Patent Publication Number: US-2023159975-A1

Title: Protein crystal production method and crystalline structure analysis method

Description:
FIELD OF INVENTION 
     The present invention relates to a production method for a protein crystal and a crystal structure analysis method. Priority is claimed on Japanese Patent Application No. 2020-027386, filed Feb. 20, 2020, the content of which is incorporated herein by reference. 
     DESCRIPTION OF RELATED ART 
     A structural analysis of proteins is important for the determination of three-dimensional structures of proteins and studies on the structural and functional correlations. In order to carry out a structural analysis of proteins, it is necessary to produce high-quality protein crystals. The quality of protein crystals has tremendous influence on the accuracy and reliability of protein crystal structure analysis. For this reason, crystallization of a protein is the largest rate-determining step in a structural analysis of the protein. 
     Crystallization of a protein involves many factors such as the concentration of the protein, purity, the type of a buffer solution, pH, the type and concentration of a precipitant, temperature, an organic solvent, metal ions, and the type and concentration of a surfactant. Therefore, in order to produce a high-quality protein crystal and perform a crystal structure analysis, it is necessary to consider numerous conditions, and a great deal of effort is needed (see, for example, Non-Patent Document 1). 
     However, on the surface of protein molecules, there are amino acid side chains having various functional groups with functionality, and the functional groups have unique chemical properties. Further, there are cases where a plurality of protein molecules are regularly arranged to form a cage-like three-dimensional structure (protein cage). 
     For example, polyhedrosis viruses are viruses that infect cells of insects such as silkworms. Polyhedrosis viruses produce, in the late stage of infection, inclusion bodies called polyhedra in infected cells in a large quantity that reach about half the total quantity of cellular proteins, and enclose a large number of virus particles in the inclusion bodies. Polyhedrin, which is a polyhedral protein, is an example of the above-mentioned protein that forms a protein cage. 
     Regarding the protein that forms a protein cage, DNA binding proteins from starved cells (DPS), capsids of viruses containing RNA, and the like are also known in addition to polyhedrin protein. Protein cages formed from these proteins may be regularly arranged to form crystals. 
     For example, Patent Document 1 describes that a modified polyhedrin having at least a portion of the amino acid sequence deleted has a polyhedron-forming ability. Furthermore, it describes that a crystal structure analysis could be performed by using a polyhedron formed from this modified polyhedrin protein. 
     CITATION LIST 
     Patent Document 
     
         
         [Patent Document 1] 
         Japanese Unexamined Patent Application, First Publication No. 2018-033404 
       
    
     Non-Patent Document 
     
         
         [Non-Patent Document 1] 
         ASHIDA, Tamaichi, Let&#39;s Begin X-ray Structure Analysis: (9) Introduction to Protein Crystal Structure Analysis, Journal of the Crystallographic Society of Japan, 38, 378-388, 1996. 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     An object of the present invention is to provide a technology for conveniently producing a crystal of a protein. 
     Solution to Problem 
     The present invention includes the following embodiments. 
     [1] A production method for a crystal of a crystalline protein, the method including: 
     a step of inducing expression of a crystalline protein in  Escherichia coli  into which an expression construct of the crystalline protein has been introduced, and incubating the  Escherichia coli  for a predetermined time until a crystal of the crystalline protein is formed inside the  Escherichia coli.    
     [2] The production method according to [1], 
     in which the  Escherichia coli  further has an expression construct of a non-crystalline protein introduced therein, 
     in the step of incubating, the non-crystalline protein is induced to be expressed together with the crystalline protein, and 
     the crystal formed inside the  Escherichia coli  is a co-crystal of the crystalline protein and the non-crystalline protein. 
     [3] The production method according to [1] or [2], 
     in which the crystalline protein is a protein described in any one of the following items (i) to (iii): 
     (i) a cytoplasmic polyhedral protein, a nuclear polyhedral protein, cathepsin B, ferritin, DNA-binding proteins from starved cells (DPS), luciferase, reovirus nonstructural protein (μNS), fusolin protein (Fusolin), Crystalline inclusion protein A (CipA) or Crystalline inclusion protein B (CipB); 
     (ii) a protein having an amino acid sequence in which one amino acid or a plurality of amino acids have been deleted, substituted, or added in an amino acid sequence of the protein of (i) and having a crystal-forming ability; and 
     (iii) a fusion protein of the protein of (i) or (ii) and a target peptide. 
     [4] The production method according to any one of [1] to [3], in which the crystalline protein is a fusion protein having a target peptide inserted between the 66th amino acid and the 67th amino acid of an amino acid sequence set forth in SEQ ID NO:1, or between an amino acid corresponding to the 66th amino acid of the amino acid sequence set forth in SEQ ID NO:1 and an amino acid corresponding to the 67th amino acid thereof in an amino acid sequence of a protein including an amino acid sequence in which one amino acid or a plurality of amino acids have been deleted, substituted, or added in the amino acid sequence set forth in SEQ ID NO:1 and having a crystal-forming ability. 
     [5] The production method according to any one of [1] to [4], in which the predetermined time is 3 to 30 hours. 
     [6] The production method according to any one of [1] to [5], in which the step of incubating is carried out at 18° C. to 40° C. 
     [7] A crystal structure analysis method, including: 
     a step of subjecting a crystal produced by the production method according to any one of [1] to [6] to an X-ray crystal structure analysis together with the  Escherichia coli.    
     Advantageous Effects of Invention 
     According to the present invention, a technology for conveniently producing a crystal of a protein can be provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1 A  is an optical microphotograph of  Escherichia coli  in which crystals (polyhedra) of CPV-derived polyhedrin were formed inside the bacterial cells in Experimental Example 1. 
         FIG.  1 B  is an optical microphotograph of  Escherichia coli  in which crystals (polyhedra) of CPV-derived polyhedrin were formed inside the bacterial cells in Experimental Example 1. 
         FIG.  1 C  is an optical microphotograph of  Escherichia coli  in which crystals of NPV-derived polyhedrin were formed inside the bacterial cells in Experimental Example 1. 
         FIG.  1 D  is an optical microphotograph of  Escherichia coli  in which crystals of NPV-derived polyhedrin were formed inside the bacterial cells in Experimental Example 1. 
         FIG.  2 A  is a photograph showing the result of observing the crystals of CPV-derived polyhedrin with a scanning electron microscope in Experimental Example 1. 
         FIG.  2 B  is a photograph showing the result of observing the crystals of NPV-derived polyhedrin with a scanning electron microscope in Experimental Example 1. 
         FIG.  3    is a photograph showing the result of SDS-polyacrylamide gel electrophoresis (PAGE) in Experimental Example 1. 
         FIG.  4 A  is a graph showing the result of a MALDI-TOF MS analysis in Experimental Example 1. 
         FIG.  4 B  is a graph showing the result of a MALDI-TOF MS analysis in Experimental Example 1. 
         FIG.  5 A  is a photograph showing the result of observing crystals of a polyhedrin variant with a scanning electron microscope in Experimental Example 2. 
         FIG.  5 B  is a photograph showing the result of observing crystals of a polyhedrin variant with a scanning electron microscope in Experimental Example 2. 
         FIG.  5 C  is a photograph showing the result of observing crystals of a polyhedrin variant with a scanning electron microscope in Experimental Example 2. 
         FIG.  6 A  is a graph showing the result of a MALDI-TOF MS analysis in Experimental Example 2. 
         FIG.  6 B  is a graph showing the result of a MALDI-TOF MS analysis in Experimental Example 2. 
         FIG.  6 C  is a graph showing the result of a MALDI-TOF MS analysis in Experimental Example 2. 
         FIG.  7 A  is a photograph showing the result of observing crystals of polyhedrin with a scanning electron microscope in Experimental Example 3. 
         FIG.  7 B  is a photograph showing the result of observing crystals of polyhedrin with a scanning electron microscope in Experimental Example 3. 
         FIG.  7 C  is a photograph showing the result of observing crystals of polyhedrin with a scanning electron microscope in Experimental Example 3. 
         FIG.  7 D  is a photograph showing the result of observing crystals of polyhedrin with a scanning electron microscope in Experimental Example 3. 
         FIG.  7 E  is a photograph showing the result of observing crystals of polyhedrin with a scanning electron microscope in Experimental Example 3. 
         FIG.  7 F  is a photograph showing the result of observing crystals of polyhedrin with a scanning electron microscope in Experimental Example 3. 
         FIG.  8    is a diagram showing the result of subjecting the crystals of CPV-derived polyhedrin to a structural analysis as remaining in  Escherichia coli  in Experimental Example 4. 
         FIG.  9 A  is a diagram in which a three-dimensional structure of a crystal of polyhedrin produced by  Escherichia coli  and a three-dimensional structure of a crystal of polyhedrin produced by an insect cell are superimposed in Experimental Example 4. 
         FIG.  9 B  is an image showing the result of a three-dimensional structural analysis of the crystals of polyhedrin produced by  Escherichia coli.    
         FIG.  9 C  is an image showing the result of a three-dimensional structural analysis of the crystals of polyhedrin produced by  Escherichia coli.    
         FIG.  10 A  is a confocal fluorescence microphotograph taken in Experimental Example 5. 
         FIG.  10 B  is a confocal fluorescence microphotograph taken in Experimental Example 5. 
         FIG.  11 A  upper part is a photograph showing the result of observing crystals of a polyhedrin variant with a scanning electron microscope in Experimental Example 6, and 
         FIG.  11 A  lower part is a diagram showing the result of subjecting the crystals of the polyhedrin variant to a structural analysis. 
         FIG.  11 B  upper part is a photograph showing the result of observing crystals of a polyhedrin variant with a scanning electron microscope in Experimental Example 6, and  FIG.  11 B  lower part is a diagram showing the result of subjecting the crystals of the polyhedrin variant to a structural analysis. 
         FIG.  12 A  is an optical microphotograph of  Escherichia coli  in which crystals of CipA were formed inside the bacterial cells in Experimental Example 7. 
         FIG.  12 B  is a photograph showing the result of observing the crystals of CipA with a scanning electron microscope in Experimental Example 7. 
         FIG.  13 A  left part is an optical microphotograph of  Escherichia coli  in which crystals of fusion proteins were formed inside the bacterial cells in Experimental Example 7.  FIG.  13 A  right part is a photograph showing the result of observing the crystals of the fusion proteins with a scanning electron microscope in Experimental Example 7. 
         FIG.  13 B  left part is an optical microphotograph of  Escherichia coli  in which crystals of fusion proteins were formed inside the bacterial cells in Experimental Example 7.  FIG.  13 B  right part is a photograph showing the result of observing the crystals of the fusion proteins with a scanning electron microscope in Experimental Example 7. 
         FIG.  13 C  is a graph showing the result of performing a MALDI-TOF MS analysis of the collected crystals of the fusion proteins in Experimental Example 7. 
         FIG.  14 A  is an image obtained by merging an optical microphotograph of crystals of a fusion protein formed inside the bacterial cells of  Escherichia coli  and collected in Experimental Example 9, with a photograph obtained by observing fluorescence of GFP. 
         FIG.  14 B  is a photograph showing the result of observing the crystals of the fusion protein with a scanning electron microscope in Experimental Example 9. 
         FIG.  14 C  is a graph showing the result of performing a MALDI-TOF MS analysis of the collected crystals of the fusion protein in Experimental Example 9. 
         FIG.  15 A  is a diagram showing a three-dimensional structure of a thalidomide-binding domain of CRBN protein. 
         FIG.  15 B  upper part is an optical microphotograph of crystals of fusion proteins synthesized and collected in  Escherichia coli  in Experimental Example 10.  FIG.  15 B  lower part is a photograph showing the result of observing the crystals of the fusion proteins with a scanning electron microscope in Experimental Example 10. 
         FIG.  15 C  upper part is an optical microphotographs of crystals of fusion proteins synthesized and collected in  Escherichia coli  in Experimental Example 10.  FIG.  15 C  lower part is a photograph showing the result of observing the crystals of the fusion proteins with a scanning electron microscope in Experimental Example 10. 
         FIG.  16 A  is a photograph showing the result of observing crystals of fragments of cytoplasmic polyhedral proteins with a scanning electron microscope in Experimental Example 11. 
         FIG.  16 B  is a graph showing the result of performing a MALDI-TOF MS analysis of the collected crystals of the fragments of the cytoplasmic polyhedral proteins in Experimental Example 11. 
         FIG.  16 C  is a photograph showing the result of observing crystals of fragments of cytoplasmic polyhedral proteins with a scanning electron microscope in Experimental Example 11. 
         FIG.  16 D  is a graph showing the result of performing a MALDI-TOF MS analysis of the collected crystals of the fragments of the cytoplasmic polyhedral proteins in Experimental Example 11. 
         FIG.  17 A  is a photograph showing the result of observing crystals of fragments of cytoplasmic polyhedral proteins with a scanning electron microscope in Experimental Example 11. 
         FIG.  17 B  is a graph showing the result of performing a MALDI-TOF MS analysis of the collected crystals of the fragments of the cytoplasmic polyhedral proteins in Experimental Example 11. 
         FIG.  17 C  is a photograph showing the result of observing crystals of fragments of cytoplasmic polyhedral proteins with a scanning electron microscope in Experimental Example 11. 
         FIG.  17 D  is a graph showing the result of performing a MALDI-TOF MS analysis of the collected crystals of the fragments of the cytoplasmic polyhedral proteins in Experimental Example 11. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     [Production Method for Protein Crystal] 
     According to an embodiment, the present invention provides a production method for a crystal of a crystalline protein, the method including a step of inducing expression of a crystalline protein in  Escherichia coli  into which an expression construct of the crystalline protein has been introduced, and incubating the  Escherichia coli  for a predetermined time until a crystal of the crystalline protein is formed inside the  Escherichia coli.    
     The crystalline protein is not particularly limited as long as it is a protein that forms a crystal in  Escherichia coli . Even for a non-crystalline protein that originally does not form a crystal, when the non-crystalline protein becomes capable of forming a crystal in  Escherichia coli  by means of techniques such as chemical modification of the protein, creation of a variant, and a fusion protein, this protein is also included in the crystalline protein according to the present specification. 
     More limited examples of the crystalline protein include the proteins described in any of the following (i) to (iii): 
     (i) a cytoplasmic polyhedral protein, a nuclear polyhedral protein, cathepsin B, ferritin, DPS, luciferase, μNS, Fusolin, CipA, or CipB; 
     (ii) a protein consisting of an amino acid sequence in which one amino acid or a plurality of amino acids have been deleted, substituted, or added in an amino acid sequence of the protein of (i) and having a crystal-forming ability; and 
     (iii) a fusion protein of the protein of (i) or (ii) and a target peptide. 
     The nuclear polyhedral protein is a polyhedrin protein derived from a nuclear polyhedrosis virus (Nucleopolyhedrovirus, NPV), which is a pathogenic virus for nuclear polyhedrosis. The cytoplasmic polyhedral protein is a polyhedrin protein derived from a cytoplasmic polyhedrosis virus (Cypovirus, CPV), which is a pathogenic virus for cytoplasmic polyhedrosis. An amino acid sequence of the cytoplasmic polyhedral protein is set forth in SEQ ID NO:2, and an amino acid sequence of the nuclear polyhedral protein is set forth in SEQ ID NO:3. 
     Cathepsin B is a protease having endopeptidase activity and exopeptidase activity. Cathepsin B is a protein that forms a crystal in cultured insect cells. An amino acid sequence of  Trypanosoma brucei -derived cathepsin B is set forth in SEQ ID NO:4. 
     Ferritin is a protein synthesized by almost all living organisms including algae, bacteria, plants, humans, and non-human animals. Ferritin forms a cage consisting of 24-mers and is responsible for in vivo iron storage by including iron ions. The outer diameter is about 12 nm. 
     Ferritin is not particularly limited, and examples thereof include horse-derived ferritin and human-derived ferritin. An amino acid sequence of horse-derived ferritin L chain is set forth in SEQ ID NO:5, and an amino acid sequence of human-derived ferritin L chain is set forth in SEQ ID NO:6. 
     DPS is a protein belonging to the ferritin superfamily, which is synthesized by many bacteria. DPS forms a cage consisting of 12-mers and protects chromosomal DNA from oxidative stress and the like by including the chromosomal DNA. The outer diameter is about 9 nm. The amino acid sequence of  Escherichia coli -derived DPS is set forth in SEQ ID NO:7. 
     Luciferase is a generic name for enzymes that have the action of catalyzing a chemical reaction in which a luminescent material emits light in bioluminescence of luminescent bacteria, fireflies, and the like. Luciferase is a protein that forms a crystal in insect cells. An amino acid sequence of firefly-derived luciferase is set forth in SEQ ID NO: 8, and an amino acid sequence of sea pansy-derived luciferase is set forth in SEQ ID NO:9. 
     μNS is reovirus nonstructural protein having crystallinity. An amino acid sequence of reovirus-derived μNS is set forth in SEQ ID NO:10. 
     Fusolin is a constituent protein of a crystalline protein inclusion body formed in host cells by Entomopoxvirus. An amino acid sequence of Entomopoxvirus-derived Fusolin is set forth in SEQ ID NO:11. 
     CipA and CipB are constituent proteins of a crystalline protein inclusion body formed in the cytoplasm by  Photorhabdus luminescens , which is an insect pathogenic bacterium. An amino acid sequence of CipA is set forth in SEQ ID NO:16, and an amino acid sequence of CipB is set forth in SEQ ID NO:17. 
     The crystalline protein may be a variant having a mutation in the above-mentioned cytoplasmic polyhedral protein, nuclear polyhedral protein, cathepsin B, ferritin, DPS, luciferase, μNS, Fusolin, CipA, CipB, and the like as long as it has a crystal-forming ability. More specifically, the crystalline protein may be, for example, a protein including an amino acid sequence in which one amino acid or a plurality of amino acids have been deleted, substituted, or added in any of the above-mentioned amino acid sequences set forth in SEQ ID NO:2 to SEQ ID NO:11, SEQ ID NO:16, and SEQ ID NO:17. 
     Here, one amino acid or a plurality of amino acids may be, for example, 1 to 50 amino acids, may be 1 to 40 amino acids for example, may be 1 to 30 amino acids for example, may be 1 to 20 amino acids for example, may be 1 to 10 amino acids for example, may be 1 to 5 amino acids for example, or may be 1 to 3 amino acids for example. As will be described later in the Examples, the inventors have confirmed that even when 38 amino acids of the polyhedrin protein are deleted, the polyhedrin protein has a crystal-forming ability. 
     Furthermore, the crystalline protein may also be the above-mentioned fusion protein of a protein and a target peptide as long as it has the crystal-forming ability. That is, the crystalline protein may also be a fusion protein of the above-mentioned cytoplasmic polyhedral protein, nuclear polyhedral protein, cathepsin B, ferritin, DPS, luciferase, μNS, Fusolin, CipA, CipB, or a variant of any of these crystalline proteins, and a target peptide. 
     Here, the target peptide may be, for example, a peptide as an object whose three-dimensional structure is to be analyzed. As will be described later, the three-dimensional structure of a target peptide can be conveniently analyzed by causing a crystalline protein, which is a fusion protein of the above-mentioned crystalline protein and a target peptide, to be expressed in  Escherichia coli  to form a crystal, and subjecting the crystal of the crystalline protein to an X-ray crystal structure analysis together with the  Escherichia coli.    
     In this case, the target peptide may be any peptide in need of analysis of the three-dimensional structure. The amino acid length of the target peptide is preferably, for example, about 5 to 50 amino acids from the viewpoint that the crystalline protein can maintain the crystal-forming ability. 
     With regard to the fusion protein of a variant of a crystalline protein and a target peptide, the crystalline protein may be the amino acid sequence set forth in SEQ ID NO:1, and the fusion protein may be a fusion protein having the target peptide inserted between the 66th amino acid and the 67th amino acid of the amino acid sequence set forth in SEQ ID NO:1. 
     Alternatively, with regard to the fusion protein of a variant of a crystalline protein and a target peptide, the crystalline protein may be a mutant protein having the amino acid sequence set forth in SEQ ID NO:1. More specifically, the mutant protein having the amino acid sequence set forth in SEQ ID NO:1 may be a protein including an amino acid sequence in which one amino acid or a plurality of amino acids have been deleted, substituted, or added in the amino acid sequence set forth in SEQ ID NO:1 and having a crystal-forming ability. Here, the phrase one amino acid or a plurality of amino acids is the same as that described above. 
     Further, the fusion protein may also be a fusion protein having a target peptide inserted between the amino acid corresponding to the 66th amino acid and the amino acid corresponding to the 67th amino acid of the amino acid sequence set forth in SEQ ID NO:1, in a mutant protein of the protein including the amino acid sequence set forth in SEQ ID NO:1. 
     The amino acid corresponding to the 66th amino acid and the amino acid corresponding to the 67th amino acid of the amino acid sequence set forth in SEQ ID NO:1 can be identified by aligning the amino acid sequence set forth in SEQ ID NO:1 with the amino acid sequence of the mutant protein by using, for example, software such as ClustalW. 
     With regard to the production method of the present embodiment, the  Escherichia coli  further has an expression construct of a non-crystalline protein introduced therein, and in the step of incubating, the non-crystalline protein may be induced to be expressed together with the crystalline protein, while the crystal formed inside the  Escherichia coli  may be a co-crystal of the crystalline protein and the non-crystalline protein. 
     According to the present specification, the term non-crystalline protein usually means a protein that does not form a crystal in  Escherichia coli . Regarding the non-crystalline protein, any protein having a molecular weight of about 1,000 to 100,000 can be used without particular limitation. 
     For example, a co-crystal may also be produced by using an unstable protein such as a membrane protein to produce a co-crystal as the non-crystalline protein. As a result, an unstable protein such as a membrane protein can be easily purified as a co-crystal. Furthermore, an unstable protein such as a membrane protein can be stably stored in the form of a co-crystal. 
     Regarding the non-crystalline protein, one kind thereof may be used alone, or two or more kinds thereof may be used as a mixture. Furthermore, when two or more kinds of the non-crystalline proteins are used, those non-crystalline proteins may form a complex. 
     Furthermore, the non-crystalline protein may be a fusion protein with a portion of the crystalline protein. As a result, there is a tendency that the non-crystalline protein is easily incorporated into the crystal of the crystalline protein, and a co-crystal is easily formed. 
     The expression method for the above-mentioned crystalline protein and non-crystalline protein in  Escherichia coli  is not particularly limited, and the proteins may be expressed by methods that are conventionally carried out. For example, the expression of a crystalline protein or a non-crystalline protein may be induced by incubating  Escherichia coli  into which an expression construct of the crystalline protein or the non-crystalline protein has been introduced, in a medium. 
     The introduction of an expression construct into  Escherichia coli  may be carried out by, for example, introducing an expression vector of a crystalline protein or a non-crystalline protein in the form of a plasmid or the like into  Escherichia coli , or may be carried out by inserting an expression construct of the crystalline protein or the non-crystalline protein into the genome of  Escherichia coli.    
     Furthermore, induction of the expression of the crystalline protein or the non-crystalline protein can be carried out by an appropriate method according to the characteristics of the expression construct of the crystalline protein or the non-crystalline protein. For example, induction of the expression of the crystalline protein or the non-crystalline protein may also be carried out by utilizing an expression control system that utilizes a lactose operon and adding isopropyl β-D-thiogalactopyranoside (IPTG) to the medium. 
     Alternatively, induction of the expression of the crystalline protein or the non-crystalline protein may also be carried out by utilizing an expression control system based on the Tet-on/Tet-off system and adding tetracycline or a derivative thereof into the medium or removing tetracycline or a derivative thereof from the medium. 
     Alternatively, an embodiment in which the expression of the crystalline protein or the non-crystalline protein is initiated at the same time as the initiation of the culture of  Escherichia coli  without implementing the expression induction control, may also be adopted. 
     In the production method of the present embodiment, the expression of the crystalline protein or the non-crystalline protein is induced in  Escherichia coli  into which an expression construct of the crystalline protein or the non-crystalline protein has been introduced. It is preferable that the expression induction be carried out, for example, after the OD 600  of  Escherichia coli  reaches 0.6 to 0.8. 
     Culturing of  Escherichia coli  may be carried out in a small amount of medium of about 10 mL. As will be described later in the Examples, the inventors clarified that a large quantity of crystals of a protein can be produced even with such a small amount of medium, and that a structural analysis of the protein can be carried out by subjecting the crystals to X-ray crystal structure analysis together with  Escherichia coli.    
     Subsequently, after inducing the expression of the crystalline protein or the non-crystalline protein, the  Escherichia coli  is incubated for a predetermined time until a crystal of the crystalline protein or a co-crystal of the crystalline protein and the non-crystalline protein is formed inside the  Escherichia coli . Here, the predetermined time may be 3 to 30 hours and may be, for example, 3 to 24 hours. As will be described later in the Examples, the inventors have clarified that a structural analysis of a protein can be performed by incubating the  Escherichia coli  for merely 20 to 24 hours to form crystals and then performing X-ray crystal structure analysis together with  Escherichia coli.    
     Furthermore, the temperature at the time of incubating until a crystal of the crystalline protein or a co-crystal of the crystalline protein and the non-crystalline protein is formed may be 18° C. to 40° C., may be 25° C. to 38° C., or may be about 30° C. When the temperature at the time of incubating until a crystal of the crystalline protein or a co-crystal of the crystalline protein and the non-crystalline protein is formed is in the above-described range, the quality of the formed protein crystals tends to be enhanced. 
     [Crystal Structure Analysis Method] 
     According to an embodiment, the present invention provides a crystal structure analysis method including a step of subjecting a crystal produced by the above-mentioned production method to an X-ray crystal structure analysis together with the  Escherichia coli.    
     As will be described later in the Examples, the inventors clarified that, surprisingly, a structural analysis of a protein can be performed by subjecting  Escherichia coli  that has formed protein crystals inside the bacterial cells to X-ray crystal structure analysis together with  Escherichia coli . That is, it is possible to perform X-ray crystal structure analysis together with  Escherichia coli  without purifying the protein crystals. According to the method of the present embodiment, a structural analysis of a protein can be performed remarkably conveniently as compared with the related art technologies. 
     Examples 
     Hereinafter, the present invention will be described by way of Examples; however, the present invention is not intended to be limited to the following Examples. 
     Experimental Example 1 
     (Crystallization of Protein in  Escherichia coli  1) 
     A gene encoding CPV-derived polyhedrin (the amino acid sequence is set forth in SEQ ID NO:2) and a gene encoding NPV-derived polyhedrin (the amino acid sequence is set forth in SEQ ID NO:3) were each inserted into a pET29b vector (Merck Millipore Corporation), and expression vectors were produced. Subsequently, each of the expression vectors was used to transform  Escherichia coli  strain BL21. 
     Subsequently, each group of the transformed  Escherichia coli  was inoculated into 10 mL of LB medium and was cultured at 37° C. until the OD 600  reached 0.6 to 0.8. Subsequently, IPTG was added thereto so as to obtain a final concentration of 0.5 mM, and expression of the polyhedrin was induced. Subsequently, the cells were cultured at 30° C. for 20 to 24 hours, and polyhedrin crystals were formed. 
       FIGS.  1 A and  1 B  are optical microphotographs of  Escherichia coli  in which crystals of (polyhedra) of CPV-derived polyhedrin were formed inside the bacterial cells. Furthermore,  FIGS.  1 C and  1 D  are optical microphotographs of  Escherichia coli  in which crystals of NPV-derived polyhedrin were formed inside the bacterial cells. In  FIGS.  1 A to  1 D , the scale bar indicates 10 μm. 
     Subsequently, each group of  Escherichia coli  was collected by centrifugation. Subsequently,  Escherichia coli  was ultrasonically crushed, and crystals of polyhedrin were collected by centrifugation.  FIG.  2 A  is a photograph showing the results of observing the crystals of CPV-derived polyhedrin with a scanning electron microscope.  FIG.  2 B  is a photograph showing the results of observing the crystals of NPV-derived polyhedrin with a scanning electron microscope. In  FIGS.  2 A and  2 B , the scale bar indicates 2 μm. As a result, cubic crystals were observed in all the samples, and it was confirmed that polyhedral crystals can be formed in  Escherichia coli.    
       FIG.  3    is a photograph showing the results obtained by submitting the collected crystals of CPV-derived polyhedrin and the collected crystals of NPV-derived polyhedrin to SDS-polyacrylamide gel electrophoresis (PAGE) and performing Coomassie Brilliant Blue staining. In  FIG.  3   , “M” denotes a molecular weight marker, and “CPV (WT)” denotes a crystal of CPV-derived polyhedrin expressed in the insect cell strain Sf21, which was submitted to SDS-PAGE as a control. 
     As a result, it was confirmed that both the CPV-derived polyhedrin and the NPV-derived polyhedrin expressed in  Escherichia coli  have the predicted molecular weights. 
       FIGS.  4 A and  4 B  are graphs showing the results of performing a MALDI-TOF MS analysis of the collected crystals of CPV-derived polyhedrin and the collected crystals of NPV-derived polyhedrin.  FIG.  4 A  shows the result for CPV-derived polyhedrin, and  FIG.  4 B  shows the result for NPV-derived polyhedrin. In  FIGS.  4 A and  4 B , “Obs” denotes the measured molecular weight, and “Cal” denotes the predicted molecular weight of the full-length protein. 
     As a result, it was confirmed that the CPV-derived polyhedrin and the NPV-derived polyhedrin have the predicted full-length molecular weights. 
     Experimental Example 2 
     (Crystallization of Protein in  Escherichia coli  2) 
     CPV-derived polyhedrin variants were expressed in  Escherichia coli  in the same manner as in Experimental Example 1. Regarding the variants, a variant in which the 29th asparagine (N29) in the amino acid sequence set forth in SEQ ID NO:2 was substituted with serine (hereinafter, referred to as “N29S”; the amino acid sequence is set forth in SEQ ID NO:12), a variant including an amino acid sequence obtained by deleting the 192nd glycine (G192), the 193rd serine (5193), and the 194th alanine (A194) from the amino acid sequence set forth in SEQ ID NO:2 (hereinafter, referred to as “A3”; the amino acid sequence is set forth in SEQ ID NO:13), and a variant including an amino acid sequence obtained by deleting the 67th alanine (A67) to the 104th alanine (A104) from the amino acid sequence set forth in SEQ ID NO:3 (hereinafter, referred to as “A38”; the amino acid sequence is set forth in SEQ ID NO:1) were used. Genes encoding the CPV-derived polyhedrin variants were each inserted into a pET29b vector (Merck Millipore Corporation), and the expression vectors were produced. Subsequently, each of the expression vectors was used to transform  Escherichia coli  strain BL21. 
     Subsequently, each group of the transformed  Escherichia coli  was inoculated into 10 mL of LB medium and was cultured at 37° C. until the OD 600  reached 0.6 to 0.8. Subsequently, IPTG was added thereto so as to obtain a final concentration of 0.5 mM, and expression of the polyhedrin variants was induced. Subsequently, the cells were cultured at 30° C. for 20 to 24 hours, and crystals of the polyhedrin variants were formed. 
     Subsequently, each group of  Escherichia coli  was collected by centrifugation. Subsequently,  Escherichia coli  was ultrasonically crushed, and crystals of the polyhedrin variants were collected by centrifugation.  FIGS.  5 A to  5 C  are photographs showing the results of observing the crystals of the polyhedrin variants with a scanning electron microscope.  FIG.  5 A  is a photograph showing the result of observing the crystals of the variant (N29S) with a scanning electron microscope,  FIG.  5 B  is a photograph showing the result of observing the crystals of the variant (Δ3) with a scanning electron microscope, and  FIG.  5 C  is a photograph showing the result of observing the crystals of the variant (Δ38) with a scanning electron microscope. In  FIGS.  5 A to  5 C , the scale bar indicates 2 μm. As a result, crystals were observed in all the samples, and it was confirmed that crystals of the polyhedrin variants can be formed in  Escherichia coli.    
       FIGS.  6 A to  6 C  are graphs showing the results of performing a MALDI-TOF MS analysis of the collected crystals of the polyhedrin variants.  FIG.  6 A  shows the result for the variant (N29S),  FIG.  6 B  shows the result for the variant (Δ3), and  FIG.  6 C  shows the result for the variant (Δ38). As a result, it was confirmed that each of the polyhedrin variants had the predicted full-length molecular weight. 
     Experimental Example 3 
     (Crystallization of Protein in  Escherichia coli  3) 
     CPV-derived polyhedrin (the amino acid sequence is set forth in SEQ ID NO:2) was expressed in  Escherichia coli  strain BL21 in the same manner as in Experimental Example 1, and crystals were formed. Here, the temperature of crystallization was set to 30° C. or 37° C. 
     Specifically, first, the transformed  Escherichia coli  was inoculated into 10 mL of LB medium and was cultured at 37° C. until the OD 600  reached 0.6 to 0.8. Subsequently, IPTG was added thereto so as to obtain a final concentration of 0.5 mM, and expression of the polyhedrin was induced. Subsequently, the cells were cultured at 30° C. or 37° C. for 20 to 24 hours, and crystals of polyhedrin were formed. 
     Subsequently, each group of  Escherichia coli  was collected by centrifugation. Subsequently,  Escherichia coli  was ultrasonically crushed, and crystals of polyhedrin were collected by centrifugation.  FIGS.  7 A to  7 C  are photographs showing the results of observing the crystals of polyhedrin crystallized at 30° C. with a scanning electron microscope. Furthermore,  FIGS.  7 D to  7 F  are photographs showing the results of observing the crystals of polyhedrin crystallized at 37° C. with a scanning electron microscope. As a result, it was clarified that polyhedrin can be crystallized at any temperature. In addition, a tendency was recognized that the quality of the crystals of polyhedrin is enhanced by performing crystallization at a lower temperature. 
     Experimental Example 4 
     (Structural Analysis of Crystals of Polyhedrin) 
       Escherichia coli  strain BL21 having an expression vector of the CPV-derived polyhedrin (the amino acid sequence is set forth in SEQ ID NO:2) produced in Experimental Example 1 was inoculated into 10 mL of LB medium and was cultured at 37° C. until the OD 600  reached 0.6 to 0.8. Subsequently, IPTG was added thereto so as to obtain a final concentration of 0.5 mM, and expression of the polyhedrin was induced. Subsequently, the cells were cultured at 30° C. for 20 to 24 hours, and polyhedrin crystals were formed. 
     Subsequently, this  Escherichia coli  was subjected to X-ray crystal structure analysis together with the bacterial cells. For the X-ray crystal structure analysis, SPring-8 BL32XU was used. As a result, a structural analysis at a resolution of 1.8 Å was successfully achieved.  FIG.  8    is a diagram showing the results of subjecting the crystals of CPV-derived polyhedrin to a structural analysis as remaining in  Escherichia coli.    
     Subsequently, X-ray crystal structure analysis was carried out in the same manner as in Experimental Example 1, by using the crystals of polyhedrin purified from  Escherichia coli . For the X-ray crystal structure analysis, SPring-8 BL32XU was used. For comparison, crystals of polyhedrin produced in insect cell strain Sf21 and purified were also similarly subjected to X-ray crystal structure analysis. As a result, a structural analysis of the purified crystals at a resolution of 1.9 Å was successfully achieved. 
       FIG.  9 A  is a diagram in which the three-dimensional structure of a crystal of polyhedrin produced by  Escherichia coli  and the three-dimensional structure of a crystal of polyhedrin produced by the insect cells are superimposed. As a result, it was confirmed that the two coincide. 
       FIGS.  9 B and  9 C  are images showing the results of a three-dimensional structural analysis of the crystals of polyhedrin produced in  Escherichia coli . In  FIGS.  9 B and  9 C , “GTP” denotes guanosine triphosphate, “ATP” denotes adenosine triphosphate, and “CTP” denotes cytosine triphosphate. As a result, as shown in  FIGS.  9 B and  9 C , it was confirmed that a nucleic acid is bound to the three-dimensional structure of the crystal of polyhedrin produced in  Escherichia coli . This result further supports that the crystal of polyhedrin produced in  Escherichia coli  is equivalent to the crystal of polyhedrin produced in an insect cell. 
     Experimental Example 5 
     (Crystallization of Protein in  Escherichia coli  4) 
     A gene encoding CPV-derived polyhedrin (the amino acid sequence is set forth in SEQ ID NO:2) was inserted into a pET29b vector (Merck Millipore Corporation), and an expression vector was produced. Furthermore, a gene encoding green fluorescent protein (sfGFP), which is a non-crystalline protein, was inserted into a pET21c vector (Merck Millipore Corporation), and an expression vector was produced. 
     sfGFP was used in the form of a fusion protein (the amino acid sequence is set forth in SEQ ID NO:15) of the H1 region of CPV-derived polyhedrin (the amino acid sequence is set forth in SEQ ID NO:14) and sfGFP. Subsequently, each of the expression vectors was mixed and used to transform  Escherichia coli  strain BL21. 
     Subsequently, the transformed  Escherichia coli  was inoculated into 10 mL of LB medium and was cultured at 37° C. until the OD 600  reached 0.6 to 0.8. Subsequently, IPTG was added thereto so as to obtain a final concentration of 0.5 mM, expression of the polyhedrin and sfGFP was induced, and the proteins were co-expressed. Subsequently, the transformed cells were cultured at 30° C. for 20 to 24 hours, and co-crystals of polyhedrin and sfGFP were formed. 
     Subsequently, each group of  Escherichia coli  was collected by centrifugation. Subsequently,  Escherichia coli  was ultrasonically crushed, and crystals of polyhedrin were collected by centrifugation.  FIGS.  10 A and  10 B  are representative photographs showing the results of observing co-crystals of polyhedrin and sfGFP crystallized at 30° C. with a confocal fluorescence microscope. In  FIGS.  10 A and  10 B , a bright field image and a fluorescence image of sfGFP are superimposed and displayed. In  FIGS.  10 A and  10 B , regions where the fluorescence of sfGFP was detected are indicated by dots. 
     As a result, it was confirmed that the formed crystals emit fluorescence of sfGFP. This result indicates that a co-crystal of polyhedrin and sfGFP was formed. 
     Experimental Example 6 
     (Structural Analysis of Crystals of Polyhedrin Variants) 
     An X-ray crystal structure analysis of the crystal of the CPV-derived polyhedrin variant (Δ3) and the crystal of the CPV-derived polyhedrin variant (Δ38), which were purified in Experimental Example 2, was performed. For the X-ray crystal structure analysis, SPring-8 BL32XU was used. 
       FIG.  11 A  upper part is a photograph showing the result of observing the crystals of the variant (Δ3) with a scanning electron microscope, and  FIG.  11 A  lower part is a diagram showing the result of a structural analysis of the crystal of the variant (Δ3). As a result, regarding the crystal of the purified variant (Δ3), a structural analysis at a resolution of 2.3 Å was successfully achieved. 
     Furthermore,  FIG.  11 B  upper part is a photograph showing the result of observing the crystals of the variant (Δ38) with a scanning electron microscope, and  FIG.  11 B  lower part is a diagram showing the result of a structural analysis of the crystal of the variant (Δ38). As a result, regarding the crystal of the purified variant (Δ38), a structural analysis at a resolution of 2.5 Å was successfully achieved. 
     Experimental Example 7 
     (Crystallization of Protein in  Escherichia coli  5) 
     A gene encoding Crystalline inclusion protein A (CipA) was inserted into a pET29b vector (Merck Millipore Corporation), and an expression vector was produced. Subsequently, the expression vector was used to transform  Escherichia coli  strain BL21. 
     Subsequently, the transformed  Escherichia coli  was inoculated into 10 mL of LB medium and was cultured at 37° C. until the OD 600  reached 0.6 to 0.8. Subsequently, IPTG was added thereto so as to obtain a final concentration of 0.5 mM, and expression of CipA was induced. Subsequently, the transformed cells were cultured at 30° C. for 20 to 24 hours, and crystals of CipA were formed.  FIG.  12 A  is an optical microphotograph of  Escherichia coli  in which crystals of CipA were formed inside the bacterial cells. The scale bar indicates 10 μm. 
     Subsequently,  Escherichia coli  was collected by centrifugation. Subsequently,  Escherichia coli  was ultrasonically crushed, and crystals of CipA were collected by centrifugation.  FIG.  12 B  is a photograph showing the results of observing the crystals of CipA with a scanning electron microscope. The scale bar indicates 2 μm. 
     Subsequently, an X-ray crystal structure analysis of the purified crystals of CipA was performed. For the X-ray crystal structure analysis, SPring-8 BL32XU was used. As a result, diffraction data at a resolution of 2.8 Å could be acquired. 
     From the above-described results, it was clarified that crystals of CipA can be formed in  Escherichia coli.    
     Experimental Example 8 
     (Crystallization of Protein in  Escherichia coli  6) 
     A gene encoding a fusion protein of ubiquitin (hereinafter, may be referred to as “Ubq”) and a cytoplasmic polyhedral protein (hereinafter, may be referred to as “PhM”) was inserted into a pET29b vector (Merck Millipore Corporation), and an expression vector was produced. 
     Expression vectors were produced for two kinds of fusion proteins, namely, a fusion protein in which Ubq was located on the N-terminal side (hereinafter, may be referred to as “Ubq-PhM”) and a fusion protein in which PhM was located on the N-terminal side (hereinafter, may be referred to as “PhM-Ubq”). In both of Ubq-PhM and PhM-Ubq, a linker “GGGS (SEQ ID NO:18)” was inserted between Ubq and PhM. An amino acid sequence of Ubq-PhM is set forth in SEQ ID NO:19, and an amino acid sequence of PhM-Ubq is set forth in SEQ ID NO:20. 
     Subsequently, each of the expression vectors was used to transform  Escherichia coli  strain BL21. Subsequently, each group of the transformed  Escherichia coli  was inoculated into 10 mL of LB medium and was cultured at 37° C. until the OD 600  reached 0.6 to 0.8. Subsequently, IPTG was added thereto so as to obtain a final concentration of 0.5 mM, and expression of both Ubq-PhM and PhM-Ubq was induced. Subsequently, the transformed cells were cultured at 30° C. for 20 to 24 hours, and crystals of both Ubq-PhM and PhM-Ubq were formed. 
       FIG.  13 A  left part is an optical microphotograph of  Escherichia coli  in which crystals of Ubq-PhM were formed inside the bacterial cells, and  FIG.  13 B  left part is an optical microphotograph of  Escherichia coli  in which crystals of PhM-Ubq were formed inside the bacterial cells. The scale bar indicates 10 μm. 
     Subsequently, each group of  Escherichia coli  was collected by centrifugation. Subsequently, each group of  Escherichia coli  was ultrasonically crushed, and crystals of each of Ubq-PhM and PhM-Ubq were collected by centrifugation.  FIG.  13 A  right part is a photograph showing the result of observing the crystals of Ubq-PhM with a scanning electron microscope, and  FIG.  13 B  right part is a photograph showing the result of observing the crystals of PhM-Ubq with a scanning electron microscope. The scale bar indicates 2 μm. 
       FIG.  13 C  is a graph showing the results of performing a MALDI-TOF MS analysis of each of the collected crystals of Ubq-PhM and PhM-Ubq.  FIG.  13 C  upper part shows the result for Ubq-PhM, and  FIG.  13 C  lower part shows the result for PhM-Ubq. In  FIG.  13 C , “Cal” denotes the predicted molecular weight of each of the fusion proteins. As a result, it was confirmed that each of Ubq-PhM and PhM-Ubq had the predicted molecular weight. 
     From the above-described results, it was clarified that crystals of Ubq-PhM and PhM-Ubq can be formed in  Escherichia coli.    
     Experimental Example 9 
     (Crystallization of Protein in  Escherichia coli  7) 
     A gene encoding a fusion protein of green fluorescent protein (hereinafter, may be referred to as “GFP”) and a cytoplasmic polyhedral protein (hereinafter, may be referred to as “PhM”) was inserted into a pET29b vector (Merck Millipore Corporation), and an expression vector was produced. 
     In the produced fusion protein, a linker “GGGS (SEQ ID NO:18)” was inserted between GFP and PhM, where GFP was located on the N-terminal side (hereinafter, may be referred to as “GFP-PhM”). The amino acid sequence of GFP-PhM is set forth in SEQ ID NO:21. 
     Subsequently, the expression vector was used to transform  Escherichia coli  strain BL21. Subsequently, the transformed  Escherichia coli  was inoculated into 10 mL of LB medium and was cultured at 37° C. until the OD 600  reached 0.6 to 0.8. Subsequently, IPTG was added thereto so as to obtain a final concentration of 0.5 mM, and expression of GFP-PhM was induced. Subsequently, the transformed cells were cultured at 30° C. for 20 to 24 hours, and crystals of GFP-PhM were each formed. 
     Subsequently,  Escherichia coli  was collected by centrifugation. Subsequently,  Escherichia coli  was ultrasonically crushed, and crystals of GFP-PhM were collected by centrifugation. 
       FIG.  14 A  is an image obtained by merging an optical microphotograph of the collected crystals of GFP-PhM with a photograph obtained by observing the fluorescence of GFP. The scale bar indicates 10 μm.  FIG.  14 B  is a photograph showing the result of observing the crystals of GFP-PhM with a scanning electron microscope. The scale bar indicates 5 μm. 
       FIG.  14 C  is a graph showing the result of performing a MALDI-TOF MS analysis of the collected crystals of GFP-PhM. In  FIG.  14 C , “SfGFP-WTPhC 2+ ” denotes a divalent ion of SfGFP-WTPhC (GFP-PhM), and “SfGFP-WTPhC + ” denotes a monovalent ion of SfGFP-WTPhC (GFP-PhM). Furthermore, “Cal” denotes the predicted molecular weight of the fusion protein. As a result, it was confirmed that GFP-PhM has the predicted molecular weight. 
     From the above-described results, it was clarified that crystals of GFP-PhM can be formed in  Escherichia coli.    
     Experimental Example 10 
     (Crystallization of protein in  Escherichia coli  8) 
     A gene encoding a fusion protein of a thalidomide-binding domain (hereinafter, may be referred to as “TBD”) of cereblon (CRBN) protein and a cytoplasmic polyhedral protein (hereinafter, may be referred to as “PhM”) was inserted into a pET29b vector (Merck Millipore Corporation), and an expression vector was produced. 
     Expression vectors were produced for the two kinds of fusion proteins, namely, a fusion protein in which TBD was located on the N-terminal side (hereinafter, may be referred to as “TBD-PhM”) and a fusion protein in which PhM was located on the N-terminal side (hereinafter, may be referred to as “PhM-TBD”). In both of TBD-PhM and PhM-TBD, a linker “GGGS (SEQ ID NO:18)” was inserted between TBD and PhM. An amino acid sequence of TBD-PhM is set forth in SEQ ID NO:22, and an amino acid sequence of PhM-TBD is set forth in SEQ ID NO:23. 
     Subsequently, each of the expression vectors was used to transform  Escherichia coli  strain BL21. Subsequently, each group of the transformed  Escherichia coli  was inoculated into 10 mL of LB medium and was cultured at 37° C. until the OD 600  reached 0.6 to 0.8. Subsequently, IPTG was added thereto so as to obtain a final concentration of 0.5 mM, and expression of both TBD-PhM and PhM-TBD was induced. Subsequently, the transformed cells were cultured at 30° C. for 20 to 24 hours, and crystals of both TBD-PhM and PhM-TBD were formed. 
     Subsequently, each group of  Escherichia coli  was collected by centrifugation. Subsequently, each group of  Escherichia coli  was ultrasonically crushed, and crystals of each of TBD-PhM and PhM-TBD were collected by centrifugation. 
       FIG.  15 A  is a diagram showing a three-dimensional structure of TBD (thalidomide-binding domain of CRBN protein).  FIG.  15 B  upper part is an optical microphotograph of the collected crystals of TBD-PhM, and  FIG.  15 C  upper part is an optical microphotograph of the collected crystals of PhM-TBD. Furthermore,  FIG.  15 B  lower part is a photograph showing the result of observing the crystals of TBD-PhM with a scanning electron microscope, and  FIG.  15 C  lower part is a photograph showing the result of observing the crystals of PhM-TBD with a scanning electron microscope. The scale bar indicates 2 μm. 
     From the above-described results, it was clarified that crystals of TBD-PhM and PhM-TBD can be formed in  Escherichia coli.    
     Experimental Example 11 
     (Crystallization of Protein in  Escherichia coli  9) 
     A gene encoding a fragment of a cytoplasmic polyhedral protein was inserted into a pET29b vector (Merck Millipore Corporation), and an expression vector was produced. 
     As the fragment of the cytoplasmic polyhedral protein, expression vectors were produced for four kinds of fragments, namely, a fragment consisting of the 1st to 114th amino acids of the amino acid sequence of the cytoplasmic polyhedral protein set forth in SEQ ID NO:2 (hereinafter, may be referred to as “fragment 1: M1 to S114”), a fragment consisting of the 1st to 155th amino acids of the amino acid sequence of SEQ ID NO:2 (hereinafter, may be referred to as “fragment 2: M1 to R155”), a fragment consisting of the 116th to 248th amino acids of the amino acid sequence of SEQ ID NO:2 (hereinafter, may be referred to as “fragment 3: S116 to Q248”), and a fragment consisting of the 58th to 248th amino acids of the amino acid sequence of SEQ ID NO:2 (hereinafter, may be referred to as “fragment 4: K58 to Q248”). 
     An amino acid sequence of fragment 1: M1 to S114 is set forth in SEQ ID NO:24, an amino acid sequence of fragment 2: M1 to R155 is set forth in SEQ ID NO:25, an amino acid sequence of fragment 3: S116 to Q248 is set forth in SEQ ID NO:26, and an amino acid sequence of fragment 4: K58 to Q248 is set forth in SEQ ID NO:27. 
     Subsequently, each of the expression vectors was used to transform  Escherichia coli  strain BL21. Subsequently, each group of the transformed  Escherichia coli  was inoculated into 10 mL of LB medium and was cultured at 37° C. until the OD 600  reached 0.6 to 0.8. Subsequently, IPTG was added thereto so as to obtain a final concentration of 0.5 mM, and expression of each of the fragment 1 to fragment 4 was induced. Subsequently, the transformed cells were cultured at 30° C. for 20 to 24 hours, and each of the crystals of the fragment 1 to fragment 4 was formed. 
     Subsequently, each group of  Escherichia coli  was collected by centrifugation. Subsequently, each group of  Escherichia coli  was ultrasonically crushed, and crystals of the fragment 1 to fragment 4 were each collected by centrifugation.  FIG.  16 A  is a photograph showing the result of observing the crystals of fragment 1: M1 to S114 with a scanning electron microscope, and  FIG.  16 C  is a photograph showing the result of observing the crystals of fragment 2: M1 to R155 with a scanning electron microscope. The scale bar indicates 2 μm. 
     Furthermore,  FIG.  17 A  is a photograph showing the result of observing the crystals of fragment 3: S116 to Q248 with a scanning electron microscope, and  FIG.  17 C  is a photograph showing the result of observing the crystals of fragment 4: K58 to Q248 with a scanning electron microscope. The scale bar indicates 5 μm. Furthermore,  FIG.  16 B  is a graph showing the result of performing a MALDI-TOF MS analysis of the collected crystals of fragment 1: M1 to S114,  FIG.  16 D  is a graph showing the result of performing a MALDI-TOF MS analysis of the collected crystals of fragments 2: M1 to R155,  FIG.  17 B  is a graph showing the result of performing a MALDI-TOF MS analysis of the collected crystals of fragment 3: S116 to Q248, and  FIG.  17 D  is a graph showing the result of performing a MALDI-TOF MS analysis of the collected crystals of fragment 4: K58 to Q248. In  FIG.  16    and  FIG.  17   , “Cal” denotes the predicted molecular weight of each fragment. As a result, it was confirmed that each of the fragment 1 to the fragment 4 has the predicted molecular weight. 
     From the above-described results, it was clarified that crystals of fragments of a cytoplasmic polyhedral protein can be formed in  Escherichia coli.    
     INDUSTRIAL APPLICABILITY 
     According to the present invention, a technology for conveniently producing a crystal of a protein can be provided.