Abstract:
The objective of the present invention is to improve the efficiency of screening/selection of a membrane protein in molecular evolutionary engineering (for example, an enzyme evolutionary method). 
     The above-described objective is achieved by providing a unilamellar liposome comprising:
       (a) a DNA comprising a promoter sequence, a translational initiation sequence, and a sequence encoding a membrane protein;   (b) an RNA polymerase;   (c) a ribonucleotide; and   (d) a cell-free protein synthesis system. In one aspect of the present invention, the membrane protein is a transporter, and the unilamellar liposome further comprises   (e) a factor that binds to a ligand transported by the membrane protein.

Description:
STATEMENT REGARDING SEQUENCE LISTING 
     The Sequence Listing associated with this application is provided in text format in lieu of a paper copy, and is hereby incorporated by reference into the specification. The name of the text file containing the Sequence Listing is 390051_407USPC_SEQUENCE_LISTING.txt. The text file is 42.6 KB, was created on Dec. 29, 2014, and is being submitted electronically via EFS-Web. 
     TECHNICAL FIELD 
     The present invention relates to the field of novel unilamellar liposomes for utilization in in-vitro molecular evolutionary engineering of membrane proteins. The present invention further relates to novel molecular evolutionary engineering, particularly enzyme evolutionary engineering, targeting membrane proteins that uses the unilamellar liposomes. 
     BACKGROUND ART 
     As a method of improving an enzyme by evolutionary engineering, a method using liposomes in which a gene library and a cell-free protein synthesis system are enclosed, and a cell sorter has been utilized. In this method, a gene library in which random mutation is introduced into an enzyme gene and a cell-free protein synthesis system are enclosed in liposomes for internal expression of an enzyme. Further, a liposome that contains an enzyme having a higher function is selected by the cell sorter to enable selection of a gene encoding an enzyme having a higher function. By repeating this selection, a gene encoding an enzyme can be evolved (Non Patent Literature 1). This conventional method is solely targeted to soluble proteins. 
     It is well known that membrane proteins play an important role in functions of cells. Thus, novel molecular evolutionary engineering, particularly enzyme evolutionary engineering, targeting membrane proteins has been required. 
     CITATION LIST 
     Non Patent Literature 
     
         
         [NPL 1] Sunami, T., Sato, K., Matsuura, T., Tsukada, K., Urabe, I., and Yomo, T. (2006) Analytical biochemistry 357, 128-136 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     The objective of the present invention is to provide a novel molecular evolutionary engineering technique, particularly an enzyme evolutionary engineering technique, targeting membrane proteins. 
     Solution to Problem 
     The above-described objective has been achieved by providing the following. 
     (Item 1) 
     A unilamellar liposome comprising: 
     (a) a DNA comprising a promoter sequence, a translational initiation sequence, and a sequence encoding a membrane protein; 
     (b) an RNA polymerase; 
     (c) a ribonucleotide; and 
     (d) a cell-free protein synthesis system. 
     (Item 2) 
     The unilamellar liposome of item 1, wherein the membrane protein is a transporter, and the unilamellar liposome further comprises 
     (e) a factor that binds to a ligand transported by the membrane protein. 
     (Item 3) 
     The unilamellar liposome of item 1 or 2, wherein the unilamellar liposome is treated with a nuclease. 
     (Item 4) 
     The unilamellar liposome of item 3, wherein the nuclease is selected from the group consisting of a ribonuclease and a deoxyribonuclease. 
     (Item 5) 
     The unilamellar liposome of item 4, wherein the nuclease is a ribonuclease. 
     (Item 6) 
     A library comprising a plurality of unilamellar liposomes, wherein the unilamellar liposome comprises: 
     (a) a DNA comprising a promoter sequence, a translational initiation sequence, and a sequence encoding a membrane protein; 
     (b) an RNA polymerase; 
     (c) a ribonucleotide; and 
     (d) a cell-free protein synthesis system. 
     (Item 7) 
     The library of item 6, wherein the membrane protein is a transporter, and the unilamellar liposome further comprises 
     (e) a factor that binds to a ligand transported by the membrane protein. 
     (Item 8) 
     The library of item 6 or 7, wherein the unilamellar liposome is treated with a nuclease. 
     (Item 9) 
     The library of item 8, wherein the nuclease is selected from the group consisting of a ribonuclease and a deoxyribonuclease. 
     (Item 10) 
     The library of item 9, wherein the nuclease is a ribonuclease. 
     (Item 11) 
     A unilamellar liposome comprising: 
     (a) an RNA comprising a translational initiation sequence, and a sequence encoding a membrane protein; and 
     (d) a cell-free protein synthesis system. 
     (Item 12) 
     The unilamellar liposome of item 11, wherein the membrane protein is a transporter, and the unilamellar liposome further comprises 
     (e) a factor that binds to a ligand transported by the membrane protein. 
     (Item 13) 
     The unilamellar liposome of item 11 or 12, wherein the unilamellar liposome is treated with a nuclease. 
     (Item 14) 
     The unilamellar liposome of item 13, wherein the nuclease is selected from the group consisting of a ribonuclease and a deoxyribonuclease. 
     (Item 15) 
     The unilamellar liposome of item 14, wherein the nuclease is a ribonuclease. 
     (Item 16) 
     A library comprising a plurality of unilamellar liposomes, wherein the unilamellar liposome comprises: 
     (a) an RNA comprising a translational initiation sequence, and a sequence encoding a membrane protein; and 
     (d) a cell-free protein synthesis system. 
     (Item 17) 
     The library of item 16, wherein the membrane protein is a transporter, and the unilamellar liposome further comprises 
     (e) a factor that binds to a ligand transported by the membrane protein. 
     (Item 18) 
     The library of item 16 or 17, wherein the unilamellar liposome is treated with a nuclease. 
     (Item 19) 
     The library of item 18, wherein the nuclease is selected from the group consisting of a ribonuclease and a deoxyribonuclease. 
     (Item 20) 
     The library of item 19, wherein the nuclease is a ribonuclease. 
     (Item 21) 
     A method of producing a unilamellar liposome treated with a nuclease, comprising: 
     (1) preparing a unilamellar liposome enclosing: 
     (a) a DNA comprising a promoter sequence, a translational initiation sequence, and a sequence encoding a membrane protein; 
     (b) an RNA polymerase; 
     (c) a ribonucleotide; and 
     (d) a cell-free protein synthesis system; and 
     (2) treating the unilamellar liposome prepared in (1) with a nuclease. 
     (Item 22) 
     A method of producing a unilamellar liposome treated with a nuclease, comprising: 
     (1) preparing a unilamellar liposome enclosing: 
     (a) a DNA comprising a promoter sequence, a translational initiation sequence, and a sequence encoding a membrane protein that is a transporter; 
     (b) an RNA polymerase; 
     (c) a ribonucleotide; 
     (d) a cell-free protein synthesis system; and 
     (e) a factor that binds to a ligand transported by the membrane protein; and 
     (2) treating the unilamellar liposome prepared in (1) with a nuclease. 
     (Item 23) 
     The method of item 21 or 22, wherein the nuclease is selected from the group consisting of a ribonuclease and a deoxyribonuclease. 
     (Item 24) 
     The method of item 23, wherein the nuclease is a ribonuclease. 
     (Item 25) 
     A method of producing a unilamellar liposome treated with a nuclease, comprising: 
     (1) preparing a unilamellar liposome enclosing: 
     (a) an RNA comprising a translational initiation sequence, and a sequence encoding a membrane protein; and 
     (d) a cell-free protein synthesis system; and 
     (2) treating the unilamellar liposome prepared in (1) with a nuclease. 
     (Item 26) 
     A method of producing a unilamellar liposome treated with a nuclease, comprising: 
     (1) preparing a unilamellar liposome enclosing: 
     (a) an RNA comprising a translational initiation sequence, and a sequence encoding a membrane protein that is a transporter; 
     (d) a cell-free protein synthesis system; and 
     (e) a factor that binds to a ligand transported by the membrane protein; and 
     (2) treating the unilamellar liposome prepared in (1) with a nuclease. 
     (Item 27) 
     The method of item 25 or 26, wherein the nuclease is selected from the group consisting of a ribonuclease and a deoxyribonuclease. 
     (Item 28) 
     The method of item 27, wherein the nuclease is a ribonuclease. 
     (Item 29) 
     A screening method using a library of unilamellar liposomes, comprising: 
     (i) providing a library of any of items 6 to 10; 
     (ii) selecting a unilamellar liposome having a desired feature from the library; 
     (iii) amplifying a DNA included in the unilamellar liposome; and 
     (iv) isolating the amplified DNA. 
     (Item 30) 
     A screening method using a library of unilamellar liposomes, comprising: 
     (i) providing a library of any of items 16 to 20; 
     (ii) selecting a unilamellar liposome having a desired feature from the library; 
     (iii) generating a DNA by operating a reverse transcriptase on an RNA included in the unilamellar liposome; 
     (iv) amplifying the generated DNA; and 
     (v) isolating the amplified DNA. 
     Advantageous Effects of Invention 
     The present invention enables an in-vitro molecular evolutionary engineering technique targeting membrane proteins that utilizes liposomes. The present invention further enables large-scale screening/selection of a gene encoding a membrane protein having a desired function. 
     If a membrane protein is a transporter, a factor that binds to a ligand transported by the membrane protein would be enclosed within a liposome to capture the transported ligand within the liposome, thereby enhancing the sensitivity of screening/selection. 
     Further, by using unilamellar liposomes that are processed by a nuclease according to the present invention, screening efficiency will be enhanced. While not wishing to be bound by theory, the following reason can be mentioned as a reason that the present invention exerts a remarkable effect. Conventionally-used liposomes are multilamellar liposomes that are prepared by a freeze-drying method, and since those liposomes internally have a multiple structure, the volume of a reaction vessel is not possible to be controlled. The volume of liposomes affects the internal enzymatic kinetics. Thus, in order to efficiently improve an enzyme, the use of unilamellar liposomes which do not have a multiple structure is preferable. However, in methods so far, when unilamellar liposomes that are prepared by a centrifugal sedimentation method are used as reaction vessels, selection and collection of a gene encoding an enzyme having a high function were not possible even by selecting liposomes that were more reactive than others by a cell sorter. In contrast, in the present invention, treatment of unilamellar liposomes with a nuclease enables further highly-efficient screening compared to unilamellar liposomes that are not treated with an enzyme and multilamellar liposomes used in conventional methods, thereby allowing selection and collection of a gene encoding a highly-functional enzyme. 
     In addition, by optimizing the composition/ratio of a lipid forming a liposome and the magnesium concentration when preparing the liposome according to the disclosure of the present invention, the sensitivity of screening/selection will be further enhanced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is the result of using a DNA comprising an EmrE-myc-his sequence (SEQ ID NO: 1) or a DNA comprising a GUS sequence (SEQ ID NO: 3), wherein a labeling anti-Myc tag antibody is added to liposomes before and after the expression of proteins, and an analysis is performed by a cell sorter. The vertical axis shows the internal volume of liposomes and the horizontal axis shows the fluorescence intensity of Alexa 488. A and B show the results of using the GUS sequence, and C and D show the results of using the EmrE-myc-his sequence. A and C are results of liposomes before the expression of proteins by incubation at 37° C., and B and D are results of liposomes that expressed proteins by an hour incubation at 37° C. 
         FIG. 2  is the result of measuring the transport activity of EtBr with different pH, in liposomes comprising a DNA comprising an EmrE-myc-his sequence (SEQ ID NO: 1;  FIG. 2A ) or a DNA comprising a GUS sequence (SEQ ID NO: 3;  FIG. 2B ), wherein proteins are expressed. 
         FIG. 3  is the result showing the percentage of expression of a membrane protein having a function when various lipid compositions are used. When hemolysin exerts the activity, Halo Tag Alexa Fluor 488 ligand is taken in with high intensity, and thus the vertical axis shows the percentage (%) of liposomes that taken in ligands with high intensity. That is, the vertical axis shows the percentage of exertion of membrane protein activity in liposomes. The results of using the mixture of POPC:Chol=9:1; the mixture of POPC:Chol=7:3; the mixture of POPC:Chol=5:5; and the mixture of POPC:Chol=3:7 are shown in order from the left. Further, POPC is an abbreviation of 1-palmitoyl-2-oleoylphosphatidylcholine, and Chol is an abbreviation of cholesterol. 
         FIG. 4  The vertical axis of  FIG. 4  shows the percentage (%) of liposomes that taken in Halo Tag Alexa Fluor 488 ligand with high intensity among all the liposomes when various lipids are used. That is,  FIG. 4  is a graph showing the relative activity of channels. The lipids that are used are as follows: EggPC is an abbreviation of phosphatidylcholine purified from a hen&#39;s egg; POPC is an abbreviation of 1-palmitoyl-2-oleoylphosphatidylcholine; PS is an abbreviation of 1-palmitoyl-2-oleoylphosphoserine; PE is an abbreviation of 1-palmitoyl-2-oleoylphosphoethanolamine; and Chol is an abbreviation of cholesterol. PC mix is an abbreviation of the mixture of 1-palmitoyl-2-oleoylphosphatidylcholine:1-palmitoyl-2-linoleoylphosphatidylcholine:1-stearoyl-2-oleoylphosphatidylcholine:1-stearoyl-2-linoleoylphosphatidylcholine=129:67:48:24 (mass ratio); EggPC/PS/PE is an abbreviation of the mixture of each of them at the ratio of 3:1:1 (mass ratio) in order; EggPC/PS/PE/Chol is an abbreviation of the mixture of each of them at the ratio of 2:1:1:1 (mass ratio) in order; PCmix/PS/PE is an abbreviation of the mixture of each of them at the ratio of 3:1:1 (mass ratio) in order; PCmix/PS/PE/Chol is an abbreviation of the mixture of each of them at the ratio of 2:1:1:1 (mass ratio) in order; POPC/PS/PE is an abbreviation of the mixture of each of them at the ratio of 3:1:1 (mass ratio) in order; and POPC/POPE/POPS/Chol is an abbreviation of the mixture of each of them at the ratio of 2:1:1:1 (mass ratio) in order. 
         FIG. 5  is a graph showing the result of an evolutionary experiment. The vertical axis shows the percentage of high intensity liposomes (the percentage of red dots). By repeating the cycle, the percentage of a group having high activity increased. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, the present invention will be described. It should be understood that unless particularly stated otherwise, the terms used in the present specification have the meanings that are conventionally used in the art. 
     Hereinafter, the definitions of the terms that are used particularly in the present specification will be listed. 
     (Definition) 
     The term “micro-compartment” as used herein refers to a closed minute space composed of a lipid layer and an internal aqueous layer. Examples of the “micro-compartment” include liposomes, but are not limited thereto. 
     The term “liposome” as used herein generally means a closed vesicle composed of a lipid layer gathered in a membrane state and an internal aqueous layer. Other than phospholipid which is representatively used, cholesterol, glycolipid and the like can be incorporated. In the present invention, a liposome preferably contains cholesterol as the component. In the present invention, in order to have a modifying group, a liposome may have a constitutional unit having a functional group that allows ester bond (for example, glycolipid, ganglioside and phosphatidylglycerol) or a constitutional unit having a functional group that allows peptide bond (for example, phosphatidylethanolamine). The liposome that is used in the present invention is a “unilamellar liposome” consisting of a single membrane consisting of a lipid bilayer. As the preparation method of the unilamellar liposome, various well-known methods can be utilized. 
     The term “promoter sequence” as used herein refers to a region on a DNA that determines an initiation site of transcription of a gene and that directly regulates the frequency thereof, which is a base sequence to which an RNA polymerase bound and starts transcription. Although a putative promoter region varies in each structural gene, a putative promoter region is generally located in the upstream of a structural gene. However, the location is not limited thereto, and a putative promoter region also may be located in the downstream of a structural gene. The promoter may be inducible, structural, site-specific or stage-specific. The promoter may be any promoter as long as the promoter is able to be expressed in a host cell such as a mammalian cell, a colon  bacillus  and yeast. Representative promoter sequences include a T7 promoter sequence, a T5 promoter sequence, a Sp6 promoter sequence and a T3 promoter sequence, but are not limited thereto. 
     The “RNA polymerase” as used herein may be any RNA polymerase as long as it adapts to a promoter sequence to be used, that is, performs transcription from the promoter to be used. Preferably, the promoter sequence and the RNA polymerase are derived from the same or close species. For example, when a promoter sequence derived from a prokaryote is used, an RNA polymerase to be used is also preferably derived from a prokaryote. Alternatively, when a promoter sequence derived from a bacteriophage is used, an RNA polymerase to be used is also preferably derived from the same or similar bacteriophage. 
     The term “translational initiation sequence” as used herein means any sequence that is able to provide a functional ribosome entry site. In the system of bacteria, this region is also referred to as Shine-Dalgarno sequence. 
     The term “cell-free protein synthesis system” as used herein is a component derived from a cell that has lost autonomous replication ability by treating the cell, and is a component that is able to synthesize a protein. As the cell-free protein synthesis system, for example, PURESYSTEM (registered trademark) (BioComber Co., Ltd.; Bunkyo-ku, Tokyo) that is commercially available can be utilized. Alternatively, the cell-free protein synthesis system is possible to be prepared by performing purification and/or recombinant expression of a component that is required for the cell-free protein synthesis system. 
     The term “operably linked” as used herein refers to a state in which the expression (operation) of a desired sequence is disposed under the control of a certain transcriptional/translational regulatory sequence (for example, a promoter and an enhancer) or a translational regulatory sequence. In order to allow for a promoter to be operably linked to a gene, the promoter is generally disposed in just upstream of the gene. However, the promoter is not necessarily adjacently disposed. 
     The term “membrane protein” as used herein refers to a protein that is attached to a lipid bilayer. The membrane protein may be a protein that contains a transmembrane region or may be a protein that does not contain a transmembrane region. 
     (Membrane Protein) 
     The present invention is applicable to various membrane proteins. Representative membrane proteins include, for example, transporters and receptors, but are not limited thereto. The sequence encoding the membrane protein of the present invention may comprise a leader sequence for inserting a protein into a membrane, as necessary. 
     (Transporter) 
     The membrane protein of the present invention may be or may not be a transporter. Examples of the transporter of the present invention include proteins related to substance transportation in cells (for example, EmrE protein) and proteins that allow permeation of a substance that does not permeate a lipid bilayer (for example, hemolysin), but are not limited thereto. 
     (Production of Unilamellar Liposome) 
     The unilamellar liposome used in the present invention is possible to be prepared by using the centrifugal sedimentation method described in the Examples. However, the preparation method is not limited thereto. For example, other than the centrifugal sedimentation method, a swelling hydration method (P. Mueller and T. F. Chien, Biophys. J., 1983, 44, 375-381) and an electro-formation method (Miglena I. Angelove and Dimiter S. Dimitrov, Faraday Discuss. Chem. Soc., 1986, 81, 303-311) can be utilized. 
     The swelling hydration method is a method that representatively encompasses the following steps: (1) a step of dissolving a lipid in a solvent for natural drying within a flask to form a lipid membrane on a surface of the flask; and (2) a step of adding an aqueous solution to enlarge the lipid membrane. By this second step, a liposome in which the lipid membrane taken in the aqueous solution floats up. 
     The electro-formation method is a method that representatively encompasses the following steps: (1) a step of applying a lipid solution on a conductive electrode for drying to form a lipid film; (2) a step of placing a conductive electrode also in the opposite side intervened by an insulating spacer and filling an aqueous solution therebetween; and (3) a step of applying an electric field between the two electrodes to remove the lipid film from the electrodes and prepare a giant thin film liposome. 
     (Component/Composition of Lipid Used in Production of Unilamellar Liposome) 
     The component/composition of a lipid used in the production of unilamellar liposomes preferably include, although not particularly limited, phospholipid and cholesterol. Examples of the lipid include L-alpha-phosphatidylcholine, cholesterol, L-alpha-dilauroylphosphatidylcholine, L-alpha-dilauroylphosphatidylethanolamine, L-alpha-dilauroylphosphatidylglycerolsodium, L-alpha-monomyristoylphosphatidylcholine, L-alpha-dimyristoylphosphatidylcholine, L-alpha-dimyristoylphosphatidylethanolamine, L-alpha-dimyristoylphosphatidylglycerol ammonium, L-alpha-dimyristoylphosphatidylglycerol sodium, L-alpha-dimyristoylphosphatidic acid sodium, L-alpha-dioleylphosphatidylcholine, L-alpha-dioleoylphosphatidylethanolamine, L-alpha-dioleoylphosphatidylserine sodium, L-alpha-monopalmitoylphosphatidylcholine, L-alpha-dipalmitoylphosphatidylcholine, L-alpha-dipalmitoylphosphatidylethanolamine, L-alpha-dipalmitoylphosphatidylglycerol ammonium, L-alpha-dipalmitoylphosphatidylglycerol sodium, L-alpha-dipalmitoylphosphatidic acid sodium, L-alpha-stearoylphosphatidylcholine, L-alpha-distearoylphosphatidylcholine, L-alpha-distearoylphosphatidylethanolamine, L-alpha-distearoylphosphatidylglycerol sodium, L-alpha-distearoylphosphatidylglycerol ammonium, L-alpha-distearoylphosphatidic acid sodium, L-alpha-dierucoylphosphatidylcholine, 1-palmitoyl-2-oleoylphosphatidylcholine, beta-oleyl-gamma-palmitoyl-L-alpha-phosphatidylethanolamine, beta-oleyl-gamma-palmitoyl-L-alpha-phosphatidylglycerol sodium, sphingomyelin and stearylamine, but are not limited thereto. 
     The proportion of the cholesterol is preferably 10% or more, more preferably 30% or more, even more preferably 50% or more, and most preferably 700 or more. 
     (Magnesium Concentration Appropriate for Production of Unilamellar Liposome) 
     The concentration of magnesium is preferably 15 mM to 50 mM, more preferably 18.88 mM to 42.48 mM, even more preferably 28.32 mM to 37.76 mM, and most preferably 33.04 mM. 
     (Nuclease) 
     Examples of the nuclease used in the present invention include a ribonuclease and a deoxyribonuclease, but are not limited thereto. The source of supply of the nuclease to be used is not particularly limited. When DNase is used as the nuclease, the enzyme activity to be used is 1 U to 20 U, more preferably 5 U to 15 U and most preferably about 12.5 U per 100 μL of a liposome solution. When RNase is used as the nuclease, enzyme activity to be used is 1 μg to 20 μg, more preferably 5 μg to 15 μg, and most preferably about 10 μg per 100 μL of a liposome solution. Those skilled in the art are able to readily determine the amount of an enzyme to be used. 
     (DNA or RNA to be Used) 
     For example, if genetic information to be included in a liposome is a DNA, a coding sequence of a protein, a translational regulatory sequence operably linked to the coding sequence, and a transcriptional/translational regulatory sequence operably linked to the coding sequence will be included in the DNA. 
     Examples of the translational regulatory sequence include a translational initiation sequence, but are not limited thereto. A translation termination codon may be included as necessary. The translational regulatory sequence to be linked preferably adapts to a cell-free protein synthesis system to be used. For example, if a cell-free protein synthesis system that is derived from  E. coli  is to be utilized, a translational regulatory sequence to be linked is preferably a translational initiation sequence of  E. coli . A translational regulatory sequence and a cell-free protein synthesis system to be used are not necessarily required to be derived from the same species. A translational regulatory sequence and a cell-free protein synthesis system to be used can be derived from any species as long as they are adaptable, that is, the cell-free protein synthesis system is able to initiate translation from the translational regulatory sequence. 
     Examples of the transcriptional/translational regulatory sequence include a promoter sequence, but are not limited thereto. An enhancer sequence, a suppressor sequence, an operator sequence, and a transcription termination site may be included as necessary. A transcriptional/translational regulatory sequence to be linked preferably adapts to an RNA polymerase to be used. For example, if an RNA polymerase derived from  E. coli  is to be utilized, a transcriptional/translational regulatory sequence to be linked is preferably a transcriptional/translational regulatory sequence of  E. coli . A transcriptional/translational regulatory sequence and an RNA polymerase to be used are not necessarily required to be derived from the same species. The transcriptional/translational regulatory sequence and the RNA polymerase to be used can be derived from any species as long as they are adaptable, that is, the RNA polymerase is able to initiate (or control) transcription from the transcriptional/translational regulatory sequence. 
     For example, if genetic information to be included in a liposome is an RNA, a coding sequence of a protein, and a translational regulatory sequence operably linked to the coding sequence will be included in the RNA. Examples of the translational regulatory sequence include a translational initiation sequence, but are not limited thereto. A translation termination codon may be included as necessary. A translational regulatory sequence to be linked preferably adapts to a cell-free protein synthesis system to be used. For example, if a cell-free protein synthesis system derived from  E. coli  is to be utilized, a translational regulatory sequence to be linked is preferably a translational initiation sequence of  E. coli . A translational regulatory sequence and a cell-free protein synthesis system to be used are not necessarily required to be derived from the same species. A translational regulatory sequence and a cell-free protein synthesis system to be used can be derived from any species as long as they are adaptable, that is, the cell-free protein synthesis system is able to initiate translation from the translational regulatory sequence. 
     (Application of Liposome of the Present Invention to Molecular Evolutionary Engineering) 
     The liposomes of the present invention can be utilized for molecular evolutionary engineering. 
     For example, unilamellar liposomes treated by a nuclease are incubated under the condition that the internal DNA or RNA generates protein products, and (1) by using the presence of proteins expressed on the surface of the liposomes as an indicator, or (2) by measuring the activity of the generated membrane proteins and using this activity as an indicator, selection (screening) of unilamellar liposomes including high-functional genetic information is performed. Activity to be utilized is representatively activity of a protein that is encoded by a DNA or an RNA within the unilamellar liposomes. For example, if a DNA or an RNA within the unilamellar liposomes encodes a transporter, activity to be utilized is representatively the transport activity thereof. If the transport activity of a transporter is used as an indicator, for example, substances that are transported into the liposomes by the transporter are labeled (for example, fluorescent labeling), and liposomes in which the labeled substances are accumulated are selected by using a cell sorter (FACS: fluorescence-activated cell sorter). For example, a factor that binds to a ligand transported by the transporter can be enclosed within the liposomes to capture the transported ligand within the liposomes, thereby enhancing the sensitivity of screening/selection. 
     Alternatively, the enzyme activity possessed by a membrane protein may be used as an indicator. 
     In order to detect phosphorylation of a protein or bonding with other proteins as an indicator of the activity of a membrane protein, for example, the following methods are used: a step of labeling an edge of a target protein with fluorescent dye that causes FRET; and when conformation is changed by phosphorylation or bonding with other proteins and the degree of FRET is changed, a step of selection by using the fluorescence change as an indicator. Alternatively, by disposing a GFP gene in the downstream of a T3RNA polymerase promoter for example, and using a T3RNA polymerase RNA at the same time, a T3RNA polymerase having higher RNA synthetic activity is possible to be obtained. 
     In addition, by introducing mutation into sequences (sequences related to the control of gene expression such as a promoter sequence, an enhancer sequence, a ribosome-binding sequence, and a translation initiation site) other than a coding sequence of a protein, and selecting the sequence to which mutation is introduced, a sequence can be evolved to have high activity (for example, high promoter activity, enhancer activity and translation activity). 
     The unilamellar liposome obtained as a result of screening is used to isolate genetic information included therein as a DNA or an RNA. If the genetic information is a DNA, the isolation can be performed by using a primer that specifically amplifies the DNA, thereby amplifying the genetic information by PCR. Alternatively, if the DNA includes a sequence that is required for autonomous replication within a host cell, the DNA can be introduced into an appropriate host cell, and the isolation can be performed after the amplification. 
     If genetic information is an RNA, (1) the RNA may be converted into a DNA using a reverse transcriptase, and then the DNA may be amplified by PCR using a thermostable DNA polymerase enzyme, or (2) genetic information of the RNA may be amplified in a single step using a thermostable reverse transcriptase. If the RNA includes a sequence that is required for autonomous replication within a host cell, the RNA can be introduced into an appropriate host cell, and the isolation can be performed after the amplification. 
     Genetic information is not necessarily required to be isolated (purified) after a first round of screening. For example, instead of obtaining a monoclonal DNA or RNA by the first round of screening, a second round of screening may be performed by obtaining a group of DNAs or RNAs and using the group as a starting material. A group of DNAs or RNAs obtained by the second round of screening or the subsequent rounds of screening may be used as a starting material of the next round. 
     Alternatively, mutagenesis may be performed on a clone (purified clone) obtained after the screening to prepare a group comprising a plurality of different clones, and the group may be used as a starting material of the screening of the next round. 
     EXAMPLES 
     Hereinafter, the present invention will be described in detail by Examples and the like. However, the present invention is not limited thereto. 
     Example 1: Preparation of Unilamellar Liposome 
     Unilamellar liposomes were prepared by the centrifugal sedimentation method described below. 
     10 mg of lipid (phosphatidylcholine:cholesterol=9:1) was dissolved into 100 μl of chloroform for mixture with 2 ml of liquid paraffin. 
     Incubation was performed for 30 minutes at 80° C. 
     An extraliposomal solution (333 mM glucose, and a solution in which a group of translated proteins and tRNA are removed from a cell-free protein synthesis system) and an intraliposomal solution (330 mM sucrose, 1 μM Transferrin Alexa 647, a cell-free protein synthesis system, 40 U/μl RNase inhibitor (Promega), 0.4 μM ribosome S1 subunit and 50 pM DNA) were prepared. A DNA comprising an EmrE-myc-his sequence (SEQ ID NO: 1; a sequence comprising a myc tag and a his tag in the C-terminus of an EmrE gene) or a DNA comprising a GUS sequence (SEQ ID NO: 3; negative control comprising a myc sequence and a GUS sequence) was used. This condition is a condition that a single molecule of DNA is enclosed in each liposome. The composition of the cell-free protein synthesis system that was used is as follows: amino acids 0.3 mM each (alanine, glycine, leucine, isoleucine, valine, serine, threonine, proline, tryptophan, phenylalanine, glutamine, glutamic acid, asparagine, aspartic acid, lysine, arginine, histidine, methionine, cysteine, tyrosine); 3.6 μg/μl tRNA; 2 mM ATP; 2 mM GTP; 1 mM CTP; 1 mM UTP; 14 mM magnesium acetate; 50 mM Hepes-KOH (pH7.8); 100 mM potassium glutamate; 2 mM spermidine; 20 mM creatine phosphate; 2 mM dithiothreitol; 10 ng/μl 10-formyl-5.6.7.8.-tetrahydrofolic acid; a group of translated proteins (2500 nM IF1, 411 nM IF2, 728 nM IF3, 247 nM RF1, 484 nM RF2, 168 nM RF3, 485 nM RRF, 727 nM AlaRS, 99 nM ArgRS, 420 nM AsnRS, 121 nM AspRS, 100 nM CysRS, 101 nM GlnRS, 232 nM GluRS, 86 nM GlyRS, 85 nM HisRS, 365 nM IleRS, 99 nM LeuRS, 115 nM LysRS, 109 nM MetRS, 134 nM PheRS, 166 nM ProRS, 99 nM SerRS, 84 nM ThrRS, 102 nM TrpRS, 101 nM TyrRS, 100 nM ValRS, 588 nM MTF, 926 nM MK, 465 nM CK, 1307 nM NDK, 621 nM Ppiase2, 1290 nM EF-G, 2315 nM EF-Tu, 3300 nM EF-Ts, 529 nM Tig, 22 nM HrpA, 1440 nM TrxC). 
     20 μl of intraliposomal solution was put into 400 μl of liquid paraffin in which a lipid is dissolved, and the solution was placed on ice for 1 minute. 
     Emulsion was prepared by stirring for 40 seconds at the maximum strength of a vortex mixer, and the emulsion was placed on ice for 10 minutes. 
     150 μl of extraliposomal solution was put into a new tube and the prepared emulsion was laminated thereon, and they were placed on ice for 10 minutes. 
     Centrifugation was performed for 30 minutes at 14 k×g, 4° C. 
     A hole was made at the bottom of the tube, and 80 μl of liposome suspension accumulated at the bottom was collected. 
     2 μl of 5 U/μl DNase or 4 mg/ml RNase was added to the liposome suspension. 
     The liposome suspension was incubated for 3 hours at 37° C., and protein synthesis was performed. 
     An antibody (anti-Myc tag antibody (mouse IgG1) labeled with Alexa Fluor 488) was diluted with a PBS+1% BSA solution and added to the liposome suspension such that the final concentration becomes 5 μg/ml (1 μl of 50 g/ml antibody was added to 9 μl of liposome solution). 
     After standing for 30 minutes at room temperature, the antibody was observed by microscopy (Ex: 470-490 Em: 510-550). 
     As a result, Alexa 488 fluorescence that is caused by an antibody bound to a polypeptide consisting of a sequence comprising a myc tag and a his tag in the C-terminus of an EmrE gene was confirmed as being localized in a liposome membrane. That is, by the above-described method, it was confirmed that a membrane protein was in-vitro synthesized within the liposome, and the membrane protein was incorporated into the liposome membrane. 
     Next, a DNA comprising an EmrE-myc-his sequence (SEQ ID NO: 1; a sequence comprising a myc tag and a his tag in the C-terminus of an EmrE gene) or a DNA comprising a GUS sequence (SEQ ID NO: 3: negative control comprising a myc sequence and a GUS sequence) was used, and an antibody (anti-Myc tag antibody (mouse IgG1) labeled with Alexa Fluor 488, final concentration 5 μg/ml) diluted with a PBS+1% BSA solution was added to liposomes before and after the expression of proteins (1 μl of 50 g/ml antibody was added to 9 μl of liposome solution) followed by 30 minutes of standing at room temperature for analysis by a cell sorter. The results are shown in  FIG. 1 . The vertical axis shows the internal volume of liposomes and the horizontal axis shows the fluorescence intensity of Alexa 488. A and B show the results of using the GUS sequence, and C and D show the results of using the EmrE-myc-his sequence. A and C are results of liposomes before the expression of proteins by incubation at 37° C., and B and D are results of liposomes that expressed proteins by an hour incubation at 37° C. As is apparent from  FIG. 1 , liposomes are prepared under the condition that a single molecule of DNA is enclosed in each liposome, and it was confirmed that a membrane protein was expressed and the membrane protein was able to be detected by an antibody. 
     Example 2: Confirmation of Function of Membrane Protein Expressed in Unilamellar Liposome 
     5 nM of a DNA comprising an EmrE-myc-his sequence (SEQ ID NO: 1; a sequence comprising a myc tag and a his tag in the C-terminus of an EmrE gene) or a DNA comprising a GUS sequence (SEQ ID NO: 3: negative control comprising a myc sequence and a GUS sequence) and a PURE system were enclosed within liposomes. The liposomes were incubated for 2 hours at 37° C. to express EmrE-myc-his and GUS-myc. After the preparation of the liposomes, external solution 1 was replaced with external solution 2 containing EtBr 5 μg/ml. Fluorescence was measured every minute, and the intake of EtBr was observed. Subsequently, the same sample was observed with a fluorescence microscope (Ex: 520-550 Em: 580-). 
     The composition of external solution 1 (that is, the external solution at the time of synthesis of liposomes) is as follows: HEPES-KOH (pH7.6) 100 mM; K-Glu 200 mM; spermidine 4 mM; magnesium acetate 25 mM; CP 40 mM; DTT 2 mM; FD 20 μg/ml; 20 types of amino acids 0.4 mM each; ATP 8 mM; GTP 8 mM; UTP 4 mM; CTP 4 mM. 
     The composition of external solution 2 (that is, the external solution for making a proton gradient) is as follows: Tris-HCl (pH9.0 or 7.6) 100 mM; K-Glu 200 mM; spermidine 4 mM, magnesium acetate 25 mM; CP 40 mM; DTT 2 mM; FD 20 μg/ml; 20 types of amino acids 0.4 mM each; ATP 8 mM; GTP 8 mM; UTP 4 mM; CTP 4 mM. 
     The results are shown in  FIG. 2 .  FIG. 2A  shows the result of using the DNA comprising the EmrE-myc-his sequence (SEQ ID NO: 1), and  FIG. 2B  shows the result of using the DNA containing the GUS sequence (SEQ ID NO: 3). In the liposomes that expressed a membrane protein from the EmrE-myc-his sequence, pH-dependent fluorescence intensity was observed. This result verifies that the membrane protein expressed in the liposomes exerted transport ability. 
     Example 3: Examination on Mg Concentration 
     DNA5 nM comprising a hemolysin sequence, a halo tag protein and a PURE system were enclosed within liposomes. At this time, liposomes were prepared under 9 conditions of Mg concentration of an intraliposomal solution and an extraliposomal solution, which are 18.88, 23.6, 28.32, 33.04, 37.76, 42.28, 47.2, 51.92, 56.64 mM. After the preparation of liposomes, incubation was performed for 16 hours at 37° C. to express hemolysin. 1 μM of Halo Tag Alexa Fluor 488 ligand was added to the extraliposomal solution to measure the function of expressed alpha hemolysin, and after 3 hours, the amount of fluorescence of Halo Tag Alexa Fluor 488 ligand accumulated within the liposomes was measured. As a result, Halo Tag Alexa Fluor 488 ligand was accumulated the most in liposomes that were prepared by the Mg concentration value of 33.04 mM. Accordingly, it was ascertained that the condition for the detection of activity of hemolysin is preferably 18.88 mM-23.6 mM, more preferably 23.6 mM-28.32 mM, and most preferably 28.32-42.48 mM. 
     Example 4: Examination on Lipid Component/Composition-1 
     Instead of the EmrE-myc-his sequence used in Example 1, a sequence encoding hemolysin (SEQ ID NO: 5) was used to express a transporter. Further, a halo tag protein (SEQ ID NO: 7) was used as a factor to which Halo Tag Alexa Fluor 488 ligand, which is the ligand transported by hemolysin, bound. Hemolysin is a membrane protein that creates a pore in a membrane, and hemolysin allows permeation of substances smaller than 3 kDa. Thus, when hemolysin is expressed, a pore is generated in liposomes, and as a result, permeation of Halo Tag Alexa Fluor 488 ligand, which is unable to permeate lipid membranes, is allowed. Halo Tag Alexa Fluor 488 ligand that permeated through the pore binds to the halo tag protein, and as a result, Halo Tag Alexa Fluor 488 ligand that moved into the liposomes accumulate within the liposomes. 
     As a lipid forming liposomes, a mixture of POPC:Chol=9:1, a mixture of POPC:Chol=7:3, a mixture of POPC:Chol=5:5, and a mixture of POPC:Chol=3:7 were used. Further, POPC is an abbreviation of 1-palmitoyl-2-oleoylphosphatidylcholine, and Chol is an abbreviation of cholesterol. As a result, as shown in  FIG. 3 , the percentage of exertion of membrane protein activity in liposomes comprising a DNA raised as the ratio of cholesterol increased. 
     Example 5: Examination on Lipid Component/Composition-2 
     Next, liposomes were synthesized using various lipids by the same technique as Example 4, and the activity of the expressed membrane protein was compared. The results are shown in  FIG. 4 . 
     The vertical axis of  FIG. 4  shows the percentage (%) of liposomes that taken in Halo Tag Alexa Fluor 488 ligand with high intensity among all the liposomes when various lipids were used. The lipids that were used are as follows: EggPC is an abbreviation of phosphatidylcholine purified from a hen&#39;s egg; POPC is an abbreviation of 1-palmitoyl-2-oleoylphosphatidylcholine; PS is an abbreviation of 1-palmitoyl-2-oleoylphosphoserine; PE is an abbreviation of 1-palmitoyl-2-oleoylphosphoethanolamine; and Chol is an abbreviation of cholesterol. PC mix is an abbreviation of the mixture of 1-palmitoyl-2-oleoylphosphatidylcholine:1-palmitoyl-2-linoleoylphosphatidylcholine:1-stearoyl-2-oleoylphosphatidylcholine:1-stearoyl-2-linoleoylphosphatidylcholine=129:67:48:24 (mass ratio); EggPC/PS/PE is an abbreviation of the mixture of each of them at the ratio of 3:1:1 (mass ratio) in order; EggPC/PS/PE/Chol is an abbreviation of the mixture of each of them at the ratio of 2:1:1:1 (mass ratio) in order; PCmix/PS/PE is an abbreviation of the mixture of each of them at the ratio of 3:1:1 (mass ratio) in order; PCmix/PS/PE/Chol is an abbreviation of the mixture of each of them at the ratio of 2:1:1:1 (mass ratio) in order; POPC/PS/PE is an abbreviation of the mixture of each of them at the ratio of 3:1:1 (mass ratio) in order; and POPC/POPE/POPS/Chol is an abbreviation of the mixture of each of them at the ratio of 2:1:1:1 (mass ratio) in order. 
     These results ascertained that change in types of phosphatidylcholine and mixture of a plurality of types, and mixture of 1-palmitoyl-2-oleoylphosphoserine and 1-palmitoyl-2-oleoylphosphoethanolamine do not significantly affect the exertion of activity of hemolysin. 
     Example 6: Concentration of Desired Nucleic Acid 
     An experiment was performed by using wild type hemolysin (SEQ ID NO: 5) and lethal mutation type hemolysin (SEQ ID NO: 8) and by using the same technique as Example 4. The proportion of wild type to lethal mutation type was set to 1:12, and tenfold or more of lethal mutation type were used. Culturing was performed for 160 minutes at 37° C. to express a membrane protein, and then liposomes that showed transport activity were selected by a cell sorter to determine the percentage of wild type genes and mutated genes included in the liposomes. The result was wild type:mutant type=8:1. This result verifies that hundredfold concentration was performed by the screening/selection of the present invention. 
     For example, by selecting a liposome showing a desired property and performing mutation induction (for example, random mutation) on the included DNA (or RNA), selection by a cell sorter can be performed by using the group to which mutation is induced as a starting material. By repeating this procedure, concentration of mutated genes having a desired property is possible. 
     Example 7: Evolutionary Experiment 
     An evolutionary experiment was performed by using the following procedures. 
     1) Liposomes are created by a centrifugal sedimentation method. 
     POPC:Chol=1:1 (wt/wt) was used as the lipid composition. As the composition of the internal solution, the same composition as the cell-free protein synthesis system described in Example 1 (except that the magnesium acetate concentration was changed to 33.04 mM) was used. Further, 100 nM T7 RNA polymerase, 200 mM sucrose, 5 mM β-glucuronidase conjugated halopeptide, 1 mM transferrin conjugated alexa fluor 647, 5 pM DNA (ORF of hemolysin was disposed under the control of a T7 promoter) were used. As the composition of the external solution, a solution containing only a small molecule having the same composition as the cell-free protein synthesis system described in Example 1 (except that the magnesium acetate concentration was changed to 33.04 mM), and 200 mM glucose was used. 
     2) The external solution was replaced to remove the intraliposomal solution that was mixed into the external solution. Centrifugation was performed for 5 minutes at 6000 G, and after the supernatant was thrown away, the precipitation was resuspend with 300 ml of new extraliposomal solution. 
     3) A hemolysin protein was synthesized within the liposomes and the hemolysin protein was presented in the lipid membrane. Incubation was performed for 16 hours at 37° C. 
     4) DNAse was added to degrade the DNA remained in the extraliposomal solution. 4 μl of DNAse (TAKARA recombinant Dnase1) was added to the liposome solution. 
     5) A fluorescent substrate was added to the external environment. 900 ml of new external solution was added to the liposome solution such that the final volume becomes 1.2 ml. The final concentration was set to 2 nM, and Halo Tag Alexa Fluor 488 ligand was added to the external solution. The fluorescence intensity of liposomes was successively measured with a flow cytometer. 
     6) The intake of the fluorescent substrate was suspended by competitive inhibitory substrate that is non-fluorescent and that is permeable to lipid bilayer. When appropriate fluorescence intensity was obtained, final concentration 200 nM halo tag biotin ligand was added to the external solution. 
     7) Concentration of the liposome solution. Centrifugation was performed for 5 minutes at 6000 G, and after the supernatant was thrown away, the precipitation was resuspended with 300 ml of new external solution. 
     8) 10,000 high-intensity liposomes were sorted from the highest intensity value with a cell sorter (BD, FACS Aria 2). 
     9) Genetic information was amplified. The sorted liposome solution was purified by using a simplified DNA purification column (QIAGEN MinElute PCR Purification Kit). Subsequently, PCR was performed for 40 cycles (TOYOBO KOD FX Neo was used for the DNA polymerase). PCR was purified by using the DNA purification column again. Subsequently, a gel band was purified by using agarose electrophoresis (life technologies, E-Gel CloneWell SYBR Safe Gel was used). After performing purification by using the DNA purification column again, PCR was performed again for 20 cycles. The PCR product was purified by DNA purification column again for reuse as the DNA stock of the next cycle. 
     The results are shown in  FIG. 5 .  FIG. 5  is a graph showing the percentage of a group of high-intensity liposomes in which the fluorescence intensity is 260 or over. The upper limit of fluorescence values in which Halo Tag Alexa Fluor 488 ligand adheres to negative-control liposomes not having hemolysin activity is 260. Thus, samples that showed a value over this fluorescence value are samples that showed specific Halo Tag Alexa Fluor 488 ligand intake by hemolysin. 
     It was shown that the percentage of genes having higher activity increased by repeating the cycle of screening/selection. Further, mutation may be introduced after the isolation of the DNA. 
     INDUSTRIAL APPLICABILITY 
     By the use of unilamellar liposomes treated with a nuclease, further highly-efficient screening is enabled, and a gene encoding a membrane protein having a desired function can be selected and obtained. 
     [Sequence Listing Free Text] 
     SEQ ID NO: 1: the nucleotide sequence of EmrE-myc-his 
     SEQ ID NO: 2: the amino acid sequence of EmrE-myc-his 
     SEQ ID NO: 3: the nucleotide sequence of GUS derived from  Escherichia coli    
     SEQ ID NO: 4: the amino acid sequence of GUS derived from  Escherichia coli    
     SEQ ID NO: 5: the nucleotide sequence encoding hemolysin derived from  Staphylococcus aureus    
     SEQ ID NO: 6: the amino acid sequence of hemolysin derived from  Staphylococcus aureus    
     SEQ ID NO: 7: the amino acid sequence of the halo tag protein 
     SEQ ID NO: 8: the nucleotide sequence encoding the lethal mutation type hemolysin derived from  Staphylococcus aureus    
     SEQ ID NO: 9: the amino acid sequence of the lethal mutation type hemolysin derived from  Staphylococcus aureus