used to evaluate the anti-fungal activity of a cryptophycin is Candida albicans. 
Anti-Cancer Activity
The anti-cancer activity of a cryptophycin can be examined using a number of different assays such as cell proliferation assays and cell cycle arrest assays. In addition, cytoskeletal structures such as tubulin can be examined using, for example, immunofluorescence assays. See, for example, U.S. Pat. No. 5,945,315.
Cryptophycins can be evaluated for anti-cancer activity against a number of different cell types. For example, murine leukemia cells (e.g., LI210 or P388), murine solid tumor cells (e.g., colon adenocarcinoma 38, pancreatic ductal adenocarcinoma 03, mammary adenocarcinoma M16/M17), human solid tumor cells (e.g., colon CX-1, HCT8, H116, lung H125, mammary MX-1, MCF-7), low malignancy fibroblast cells (e.g., LML), human nasopharyngeal carcinoma cells (e.g., KB), human colon carcinoma cells (e.g., LoVo), and human ovarian carcinoma cells (e.g., SKOV3) can be used to evaluate the anti-cancer activity of a cryptophycin. For example, a disk diffusion assay much like the Corbett assay (Kemp, supra) commonly used in antifungal and antibacterial testing can be used to evaluate the anti-cancer activity of a cryptophycin. A zone of inhibition can be correlated with the anti-cancer activity of a cryptophycin.
Nucleic Acids and Polypeptides Involved in Cryptophycin Biosynthesis
Approximately 45 kb of DNA corresponding to the genes predicted to be involved in cryptophycin biosynthesis were cloned into a cosmid designated pDAM163 and sequenced. FIG. 4 shows a schematic of pDAM163, while FIG. 5 shows the nucleotide sequence of the cloned insert of pDAM163 (SEQ ID NO:1). This cosmid replicated efficiently and stably in well-developed fermentation strains such as E. coli B (E. coli BL21pLys) and E. coli K (DH5α) derivatives. Expressing the coding regions contained within pDAM163 can result in the production of cryptophycin in the E. coli strains. A variety of microorganisms such as bacteria (e.g., Escherichia coli), yeast (e.g., Pichia pastoris or Saccharomyces cerevisiae), or fungi (e.g., Neurospora crassa) that include expression constructs such as pDAM163 or variants thereof can be used to generate cryptophycins.
The components of the biosynthetic pathway are summarized in Table 1, which provides information related to the putative function of each polypeptide.







TABLE 1







Nucleic Acids and Polypeptides Involved


in Cryptophycin Biosynthesis









Designation




(SEQ ID NO: Nucleic
Length


Acid/Polypeptide)
(amino acids)
Putative Function












crpA (SEQ ID NO: 42/43)
2942
Polyketide synthetase




(PKS)


crpB (SEQ ID NO: 44/45)
3470
Polyketide synthetase




(PKS)


crpC (SEQ ID NO: 4/5)
1944
Nonribosomal Peptide




Synthetase (NRPS)


crpD (SEQ ID NO: 6/7)
3344
NRPS


crpE (SEQ ID: NO: 8/9)
451
Cytochrome p450




(epoxidase)


crpF (SEQ ID NO: 10/11)
295
Iron-dependent non-heme




hydroxylase


crpG (SEQ ID NO: 12/13)
114
Aspartate decarboxylase


crpH (SEQ ID NO: 14/15)
492
Non-heme halogenase


crpI
a
IS1327 Transposase


crpK
a
IS892-orf2


crpL
a
IS892-orf1


crpM (SEQ ID NO: 16/17)
461
ISRSO13 Transposase


crpN (SEQ ID NO: 18/19)
314
Benzoyl-CoA reductase/




2-Hydroxyglutaryl-CoA




dehydratase


crpO
b
pvdE type regulator


crpP (SEQ ID NO: 20/21)
2I0
Thioredoxin


crpU (SEQ ID NO: 22/23)
155
N-acetyltransferase


crpV (SEQ ID NO: 24/25)
784
Large exoprotein




involved in heme




utilization


crpW
a
TPR repeat protein


crpX (SEQ ID NO: 26/27)
132
Chorismate mutase/Pre-




phenate dehydrogenase


crpY (SEQ ID NO: 28/29)
214
2-Hydroxychromenene-2-




carboxylate isomerase


crpZ (SEQ ID NO: 30/31)
90
3-Dehydroquinate




synthase





a, no open reading frame identified;


b, multiple open reading frames identified.






Nucleic Acid Molecules
The present invention is based, in part, on the identification of nucleic acid molecules that encode polypeptides involved in cryptophycin synthesis. Particular nucleic acid molecules of the invention include the sequences shown in SEQ ID NOs:1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 42 and 44. As used herein, the term “nucleic acid molecule” can include DNA molecules and RNA molecules and analogs of the DNA or RNA molecule generated using nucleotide analogs. A nucleic acid molecule of the invention can be single-stranded or double-stranded, and the strandedness will depend upon its intended use.
The invention further encompasses nucleic acid molecules that differ from the nucleotide sequence of SEQ ID NOs: 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 42 and 44. Nucleic acid molecules of the invention include molecules that are at least 10 nucleotides in length and that have at least 75% sequence identity (e.g., at least 80%, 85%, 90%/, 95%/, or 99% sequence identity) to any of SEQ ID NOs: 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 42 and 44. Nucleic acid molecules that differ in sequence from the nucleic acid sequences shown in SEQ ID NOs: 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 42 and 44 can be generated by standard techniques, such as site-directed mutagenesis or PCR-mediated mutagenesis. In addition, nucleotide changes can be introduced randomly along all or part of a nucleic acid molecule of the invention, such as by saturation mutagenesis. Alternatively, nucleotide changes can be introduced into a sequence by chemically synthesizing a nucleic acid molecule having such changes.
In calculating percent sequence identity, two sequences are aligned and the number of identical matches of nucleotides or amino acid residues between the two sequences is determined. The number of identical matches is divided by the length of the aligned region (i.e., the number of aligned nucleotides or amino acid residues) and multiplied by 100 to arrive at a percent sequence identity value. It will be appreciated that the length of the aligned region can be a portion of one or both sequences up to the full-length size of the shortest sequence. It also will be appreciated that a single sequence can align with more than one other sequence and hence, can have different percent sequence identity values over each aligned region. It is noted that the percent identity value is usually rounded to the nearest integer. For example, 78.1%, 78.2%, 78.3%, and 78.4% are rounded down to 78%, while 78.5%, 78.6%, 78.7%, 78.8%, and 78.9% are rounded up to 79%. It is also noted that the length of the aligned region is always an integer.
The alignment of two or more sequences to determine percent sequence identity is performed using the algorithm described by Altschul et al. (1997, Nucleic Acids Res., 25:3389-3402) as incorporated into BLAST (basic local alignment search tool) programs, available at ncbi.nlm.nih.gov on the World Wide Web. BLAST searches can be performed to determine percent sequence identity between a nucleic acid molecule of the invention and any other sequence or portion thereof aligned using the Altschul et al. algorithm. BLASTN is the program used to align and compare the identity between nucleic acid sequences, while BLASTP is the program used to align and compare the identity between amino acid sequences. When utilizing BLAST programs to calculate the percent identity between a sequence of the invention and another sequence, the default parameters of the respective programs are used. Sequence analysis of the nucleic acid sequences as performed herein used BLAST version 2.2.8 (updated on Feb. 10, 2004).
The sequences of representative nucleic acids of the invention having 75%, 80%, 85%, 90%, 95%, and 99% sequence identity to SEQ ID NO:2 are shown in FIG. 7 (SEQ ID NOs:32-37, respectively). Such sequences can be generated using a computer or by hand. The nucleic acid sequences shown in SEQ ID NOs:32-37 were generated by hand by randomly changing 25 nucleotides out of every 100 nucleotides of SEQ ID NO:2, 2 out of every 10, 15 out of every 100, 1 out of every 10, 5 out of every 100, or 1 nucleotide out of every 100 nucleotides of SEQ ID NO:2, respectively. By “changing,” it is meant that the nucleotide at a particular position is replaced randomly with one of the other three nucleotides. It is apparent to those of ordinary skill in the art that any nucleic acid molecule within the scope of the invention can be generated using the same method described herein (i.e., by similarly changing nucleotides within the sequence of SEQ ID NOs: 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 42 or 44).
Nucleic acid fragments are included in the invention. Nucleic acid fragments suitable for use in the invention are those fragments that encode a polypeptide having functional activity. These fragments can be called “functional fragments,” although it is understood that it is not the nucleic acid that possesses functionality.
For example, nucleic acid fragments of crpA (SEQ ID NO:42) can be at least 50 nucleotides in length (e.g., 100, 246, 298, 356, 387, 455, 578, 621, 780, 881, 972, 1040, 1128, 1287, 1344, 1468, 1579, 1622, 1734, 1849, 1931, 2006, 2143, 2267, 2389, 2467, 2590, 2671, 2742, 2862, 2944, 3050, 3133, 3266, 3378, 3496, 3577, 3658, 3792, 3846, 3930, 4002, 4127, 4239, 4347, 4437, 4569, 4672, 4722, 4892, 4956, 5012, 5168, 5234, 5378, 5499, 5546, 5623, 5749, 5832, 5911, 6049, 6128, 6226, 6359, 6439, 6533, 6677, 6788, 6873, 6912, 7019, 7128, 7246, 7359, 7456, 7599, 7688, 7744, 7834, 7926, 8022, 8156, 8277, 8359, 8466, 8523, 8611, 8752, 8801, or 8820); nucleic acid fragments of crpB (SEQ ID NO:44) can be at least can be at least 50 nucleotides in length (e.g., 100, 233, 256, 389, 363, 443, 560, 622, 789, 832, 976, 1056, 1158, 1264, 1322, 1489, 1552, 1619, 1729, 1838, 1974, 2077, 2129, 2202, 2376, 2433, 2511, 2683, 2794, 2805, 2905, 3078, 3124, 3285, 3385, 3424, 3536, 3615, 3728, 3894, 3973, 4084, 4121, 4290, 4357, 4483, 4549, 4614, 4799, 4843, 4983, 5003, 5177, 5245, 5308, 5446, 5577, 5636, 5763, 5878, 5939, 6049, 6177, 6275, 6393, 6429, 6504, 6648, 6793, 6847, 6914, 7028, 7148, 7258, 7324, 7427, 7558, 7677, 7742, 7812, 7978, 8093, 8178, 8262, 8393, 8455, 8522, 8644, 8775, 8848, 8954, 9032, 9168, 9256, 9356, 9419, 9521, 9628, 9720, 9811, 9920, 10025, 10257, 10368, or 10400); nucleic acid fragments of crpC (SEQ ID NO:4) can be at least 292 nucleotides in length (e.g., 292, 306, 382, 461, 592, 715, 825, 947, 1059, 1172, 1236, 1358, 1496, 1590, 1671, 1774, 1889, 1923, 2047, 2135, 2265, 2346, 2477, 2588, 2667, 2754, 2863, 2954, 3084, 3126, 3278, 3345, 3412, 3551, 3670, 3781, 3890, 3910, 4044, 4123, 4266, 4378, 4423, 4513, 4622, 4783, 4822, 4989, 5002, 5156, 5237, 5368, 5486, 5572, 5691, 5765, or 5831); nucleic acid fragments of crpD (SEQ ID NO:6) can be at least 502 nucleotides in length (e.g., 502, 624, 738, 829, 914, 1026, 1138, 1257, 1318, 1452, 1525, 1637, 1768, 1828, 1987, 2074, 2183, 2294, 2338, 2444, 2557, 2637, 2789, 2816, 2942, 3067, 3178, 3227, 3348, 3459, 3504, 3684, 3759, 3812, 3943, 4005, 4276, 4495, 4658, 4827, 5048, 5276, 5424, 5608, 5877, 6034, 6269, 6447, 6632, 6874, 7006, 7284, 7472, 7647, 7814, 8038, 8246, 8459, 8644, 8888, 9053, 9298, 9436, 9666, 9878, or 10,032); nucleic acid fragments of crpE (SEQ ID NO:8) can be at least 68 nucleotides in length (e.g., 68, 74, 82, 88, 95, 105, 168, 235, 367, 489, 524, 665, 784, 863, 925, 1064, 1138, 1279, or 1352); nucleic acid fragments of crpF (SEQ ID NO:10) can be at least 44 nucleotides in length (e.g., 44, 54, 58, 67, 74, 83, 97, 107, 189, 267, 345, 457, 536, 679, 772, or 884); nucleic acid fragments of crpG (SEQ ID NO:12) can be at least 33 nucleotides in length (e.g., 33, 45, 52, 68, 73, 84, 93, 108, 168, 216, 248, 293, 312, or 332); nucleic acid fragments of crpH (SEQ ID NO:14) can be at least 74 nucleotides in length (e.g., 74, 106, 187, 254, 304, 379, 467, 522, 592, 667, 714, 781, 859, 911, 978, 1049, 1138, 1273, 1347, 1405, or 1475); nucleic acid fragments of crpM (SEQ ID NO:16) can be at least 69 nucleotides in length (e.g., 69, 136, 216, 362, 486, 592, 647, 781, 844, 919, 1049, 1138, 1274, or 1382); nucleic acid fragments of crpN (SEQ ID NO:18) can be at least 94 nucleotides in length (e.g., 94, 182, 261, 358, 442, 580, 625, 740, 862, or 941); nucleic acid fragments of crpP (SEQ ID NO:20) can be at least 32 nucleotides in length (e.g., 32, 85, 120, 175, 232, 286, 310, 379, 433, 561, or 632); nucleic acid fragments of crpU (SEQ ID NO:22) can be at least 23 nucleotides in length (e.g., 23, 74, 112, 178, 215, 280, 315, 369, 402, or 467); nucleic acid fragments of crpV (SEQ ID NO:24) can be at least 118 nucleotides in length (e.g., 118, 235, 366, 440, 521, 636, 783, 852, 918, 1044, 1168, 1238, 1350, 1448, 1569, 1722, 1838, 1924, 2052, 2167, 2288, or 2354); nucleic acid fragments of crpX (SEQ ID NO:26) can be at least 60 nucleotides in length (e.g., 60, 98, 137, 182, 214, 278, 308, 357, or 398); nucleic acid fragments of crpY (SEQ ID NO:28) can be at least 32 nucleotides in length (e.g., 32, 74, 121, 169, 204, 263, 298, 355, 391, 426, 484, 523, 577, 624, or 644); and nucleic acid fragments of crpZ (SEQ ID NO:30) can be at least 27 nucleotides in length (e.g., 27, 68, 103, 158, 193, 243, or 272). Based on contemporaneous public database searches, such fragments appear not to have more than 85% sequence identify to sequences in the public databases.
As used herein, an “isolated” nucleic acid molecule is a nucleic acid molecule that is separated from other nucleic acid molecules that are usually associated with the reference nucleic acid molecule in the genome. Thus, an “isolated” nucleic acid molecule includes, without limitation, a nucleic acid molecule that is free of sequences that naturally flank one or both ends of the nucleic acid in the genome of the organism from which the isolated nucleic acid molecule is derived (e.g., a cDNA or genomic DNA fragment produced by PCR or restriction endonuclease digestion). Such an isolated nucleic acid molecule is generally introduced into a vector (e.g., a cloning vector, or an expression vector) for convenience of manipulation or to generate a fusion nucleic acid molecule. In addition, an isolated nucleic acid molecule can include an engineered nucleic acid molecule such as a recombinant or a synthetic nucleic acid molecule. A nucleic acid molecule existing among hundreds to millions of other nucleic acid molecules within, for example, a nucleic acid library (e.g., a cDNA, or genomic library) or a portion of a gel (e.g., agarose, or polyacrylamine) containing restriction-digested genomic DNA is not to be considered an isolated nucleic acid.
Isolated nucleic acid molecules of the invention can be obtained using techniques routine in the art. For example, isolated nucleic acids within the scope of the invention can be obtained using any method including, without limitation, recombinant nucleic acid technology, and/or the polymerase chain reaction (PCR). General PCR techniques are described, for example in PCR Primer: A Laboratory Manual, Dieffenbach & Dveksler, Eds., Cold Spring Harbor Laboratory Press, 1995. Recombinant nucleic acid techniques include, for example, restriction enzyme digestion and ligation, which can be used to isolate a nucleic acid molecule of the invention. Isolated nucleic acids of the invention also can be chemically synthesized, either as a single nucleic acid molecule or as a series of oligonucleotides. In addition, isolated nucleic acid molecules of the invention also can be obtained by mutagenesis. For example, an isolated nucleic acid that shares identity with an art known sequence can be mutated using common molecular cloning techniques (e.g., site-directed mutagenesis). Possible mutations include, without limitation, deletions, insertions, substitutions, and combinations thereof.
Vectors containing nucleic acid molecules that encode polypeptides involved in cryptophycin synthesis also are provided by the invention. Vectors, including expression vectors, suitable for use in the present invention are commercially available and/or produced by recombinant DNA technology methods routine in the art. A vector containing a nucleic acid molecule of the invention can have elements necessary for expression operably linked to such a nucleic acid molecule, and further can include sequences such as those encoding a selectable marker (e.g., an antibiotic resistance gene), and/or those that can be used in purification of a polypeptide involved in cryptophycin synthesis (e.g., 6×His tag).
Elements necessary for expression include nucleic acid sequences that direct and regulate expression of nucleic acid coding sequences. One example of an element necessary for expression is a promoter sequence. Elements necessary for expression also can include introns, enhancer sequences, response elements, or inducible elements that modulate expression of a nucleic acid molecule of the invention. Elements necessary for expression can be of bacterial, yeast, insect, mammalian, or viral origin and vectors can contain a combination of elements from different origins. Elements necessary for expression are described, for example, in Goeddel, 1990, Gene Expression Technology: Methods in Enzymology, 185, Academic Press, San Diego, Calif. As used herein, operably linked means that a promoter and/or other regulatory element(s) are positioned in a vector relative to a nucleic acid molecule of the invention in such a way as to direct or regulate expression of the nucleic acid molecule. Many methods for introducing nucleic acids into host cells, both in vivo and in vitro, are well known to those skilled in the art and include, without limitation, calcium phosphate precipitation, electroporation, heat shock, lipofection, microinjection, and viral-mediated nucleic acid transfer.
Another aspect of the invention pertains to host cells into which a vector of the invention, e.g., an expression vector, or an isolated nucleic acid molecule of the invention has been introduced. The term “host cell” refers not only to the particular cell but also to the progeny or potential progeny of such a cell. A host cell can be any prokaryotic or eukaryotic cell. For example, nucleic acid molecules of the invention can be expressed in bacterial cells such as E. coli, or in insect cells, yeast or mammalian cells (such as Chinese hamster ovary cells (CHO) or COS cells). Other suitable host cells are known to those skilled in the art.
Conditions for amplification of a nucleic acid and detection of an amplification product are known to those of skill in the art (see, e.g., PCR Primer: A Laboratory Manual, 1995, Dieffenbach & Dveksler, Eds., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.; and U.S. Pat. Nos. 4,683,195; 4,683,202; 4,800,159; and 4,965,188). Modifications to the original PCR also have been developed. For example, anchor PCR, RACE PCR, or ligation chain reaction (LCR) are additional PCR methods known in the art (see, e.g., Landegran et al., 1988, Science, 241:1077-1080; and Nakazawa et al., 1994, Proc. Natl. Acad. Sci. USA, 91:360-364).
Hybridization between nucleic acid molecules is discussed in detail in Sambrook et al. (1989, Molecular Cloning: A Laboratory Manual, 2nd Ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.; Sections 7.37-7.57, 9.47-9.57, 11.7-11.8, and 11.45-11.57). For oligonucleotide probes less than about 100 nucleotides, Sambrook et al. discloses suitable Southern blot conditions in Sections 11.45-11.46. The Tm between a sequence that is less than 100 nucleotides in length and a second sequence can be calculated using the formula provided in Section 11.46. Sambrook et al. additionally discloses prehybridization and hybridization conditions for a Southern blot that uses oligonucleotide probes greater than about 100 nucleotides (see Sections 9.47-9.52). Hybridizations with an oligonucleotide greater than 100 nucleotides generally are performed 15-25° C. below the Tm. The Tm between a sequence greater than 100 nucleotides in length and a second sequence can be calculated using the formula provided in Sections 9.50-9.51 of Sambrook et al. Additionally, Sambrook et al. recommends the conditions indicated in Section 9.54 for washing a Southern blot that has been probed with an oligonucleotide greater than about 100 nucleotides.
The conditions under which membranes containing nucleic acids are prehybridized and hybridized, as well as the conditions under which membranes containing nucleic acids are washed to remove excess and non-specifically bound probe can play a significant role in the stringency of the hybridization. Such hybridizations and washes can be performed, where appropriate, under moderate or high stringency conditions. Such conditions are described, for example, in Sambrook et al. section 11.45-11.46. For example, washing conditions can be made more stringent by decreasing the salt concentration in the wash solutions and/or by increasing the temperature at which the washes are performed. In addition, interpreting the amount of hybridization can be affected, for example, by the specific activity of the labeled oligonucleotide probe, by the number of probe-binding sites on the template nucleic acid to which the probe has hybridized, and by the amount of exposure of an autoradiograph or other detection medium.
It will be readily appreciated by those of ordinary skill in the art that although any number of hybridization and washing conditions can be used to examine hybridization of a probe nucleic acid molecule to immobilized target nucleic acids, it is more important to examine hybridization of a probe to target nucleic acids under identical hybridization, washing, and exposure conditions. Preferably, the target nucleic acids are on the same membrane.
A nucleic acid molecule is deemed to hybridize to a nucleic acid of the invention but not to another nucleic acid if hybridization to a nucleic acid of the invention is at least 5-fold (e.g., at least 6-fold, 7-fold, 8-fold, 9-fold, 10-fold, 20-fold, 50-fold, or 100-fold) greater than hybridization to another nucleic acid. The amount of hybridization can be quantitated directly on a membrane or from an autoradiograph using, for example, a PhosphorImager or a Densitometer (Molecular Dynamics, Sunnyvale, Calif.).
Detection of an amplification product or a hybridization complex is usually accomplished using detectable labels. The term “labeled” with regard to an agent (e.g., an oligonucleotide or a polypeptide) is intended to encompass direct labeling of the agent by coupling (i.e., physically linking) a detectable substance to the agent, as well as indirect labeling of the agent by reactivity with another reagent that is directly labeled with a detectable substance. Detectable substances include various enzymes, prosthetic groups, fluorescent materials, luminescent materials, bioluminescent materials, and radioactive materials.
Polypeptides
One aspect of the invention pertains to purified polypeptides involved in cryptophycin synthesis as well as polypeptide fragments, particularly those that possess enzymatic activity (i.e., functional fragments). Predicted amino acid sequences of polypeptides involved in cryptophycin synthesis are shown in SEQ ID NOs:3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 43 and 45.
The term “purified” polypeptide as used herein refers to a polypeptide that has been separated or purified from cellular components that naturally accompany it. Typically, the polypeptide is considered “purified” when it is at least 70% (e.g., at least 75%, 80%, 85%, 90%, 95%, or 99%) by dry weight, free from the proteins and naturally occurring molecules with which it is naturally associated. Since a polypeptide that is chemically synthesized is, by nature, separated from the components that naturally accompany it, a synthetic polypeptide is “purified.”
Polypeptides involved in cryptophycin synthesis can be purified from natural sources (e.g., a biological sample) by known methods such as DEAE ion exchange, gel filtration, and hydroxyapatite chromatography. A purified polypeptide also can be obtained, for example, by expressing a nucleic acid molecule of the invention in an expression vector. In addition, a purified polypeptide can be obtained by chemical synthesis. The extent of purity of a polypeptide can be measured using any appropriate method, e.g., column chromatography, polyacrylamide gel electrophoresis, or HPLC analysis.
In addition to the naturally-occurring polypeptides involved in cryptophycin biosynthesis, the skilled artisan will further appreciate that changes can be introduced into a nucleic acid molecule (e.g., those having the sequence shown in SEQ ID NOs: 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 234, 26, 28, 30, 42 and 44) as discussed herein, thereby leading to changes in the amino acid sequence of the encoded polypeptide. For example, changes can be introduced into nucleic acid coding sequences leading to conservative and/or non-conservative amino acid substitutions at one or more amino acid residues. A “conservative amino acid substitution” is one in which one amino acid residue is replaced with a different amino acid residue having a similar side chain. Similarity between amino acid residues has been assessed in the art. For example, Dayhoff et al. (1978, in Atlas of Protein Sequence and Structure, 5(Suppl. 3):345-352) provides frequency tables for amino acid substitutions that can be employed as a measure of amino acid similarity. A non-conservative substitution is one in which an amino acid residue is replaced with an amino acid residue that does not have a similar side chain.
The invention also provides for chimeric or fusion polypeptides. As used herein, a “chimeric” or “fusion” polypeptide includes a polypeptide involved in cryptophycin synthesis operatively linked to a heterologous polypeptide. A heterologous polypeptide can be at either the N-terminus or C-terminus of a polypeptide involved in cryptophycin synthesis. Within a chimeric or fusion polypeptide, the term “operatively linked” is intended to indicate that the two polypeptides are encoded in-frame relative to one another. In a fusion polypeptide, the heterologous polypeptide generally has a desired property such as the ability to purify the fusion polypeptide (e.g., by affinity purification). A chimeric or fusion polypeptide of the invention can be produced by standard recombinant DNA techniques, and can use commercially available vectors.
A polypeptide commonly used in a fusion polypeptide for purification is glutathione S-transferase (GST), although numerous other polypeptides are available and can be used. In addition, a proteolytic cleavage site can be introduced at the junction between a polypeptide and a heterologous polypeptide to enable separation of the two polypeptides subsequent to purification of the fusion polypeptide. Enzymes that cleave such proteolytic sites include Factor Xa, thrombin, or enterokinase. Representative expression vectors encoding a heterologous polypeptide that can be used in affinity purification of a polypeptide involved in cryptophycin synthesis include pGEX (Pharmacia Biotech Inc; Smith & Johnson, 1988, Gene, 67:31-40), pMAL (New England Biolabs, Beverly, Mass.) and pRIT5 (Pharmacia, Piscataway, N.J.).
Antibodies can be used to detect the presence or absence of polypeptides involved in cryptophycin synthesis. Techniques for detecting polypeptides using antibodies include enzyme linked immunosorbent assays (ELISAs), Western blots, immunoprecipitations and immunofluorescence. An antibody can be polyclonal or monoclonal, and usually is detectably labeled. An antibody having specific binding affinity for a polypeptide involved in cryptophycin synthesis can be generated using methods well known in the art. The antibody can be attached to a solid support such as a microtiter plate using methods known in the art (see, for example, Leahy et al., 1992, BioTechniques, 13:738-743). In the presence of a polypeptide involved in cryptophycin synthesis, an antibody-polypeptide complex is formed.
Detection of a polypeptide-antibody complex is usually accomplished by detectably labeling the antibody. The term “labeled” with regard to an antibody is intended to encompass direct labeling of the antibody by coupling (i.e., physically linking) a detectable substance to the antibody, as well as indirect labeling of the antibody by reactivity with another reagent that is directly labeled with a detectable substance. Detectable substances are described above.
Biosynthesis of Cryptophycin
FIG. 3 shows the modular structure of cryptophycins. Cryptophycin biosynthesis is a result of a mixed Type I PKS/NRPS system.
Unit A is a polyketide synthetase derived unit. Incorporation and linkage of unnatural amino acids such as chlorinated methoxy D-tyrosine amino acid (Unit B) and β-methyl β-alanine (Unit C) are consistent with activities of non-ribosomal peptide synthetase domains. The final terminating unit, the rare carboxylic acid of leucine, leucic acid, could be the result of a NRPS system. However, the ester linkage between Unit C and D is not consistent with a peptide bond forming condensation domain of such a system. It is possible that incorporation of this ester occurs by a novel domain as part of a larger NRPS system. Alternatively, incorporation of the ester may be directed by an enzyme that previously has not been described. The generation of the macrocycle to form the core cryptophycin chemical skeleton involves a chain-terminating cyclization step, likely completed by a member of the hydrolase superfamily of enzymes or domains. The lactone formed between Unit A (the hydroxyl group) and Unit D points to a classic thioesterase dependent mechanism. Additional enzymes such as a cytochrome p450-dependent hydroxylase (likely a cryptophycin epoxidase), a non-heme dependent halogenase, o-methyltransferase, and enzymes involved in activation and methylation of the β-carbon of 3-amino propanoic acid are involved in Unit A, B, C, or D synthesis or in final structural components of cryptophycin. Many of these types of enzymes have been previously described from other polyketide and nonribosomal peptide synthetases. For an overview of the predicted pathway of cryptophycin biosynthesis, see FIG. 8. See also, FIG. 2.
Polyketide Synthetase
Based on homology searches of the GenBank database, the nucleotide sequences designated crpA (SEQ ID NO:42) and crpB (SEQ ID NO:44) appear to encode PKSs (CrpA, SEQ ID NO:43; CrpB, SEQ ID NO:45). With respect to SEQ ID NO:2, a portion of which has homology to SEQ ID NO:42, sequence analysis indicated that SEQ ID NO:2 contains a PKS domain (positioned at approximately nucleotides 1-450 of SEQ ID NO:2), an acyltransferase domain (positioned at approximately nucleotides 1-220 of SEQ ID NO:2), a dehydrogenase domain (positioned at approximately nucleotides 760-1000 or 860-1000 of SEQ ID NO:2), a ketoreductase domain (positioned at approximately nucleotides 850-1000 of SEQ ID NO:2), and an acyl carrier protein domain.
Polyketides are diverse biologically active molecules with a wide variety of structures. Polyketides are synthesized from 2-carbon units through a series of condensations and subsequent modifications, and occur in many types of organisms including fungi and mycelial bacteria. Polyketide synthetases (PKSs) catalyze the biosynthesis of polyketides through repeated, decarboxylative Claisen condensations between acylthioester building blocks. The building blocks used to form complex polyketides are typically acylthioesters such as acetyl, butyryl, propionyl, malonyl, hydroxymalonyl, methylmalonyl, and ethylmalonyl CoA.
The sequencing of several genes encoding enzymes that produce type 1 modular PKSs has revealed a linear organization of modules, each of which contains the activities needed for one cycle of polyketide chain elongation. The minimal module contains a ketosynthase (KS), an acyltransferase (AT), and an acyl carrier protein (ACP) that together catalyze a 2-carbon extension of the chain similar to the condensation of 2-carbon units in the biosynthesis of fatty acids. In PKS polypeptides, the regions that encode enzymatic activities are separated by linker regions, also called scaffold regions. These scaffold regions encode amino acid sequences that space the enzymatic activities at the appropriate distances and in the correct order.
PKS is likely responsible for synthesis of the Unit A region, which is one of the most challenging aspects in the chemical synthesis of cryptophycins. The Unit A portion of the molecule is a dioxadiazacyclo, hexadecenetetrone moiety and represents the beginning polyketide unit (FIG. 3).
Non-Ribosomal Peptide Synthetase
Based on homology searches of the GenBank database, the nucleotide sequence designated crpC (SEQ ID NO:4) appears to encode a non-ribosomal peptide synthetase (NRPS) (CrpC; SEQ ID NO:5) involved in production of the Unit B peptide portion of cryptophycin. Sequence analysis indicated that CrpC may contain one or more NRPS domains (positioned at approximately nucleotides 300-950 and 1290-1425 of SEQ ID NO:4), one or more condensation domains (positioned at approximately nucleotides 50-350 and 1475-1780 of SEQ ID NO:4), an adenylation domain, an o-methyltransferase domain (positioned at approximately nucleotides 1000-1200 of SEQ ID NO:4), one or more peptidyl carrier protein domains, an epimerase domain, and one or more acyl CoA synthetase (positioned at approximately nucleotides 525-1000 of SEQ ID NO:4).
Based on homology searches of the GenBank database, the nucleotide sequence designated crpD (SEQ ID NO:6) appears to encode a NRPS (Crp D; SEQ ID NO:7) involved in production of the Units C and D peptide portions of cryptophycin. CrpD also apparently generates a 16-membered peptolide ring during cryptophycin biosynthesis. Sequence analysis indicated that CrpD contains one or more NRPS domains (positioned at approximately nucleotides 250-975, 1350-1600, 1850-2300, and 2950-3100 of SEQ ID NO:6), one or more condensation domains (positioned at approximately nucleotides 1-300 and 1150-1450 of SEQ ID NO:6), an adenylation domain, one or more peptidyl carrier protein domains, one or more acyl CoA ligase domains (positioned at approximately nucleotides 500-1000 and 1900-2400 of SEQ ID NO:6), one or more acyl CoA synthetase domains (positioned at approximately nucleotides 475-1000 and 1900-2400 of SEQ ID NO:6), and a thioesterase domain.
NRPSs are modular in nature, where a module is usually defined as a segment of the NRPS necessary to catalyze the activation of a specific amino acid and result in the incorporation of that amino acid into a non-ribosomal peptide. A minimal module typically contains three domains: (1) an adenylation domain (about 60 kDa) responsible for selecting and activating an amino acid and transferring the aminoacyl adenylate to a peptidyl carrying center; (2) a thiolation domain, also referred to as a peptidyl carrier protein (8-10 kDa), containing a serine residue that is post-translationally modified with a 4-phosphopantetheine group (Ppant) and acts as an acceptor for the aminoacyl adenylate; and (3) a condensation domain (50-60 kDa), which catalyzes peptide bond-forming chain-translocating steps between an upstream peptidyl-s-Ppant and the downstream aminoacyl-Ppant of the adjacent module. This minimal module for chain extension is typically repeated within a NRPS. A co-linear relationship exists between the number of modules present and the number of amino acids in the final product, with the order of the modules in the synthetase determining the order of the amino acids in the peptide.
Thioesterase Domain
Based on homology searches of the GenBank database, a thioesterase domain is positioned at approximately nucleotide 9,199 to nucleotide 10,032 of CrpD (SEQ ID NO:6).
The cryptophycin thioesterase is likely responsible for the cyclization and release of the cryptophycins from the phosphopantethienyl group of the C-terminal phosphopantethienyl carrier protein (PCP) of a NRPS. The synthetic methods used for ring closure of cryptophycin thus far limit the scope and ease of derivatization of cyptophycins.
The utility of thioesterase domains as semi-synthetic tools for cyclization of synthetic molecules has been demonstrated for gramicidin, epothilone C, and tyrocidine semi-synthesis. See, for example, Wu et al., 2003, Org. Lett., 5:1749; Kohli et al. 2003, J. Am. Chem. Soc., 125:7160; Kohli et al., 2002, Nature, 418:658; and Boddy et al., 2003, J. Am. Chem. Soc., 125:3428. Use of the cryptophycin thioesterase for semi-synthesis of cryptophycin provides a new route to synthesis of cryptophycin and its analogues that allows for rapid generation in diversity throughout the entire cryptophycin molecule. Use of a thioesterase domain of the invention to cyclize a cryptophycin chain elongation intermediate (e.g., a seco-SNAC-cryptophycin thioester) provides an approach for generating novel cryptophycins.
Cytochrome p450
Based on homology searches of the GenBank database, the crpE nucleic acid sequence (SEQ ID NO:8) appears to encode a cytochrome p450 (CrpE; SEQ ID NO:9), which is likely an epoxidase involved in cryptophycin biosynthesis.
A survey of the structure-activity relationship of cryptophycins has demonstrated the necessity of the epoxide for high-level tubulin depolymerization and anti-proliferative activities toward tumor cells. Opening of the epoxide, however, is one of the major problems encountered in clinical uses of cryptophycins. A new generation of compounds has been synthesized containing a chlorohydrin. Chlorohydrin analogs are generated from cryptophycins containing an epoxide, and act as pro-drugs. Once chlorohydrins are injected into the serum, the compounds are rapidly converted back to the corresponding epoxides.
High-level tubulin depolymerization and anti-proliferative activities toward tumor cells also requires proper stereochemistry of the epoxide group (β epoxide). Synthesis of cryptophycins containing an epoxide often results in a mixture of two diastereomers. One of the diastereomers is usually inactive, thereby requiring reverse-phase HPLC to separate the two compounds. See, for example, FIG. 2. In addition to the extra expense and time required for separation, separation of the diastereomers results in a significant loss of starting material.
Using a recombinant cell line expressing an epoxidase or a purified form of an epoxidase could dramatically increase overall yields, eliminate a separation step (e.g., HPLC), and allow more flexibility in synthetic strategies. Since no known natural cryptophycin contains the α-epoxide, the native epoxidase enzyme seems to be highly efficient at generating the desired epoxide diastereomer (see FIG. 9). Further, the cryptophycin epoxidase apparently exhibits a high degree of flexibility since it is able to use various substrates (e.g., those having different ring sizes).
Additional Enzymes
Additional enzymes having a variety of functions are involved in cryptophycin biosynthesis. In addition to the PKS, the NRPS, and the epoxidase discussed above, sequence analysis indicated that the following types of enzymes are likely involved in cryptophycin biosynthesis.
Based on homology searches of the GenBank database, crpF (SEQ ID NO:10) appears to encode an iron-dependent non-heme hydroxylase (CrpF; SEQ ID NO:11), which is a member of the γ-butyrobetaine hydroxylase group. Non-heme iron-dependent enzymes generally catalyze a wide variety of O2 reactions. An iron-dependent non-heme hydroxylase is likely involved in hydroxylation of cryptophycins.
Based on homology searches of the GenBank database, crpG (SEQ ID NO:12) appears to encode an aspartate decarboxylase (CrpG; SEQ ID NO:13). An aspartate decarboxylase (EC 4.1.1.1.1) is likely involved in production of β-alanine or methyl-β-alanine, which is a precursor for NRPS. See, for example, Williamson & Brown, 1979, J. Biol. Chem., 254:8074-82; and Ramjee et al., 1997, Biochem. J, 323:661-9.
Based on homology searches of the GenBank database, crpI appears to be the remnants of an IS1327 transposition event. The sequences identified as having homology to IS1327 are positioned at approximately nucleotides 9154-8514 of SEQ ID NO:1 (pDAM163). No open reading frame or coding sequences, however, were identified.
Based on homology searches of the GenBank database, crpH (SEQ ID NO:14) appears to encode a non-heme-dependent, flavin-dependent halogenase (CrpH; SEQ ID NO:15). See, for example, van Pee & Unversucht, 2003, Chemosphere, 52:299-312; and Littlechild, 1999, Curr. Opin. Chem. Biol., 3:28-34. A halogenase is likely involved in chlorination of the Unit B amino acid, o-methyl tyrosine.
Based on homology searches of the GenBank database, crpK appears to be the remnants of an IS892-orf2 transposition event. The sequences identified as having homology to IS892-orf2 are positioned at approximately nucleotides 4730-7039 of SEQ ID NO:1 (pDAM163). No open reading frame or coding sequences, however, were identified.
Based on homology searches of the GenBank database, crpL appears to be the remnants of an IS892-orf1 transposition event. The sequences identified as having homology to IS892-orf2 are positioned at approximately nucleotides 4730-7039 of SEQ ID NO:1 (pDAM163). No open reading frame or coding sequences, however, were identified.
Based on homology searches of the GenBank database, crpM appears to be an ISRSO13 transposase sequence. The identified coding sequence (crpM; SEQ ID NO:16) encodes a polypeptide designated CrpM (SEQ ID NO:17) with unknown function.
Based on homology searches of the GenBank database, crpN (SEQ ID NO:18) appears to encode a non-heme-dependent, iron-dependent hydroxylase (CrpN, SEQ ID NO:19). See, for example, Solomon et al., 2003, PNAS USA, 100:3589-94; and Ryle et al., PNAS USA, 100:3790-5.
Based on homology searches of the GenBank database, crpO appears to encode a pvdE-type regulator (CrpO). The sequences identified as having homology to a pvdE-type regulator are positioned at approximately nucleotides 786-1768 of SEQ ID NO:1 (pDAM163). A pvdE-type regulator is likely involved in regulating cryptophycin biosynthesis. See, for example, Wilson et al., 2001, J. Bacteriol., 183:2151-5.
Based on homology searches of the GenBank database, crpP (SEQ ID NO:20) appears to encode a thioredoxin (CrpP, SEQ ID NO:21). Thioredoxins are generally reduction/oxidation (redox)-regulatory proteins thought to have anti-apoptotic effects. Thioredoxin is likely involved in redox reactions (e.g., cytochrome p450-dependent hydroxylations) associated with cryptophycin biosynthesis.
Based on homology searches of the GenBank database, crpU (SEQ ID NO:22) appears to encode an N-acetyltransferase (EC 2.3.1.5) (CrpU, SEQ ID NO:23). N-acetyltransferases usually catalyze the transfer of acetyl groups from acetyl-CoA to arylamines.
Based on homology searches of the GenBank database, crpV (SEQ ID NO:24) appears to encode a large exoprotein involved in heme utilization (CrpV, SEQ ID NO:25). A large exoprotein involved in heme utilization may be involved in redox reactions associated with cryptophycin formation (i.e., cytochrome p450-dependent hydroxylations).
Based on homology searches of the GenBank database, crpW appears to encode a tetratricopeptide repeat (TPR) protein (CrpW). A TPR is a 34 amino acid repeated sequence motif found in a number of diverse proteins that may be involved in transcriptional repression, mitochondrial and/or peroxisomal protein transport, cell cycle regulation, protein kinase inhibition, heat shock response, and/or mediating protein-protein interactions. See, for example, Sikorski et al., 1991, Cold Spring Harbor Symp. Quant. Biol., 56:663-73; and Lamb et al., 1995, Trends Biosci., 20:257-9.
Based on homology searches of the GenBank database, crpX (SEQ ID NO:26) appears to encode a chorismate mutase-prephenate dehydrogenase (CrpX, SEQ ID NO:27). A chorismate mutase-prephenate dehydrogenase (EC 1.3.1.12) usually catalyzes the first two steps in the biosynthesis of tyrosine (the chorismate mutase activity) and the conversion of prephenate to p-hydroxyphenylpyruvate in the presence of NAD (the prephenate dehydrogenase activity). A chorismate mutase-prephenate dehydrogenase is likely involved in the production of shikimate-derived PKS starter units in cryptophycin biosynthesis.
Based on homology searches of the GenBank database, crpY (SEQ ID NO:28) appears to encode a 2-hydroxychromene-2-carboxylate isomerase (CrpY, SEQ ID NO:29). A 2-hydroxychromene-2-carboxylate isomerase is involved in the naphthalene catabolic pathway and catalyzes the reaction of 2-hydroxychromene-2-carboxylate into trans-o-hydroxybenzylidenepyruvate. See, for example, Eaton, 1994, J. Bacteriol., 176:7757-62; and Zylstra et al., 1997, FEMS Microbiol. Lett., 153:479-84. A 2-hydroxychromene-2-carboxylate isomerase is likely involved in the production of shikimate-derived PKS starter units in cryptophycin biosynthesis.
Based on homology searches of the GenBank database, crpZ (SEQ ID NO:30) appears to encode a 3-dehydroquinate synthase (CrpZ, SEQ ID NO:31). A 3-dehydroquinate synthase (EC 4.2.3.4) usually catalyzes the cyclization of 3-deoxy-D-arabino-heptulosonic acid 7-phosphate (DAHP) to dehydroquinate. A 3-dehydroquinate synthase may be involved in the production of shikimate-derived PKS starter units.
Combinatorial Techniques and Domain Swapping
It will be apparent to one of skill in the art that any number and/or combination of nucleic acid molecules of the invention (e.g., SEQ ID NOs:2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 42 and/or 44) can be joined together to generate a longer nucleic acid molecule (e.g., pDAM163; shown in FIGS. 4 and 5 and SEQ ID NO:1). In addition, the nucleic acid molecules (SEQ ID NOs:2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 42 and 44) can be manipulated using standard techniques to delete or inactivate activity encoding regions, insert regions from different molecules encoding corresponding activities from the same or different biosynthesis systems, or be otherwise mutated using standard procedures for obtaining genetic alterations. Mutations can be made to the native sequences using conventional techniques such as those described above.
Chemical approaches have lead to highly informative structure-activity relationships. Therefore, the regions suggested for modifications are well defined, particularly in view of the modular-type structure of the PKSs and NRPSs. In addition to approaches that provide mutated polypeptides, it is possible to manipulate entire domains or portions of domains. For example, a domain having a particular activity from one biosynthetic pathway can be exchanged or replaced with a domain having a corresponding activity from a different biosynthetic pathway. Alternatively, a domain having a particular activity from a biosynthetic pathway can be exchanged or replaced with a domain having an unrelated activity from the same or a different biosynthetic pathway.
If replacement of a particular nucleic acid region encoding a host enzyme is to be made, this replacement can be conducted in vitro using suitable restriction enzymes and cloning techniques or can be effected in vivo using recombinant techniques involving homologous sequences framing the replacement region in a donor plasmid and a receptor region in a recipient plasmid. A representative exchange system that involves plasmids that have different temperature sensitivities is described in PCT Publication No. WO 96/40968.
The various nucleic acid molecules involved in cryptophycin biosynthesis, individually or as a cocktail of such molecules, can be cloned into one or more recombinant vectors. When more than one molecule is cloned together, such elements can be under the control of a single element for expression (e.g., a promoter) or each molecule can be under the control of an element for expression. The nucleotide sequences encoding an enzymatic subunit or a cocktail of such molecules can include flanking restriction sites to allow for the easy deletion and insertion of other molecules or regions of a molecule. In this manner, nucleotide sequences encoding hybrid or chimeric enzymes can be generated. The design of such unique restriction sites is known to those of skill in the art and can be accomplished using the techniques described above such as site-directed mutagenesis and PCR.
Expression vectors containing nucleotide sequences encoding a variety of enzymatic activities can be transformed into an appropriate host cell to construct a library. In one approach, a mixture of such vectors is transformed into host cells and the resulting cells plated into individual colonies and selected for successful transformants. Each individual colony represents a colony expressing an enzyme having a particular activity and, ultimately, the ability to produce a particular product. Alternatively, expression vectors can be used individually to transform host cells, which are then assembled into a library. Methods are known for screening a library or isolates from a library for substrate-specificity and/or production of a particular product. Another strategy for preparing a variety of products is by random digestion-religation leading to chimeric domains or modules. A similar such method has been described as a “DNA shuffling method” (see Patten et al., 1997, Curr Op. Biotechnol., 8: 724-733).
As one non-limiting example, the creation of novel macrolides can be achieved through genetic manipulation of polyketide synthetases. The module nature of polyketide synthetases allows for domain exchange between different polyketide synthetase genes, resulting in hybrid genes that produce polyketide synthetases with altered properties that, in turn, produce modified macrolide structures. Thus, it is possible to control chain length, choice of chain extender unit, degree of β-carbon oxidation level, and stereochemistry. See, for example, PCT publication Nos. WO 93/13663; WO 95/08548; WO 97/02358; WO 98/27203; and WO 98/49315; U.S. Pat. Nos. 4,874,748; 5,063,155; 5,098,837; 5,149,639; 5,672,491; 5,712,146; 5,830,750; and 5,843,718; and Fu et al., 1994, Biochemistry, 33:9321-9326; McDaniel et al., 1993, Science, 262:1546-1550; and Rohr, 1995, Angew. Chem. Int. Ed. Engl., 34(8):881-888.
The application of innovative combinatorial techniques to this type of genetic organization has prompted the generation of novel natural products, by adding, deleting, or exchanging domains or entire modules. See, for example, U.S. Pat. Nos. 5,672,491; 5,712,146; 5,830,750; 5,843,718; 5,962,290; and 6,022,731; and Tang et al., 2000, Science, 287:640-2). The invention allows for combinatorial biosynthesis technology to produce a diversity of cryptophycin analogues in addition to those cryptophycin analogues produced to date.
The invention will be further described in the following examples, which do not limit the scope of the invention described in the claims.
EXAMPLES
Example 1
Cloning and Sequencing the crp Gene Cluster Contained within pDAM163
Primer synthesis and cosmid sequencing was preformed at the University of Minnesota Advanced Genetic Sequencing and Analysis Center-AGAC (St. Paul, Minn.). Degenerate PCR primers specific for conserved core motifs of peptide synthetase adenylation domains A2 and A8 (Marahiel et al., 1997, Chem. Rev., 97:2651-74) were used and consisted of the following sequences: MTF2′ forward primer (5′-GCNGG (ct) GG (ct) GCNTA (ct) GTNCC-3′ (SEQ ID NO:38)) and MTR reverse primer (5′-CCNGG (agt) AT (tc) TTNAC (tc) TG-3′ (SEQ ID NO:39)) (Neilan et al., 1999, J. Bacteriol., 181.4089-97). Adenylation domain containing DNA fragments of approximately 1100 bp in length were synthesized by PCR using a Hybaid Express PCR thermocycler (30 cycles: 95° C. for 1 min, 55° C. for 1 min, 72° C. for 1 min) with Nostoc sp ATCC 53789 genomic DNA as a template. End sequencing of one fragment, pNAM124, using an Applied Biosystems, Inc. ABI3700 sequencer (Foster City, Calif.) confirmed that the fragment contained an adenylation domain. Prediction of its substrate specificity (aromatic amino acid activating) was determined using methods described previously (Challis et al., 2000, Chem. Biol., 7:211-24). The fragment was radiolabeled using the RadPrime labeling kit (Pharmacia) with [α-32P] dCTP (Amersham) according to the manufacturer's directions. The radiolabeled fragment was used to probe the genomic library using standard colony hybridization protocols (Sambrook & Russell, 2000, Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory Press). One cosmid, pDAM163, was selected because it hybridized to the adenylation domain encoding DNA probe contained within pNAM124. The DNA sequence of pDAM163 was obtained by creating a shotgun library of the cosmid within the sequencing vector, pUC18. Sequences obtained were assembled using SeqMan version 5.06 (DNAStar, Madison, Wis.) and Frameplot 2.3.2 (Ishikawa & Hotta, 1999, FEMS Microbiol Lett., 174:251-3) used to identify individual open reading frames. The putative functions of the crp biosynthesis genes were assessed by using the open reading frames and their putative protein products versus genes/proteins contained within the GenBank database using BlastN and BlastP.
Example 2
Cloning Genes Involved in Cryptophycin Biosynthesis
DNA encoding a putative cryptophycin biosynthetic gene cluster was contained on a cosmid designated pDAM163. pDAM163 DNA was prepared using a Qiagen large construct DNA extraction kit from a 500 mL culture grown overnight at 25° C. in LB media containing 50 μg/mL ampicillin.
Example 3
Cryptophycin Production
The cosmid, pDAM163, or sub-vectors such as cosmid, plasmids, yeast artificial chromosomes, bacterial artificial chromosomes, or phage vectors containing pDAM163 sequences can be used to biosynthetically prepare cryptophycins in a non-Nostoc spp. host. pDAM163 is introduced into an Escherichia coli strain that harbors a phosphopantetheinyl transferase gene required for expressing active polyketide synthase and nonribosomal peptide synthetase enzymes. Fermentation of the resulting strain on a large scale, and extracting and detecting cryptophycins are performed as described previously (Subbaraju et al., 1997, J. Nat. Prod., 60:302-5; and Golakoti et al., 1994, J. Am. Chem. Soc., 116:4729-37).
Example 4
Cloning Strategy of the Thioesterase Domain
DNA encoding the cryptophycin thioesterase domain is contained at the 3′-end of the 3′-terminal open reading frame of CrpC, which also codes for domains necessary for incorporation of units C and D of cryptophycin. Therefore, truncation of the DNA in the final ORF was necessary in order to isolate the cryptophycin thioesterase. Identification of the DNA encoding the cryptophycin thioesterase was elucidated through use of the NCBI “CDART” program for identification of conserved domains. The “nnpredict” secondary structure prediction program (Kneller et al., 1990, J. Mol. Biol., 214:171) was used to determine the putative secondary structure of the gene product of the putative thioesterase domain and a domain capable of being phosphopantetheinlyated. The forward primer, 5′-ATT TAT CAT ATG GGT TCC GAT TCC GGA GCC GA-3′ (SEQ ID NO:40), was designed to a position immediately 3′ of a nucleic acid sequence predicted to encode a protein capable of being phosphopantethienylated in a region appearing to lack secondary structure based on the “nnpredict” program results and contained an NdeI restriction site. The reverse primer, 5′-AAA TAA GAA TCC TCA TCA TTT TTC CAA TTG ATG GGT-3′ (SEQ ID NO:41), was constructed to anneal to the 3′ end of the open reading frame and contained a BamHI restriction site.
PCR reactions were performed with 0.1 μL of pDAM163 DNA from the extraction, 1 μM forward primer, 1 μM reverse primer, 1×ExTaq buffer (Takara), 1 μL ExTaq polymerase (Takara), and 1 μM dNTP (Takara) to a final volume of 50 μL with water. The PCR program consisted of 30 cycles of the following amplification conditions: denaturation 1 min at 95° C., 1 min annealing at 50° C., 1.5 min extension at 72° C. PCR fragments corresponding to the desired length were separated on a 1% agarose gel and purified from the gel using a Qiagen gel extraction kit. The PCR fragment was cloned into a pGEM T-Easy vector (Promega) using T-overhang cloning with the pGEM T-Easy kit (Promega).
Clones were transformed into XL-1 Blue competent cells using heat shock protocols as described in the pGEM T-Easy kit. Constructs containing inserts were identified using blue/white screening according to the pGEM T-Easy kit protocol. Five clones containing insert were re-plated and half of the colony was subjected to PCR to verify insert of the desired DNA size using the same PCR condition listed above, with the exception of a 5 min incubation of each clone at 96° C. prior to the amplification cycles.
One clone containing the desired size insert was grown in a 2 mL culture overnight in LB media containing ampicillin (50 μg/mL; Research Products International Corp). DNA was purified using a Qiagen mini-prep kit. DNA was submitted for sequencing to the University of Michigan DNA Sequencing Core Lab and sequenced 3 times from the 5′ end using the T7 primer binding site and 3 times from the 3′ end using the SP6 primer-binding site. DNA from the sequenced clone was ligated into the NdeI and BamHI sites in pET28b (Novagen) and transformed into BL21 competent cells using electroporation. All cells were plated on LB plates containing kanamycin (50 μg/mL; Research Products International Corp) and incubated overnight at 37° C. Ten colonies were subjected to PCR verification of the desired DNA insert using the primers and protocols listed above.
Example 5
Expression and Purification of the Cryptophycin Thioesterase Domain
A clone containing the desired insert size, as visualized by agarose gel electrophoresis, was grown overnight in 25 mL of 2YT broth (16 g tryptone, 10 g yeast extract, 10 g NaCl) containing 50 μg/mL kanamycin at 37° C. 5 mL of the overnight culture were used to inoculate 1 L of 2YT media containing 50 μg/mL kanamycin, which was grown at 37° C. The culture was induced at an OD595 of 0.7 with 0.2 mM IPTG and grown overnight at 30° C. Cells were harvested at 5000 g for 30 min. The pellet was resuspended in 20 mL 0.1 M sodium phosphate buffer (pH 8) containing 20 mM imidazole and 300 mM NaCl. 4 mg of lysozyme and 2 g sucrose were added to the cell suspension and incubated at room temperature for 30 min until the viscosity of the solution increased. The solution was put on ice and subjected to sonication (5 times for 20 sec) at a level of 6 on the sonicator until the solution became less viscous. The suspension was centrifuged at 17,000 g for 1 hour at 4° C.
The supernatant was collected and incubated with 7 mL of Qiagen Ni-Agarose overnight at 4° C. The agarose was then loaded into a column and washed with 10 column volumes of 0.1 M sodium phosphate buffer (pH 8) containing 20 mM imidazole and 300 mM NaCl. The column was washed with 10-column volumes wash buffer containing 50 mM imidazole. Protein was eluted with wash buffer containing 100 mM imidazole. The eluted sample contained 50 mg of protein as determined using a BioRad Bradford assay kit. Samples were run on a 4-20% SDS-PAGE gel to check for purity. A band corresponding the expected molecular weight was observed at >95% purity. Protein was subjected to a PD-10 column prior to kinetic assays for buffer exchange to 100 mM sodium phosphate buffer (pH 8).
Example 6
Preparation of Substrates
Referring to FIG. 10, substrate 3 represents the tri-depsipeptide sector of cryptophycin except that the methyl β-alanine residue has been replaced by β-alanine. The remaining functionality has been preserved. The halogenation of the tyrosine residue likely is a tailoring modification, which is performed after thioesterase-mediated cyclization. Therefore, a simple tyrosine methyl ether was employed. The SNAC thioester substrate 3 was prepared from known tri-depsipeptide 1 (Georg et al., J. Org. Chem., 2000, 65:7792-7799) by PyBOP coupling of N-acetylcysteamine followed by Boc deprotection with 4 N HCl in 1,4-dioxane to provide 3 as the hydrochloride salt (FIG. 10).
Similarly, the Unit A analogs 6 and 9 were prepared by stepwise deprotection of the t-butyl ester with TFA containing 1% triethylsilane followed by TBS cleavage with 5% hydrofluoric acid in acetonitrile from known Unit A fragments 4 and 7 (Georg et al., supra). PyBOP mediated coupling of subunit 3 with fragments 6 and 9 afforded the seco-SNAC-cryptophycin thioester substrates 10 and 11 respectively, which were purified by reverse-phase semi-preparative HPLC (C18, Alltech Econosil 10×250 mm, 5 mL/min, 10-100% AcCN/H2O+0.1% TFA, 30 minutes).
Example 7
Kinetic Characterization of Cryptophycin Thioesterase Activity with a Substrate
A standard curve of the cleaved product was determined on a 10-67% acetonitrile/water (0.1% TFA) gradient over 30 min. Cleavage reactions were run for 15 min at 30° C. with 1.4 μM cryptophycin thioesterase with substrate concentrations of 0.3125, 0.625, 1.25, 2.5, and 5 mM substrate containing 4% DMSO in 0.1 M NaH2PO4 buffer at pH 7, 8, and 8.75. The hydrolyzed version of substrate 3 was monitored in order to determine the rate of hydrolysis for the reactions. All reactions were run in triplicate.
Example 8
Cyclization of Cryptophycin Substrates
A 1 mL solution containing 100 μM substrate 10 or substrate 11, with 7 μM cryptophycin thioesterase, 0.095 M NaH2PO4 buffer (pH 7), and 5% DMSO was incubated for 1 hour at 30° C. Negative control reactions containing all reagents except for the cryptophycin thioesterase were run in parallel. The total contents of each reaction were separated using reverse phase chromatography with a 10-100% gradient (acetonitrile+0.1% TFA/water+0.1% TFA) over 37 min on an Alltech Econosil 10 U C18 column with dimensions 250 mm×4.6 mm. The products were analyzed by electrospray mass spectrometry (ES+). The relative concentration of the products was determined by comparing absorption at 245 nM, which corresponds to the enone functionality contained within each molecule examined.
Example 9
Results
Immediately 5′ of the nucleotide sequences encoding the cryptophycin thioesterase are sequences that putatively encode a phosphopantetheinylation domain. The thioesterase domain was, therefore, constructed to begin immediately following the 3′ end of DNA predicted to encode the phosphopantetheinylation domain.
The molecular weight of cryptophycin TE was determined to be 35,424 Da by ES+mass spectrometry and 35,410 by MALDI-TOF mass spectrometry. The calculated average mass for the cryptophycin TE was 35,550.08, and the monoisoptopic mass was determined to be 35527.66. The mass spectrometry determined that the molecular weight of cryptophycin thioesterase corresponds to a thioesterase that is missing its N-terminal methionine. Processing of the N-terminal methionine commonly occurs when proteins are expressed small amino acids adjacent to the N-terminal methionine, such as the glycine that is located adjacent to the N-terminal methionine in the engineered construct.
The cyclized cryptophycins are fairly insoluble in water and, therefore, kinetic characterization of hydrolytic rate of the cryptophycin thioesterase was determined using a substrate modeled after the depsipeptide fragment corresponding to Units B, C and D of Cryptophycin 1.
Characterization of the cryptophycin thioesterase-catalyzed hydrolysis of the substrate 3 was monitored by HPLC. The two hydrolysis products produced by the reaction were determined using ES+ mass spectrometry to be N-acetyl cystamine and molecule 12 (FIG. 11).
Initially, the cryptophycin thioesterase was stored in 5% glycerol containing buffer. However, analysis by HPLC/MS of hydrolysis of the substrate 3 with cryptophycin thioesterase containing 5% glycerol revealed that the glycerol adduct was the major product of the reaction with a minor product of the hydrolyzed substrate. Therefore, the expression strain containing cryptophycin thioesterase was recultured and the cryptophycin thioesterase was purified in the absence of glycerol. Subsequent analysis of the cryptophycin thioesterase-catalyzed hydrolysis of the substrate 3 did not reveal a glycerol adduct peak. The generation of the glycerol adduct (molecule 13, FIG. 11) warrants caution when determining kinetics using buffers containing glycerol (especially using indirect methods).
The hydrolytic activity of cryptophycin thioesterase was determined for the substrate 3 using steady state kinetic analysis utilizing HPLC analytical methods. FIG. 12 outlines the catalytic rate constants for hydrolysis of the substrate 3 with cryptophycin thioesterase at pH 7, pH 8, and pH 8.75.
The ability of the cryptophycin thioesterase to cyclize substrates was examined using seco-SNAC-des-epoxy-arenastatin 11 and the des-benzyl derivative of seco-SNAC-des-epoxy-arenastatin 10 as substrates (FIG. 13).
The partition ratio of cyclization to hydrolysis for the cryptophycin catalyzed reaction with seco-SNAC-des-epoxy arenastatin 11 was 5:1, while the partition ratio of cyclization to hydrolysis with the seco-SNAC-des-benzyl-des-epoxy-arenastatin 10 was 1:8.3, as determined by HPLC/MS with quantitation of the quantity of enone functionality at 245 nM. Therefore, the cryptophycin thioesterase preferentially cyclized the SNAC thioester of seco-des-epoxy-arenastatin over the SNAC thioester of seco-des-benzyl-des-epoxy-arenastatin.
The specificity constant (kcat/KM) for the cryptophycin thioesterase catalyzed hydrolysis of the substrate 3 increased over the pH range from 7 to 8.75 (FIG. 12). The increase in the specificity constant was due to an increase in the kcat from pH 7 to pH 8, and a decrease in kcat from pH 8 to pH 8.75. The KM for the hydrolysis of the substrate 3 decreased slightly from pH 8 to pH 8.75, although the kcat for hydrolysis also decreased, resulting in an overall increase in the specificity constant.
Interestingly, although substrate 3 contains both a thioester bond and an ester bond, hydrolysis occurred specifically at the thioester, even after complete hydrolysis of the thioester, indicating a selective preference for that site.
Example 10
Identification of Coding Sequences within the Polyketide Portion of a Biosynthetic Operon
Two additional coding sequences designated crpA and crpB were identified in the polyketide portion of the operon. crpA (SEQ ID NO:42) and the encoded amino acid sequence (SEQ ID NO:43) are shown in FIG. 14, while crpB (SEQ ID NO:44) and the encoded amino acid sequence (SEQ ID NO:45) are shown in FIG. 15.
OTHER EMBODIMENTS
It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.

## Claims

1. An isolated nucleic acid molecule comprising a nucleic acid sequence having at least 95% sequence identity to the sequence shown in SEQ ID NO:42, wherein said nucleic acid sequence encodes a polypeptide that exhibits polyketide synthetase activity in the biosynthesis of cryptophycin under appropriate conditions.

2. The nucleic acid molecule of claim 1, wherein said nucleic acid sequence has at least 99% sequence identity to the sequence shown in SEQ ID NO:42.

3. The nucleic acid molecule of claim 1, wherein said nucleic acid sequence is SEQ ID NO:42.

4. A vector comprising the nucleic acid molecule of claim 1.

5. An isolated host cell comprising the vector of claim 4.