Patent Publication Number: US-2019194403-A1

Title: Molded Article and Method for Producing Molded Article

Description:
TECHNICAL FIELD 
     The present invention relates to a molded article and a method for producing a molded article. 
     BACKGROUND ART 
     Attempts are being made to replace metallic materials with organic materials for the purpose of weight reduction, cost reduction, the facilitation of molding, and the like. As such organic materials, phenolic resins having a high hardness are often used, and, in order to further increase the bending elastic modulus or the bending strength, a method in which a phenolic resin fiber is added to a matrix resin containing a phenolic resin is known (for example, refer to Patent Literature 1). In addition, due to a rising demand for environmental protection, bioplastics which are non-petroleum-based materials have been drawing attention, and, for example, it is known that a molded article having a bending elastic modulus of 4.5 GPa can be obtained by resinifying silk powder (for example, refer to Non-Patent Literature 1). 
     CITATION LIST 
     Patent Literature 
     
         
         [Patent Literature 1] IP 2014-80491 A 
       
    
     Non Patent Literature 
     
         
         [Non-Patent Literature 1] Shinji Hirai, Function &amp; Materials, June 2014, “Environment-conscious silk and wool resins using waste-derived animal protein” 
         [Non-Patent Literature 2] Science, 2002, Vol. 295, pp. 472 to 476 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the material as disclosed by Patent Literature 1 is a material exhibiting a stiffening effect by containing a fiber (reinforcement fiber) in the matrix resin, and it is difficult to achieve a high strength with the matrix resin alone. In addition, it is not possible to obtain a molded article having a bending elastic modulus of more than 4.5 GPa using a biodegradable material. 
     Therefore, an object of the present invention is to provide a biodegradable molded article having an excellent bending elastic modulus and an excellent bending strength and a method for producing the same. 
     Solution to Problem 
     The present invention provides the following [1] to [9]. 
     [1] A method for producing a molded article, comprising a step 1 of adding water to a composition including a structural protein, a step 2 of molding a water-containing composition obtained in the step 1, and a step 3 of drying the molded composition obtained in the step 2 to obtain the molded article. 
     [2] The method according to [1], in which the step 2 comprises hot pressing. 
     [3] The method according to [1] or [2], in which the structural protein comprises a spider silk protein or a recombinant polypeptide thereof. [4] A molded article which is obtained by using the method according to any one of [1] to [3]. 
     [5] A molded article which is a molded article of a composition including a structural protein and has a bending elastic modulus of more than 6.9 GPa. 
     [6] The molded article according to [5] which is a hot press-molded article. 
     [7] The molded article according to [5] or [6], in which the structural protein includes a spider silk protein or a recombinant polypeptide thereof. 
     [8] The molded article according to any one of [5] to [7], in which a bending strength is 140 MPa or more. 
     [9] The molded article according to any one of [5] to [8], in which a Vickers hardness is 45 HV or more. 
     The molded article has characteristics of being biodegradable, being obtained from a structural protein and/or a recombinant polypeptide thereof as a raw material, and being formed by molding the raw material. The molded article exhibits an extremely high bending elastic modulus (for example, more than 6.9 GPa) even without using an additive material such as a reinforced fiber due to the above-described characteristics. Furthermore, the molded article can also be imparted with transparency and thus has an advantage of having a range of applicable uses which can be extremely broadening compared with resins which have a high strength but are not transparent such as a phenolic resin or polyether ether ketone (PEEK). In addition, the structural protein allows a variety of inheritable genetic modifications, and thus it is possible to easily optimize the performance in accordance with the final uses. 
     In a manufacturing process of the molded article, water is added to the composition, and the composition is molded and then dried, whereby a molded article having an excellent bending elastic modulus and an excellent bending strength is produced. The molded article refers to an article or the like molded using a casting mold (mold), but hot pressing provides a superior bending elastic modulus to the molded article. 
     Advantageous Effects of Invention 
     According to the present invention, a biodegradable molded article exhibiting an excellent bending elastic modulus and an excellent bending strength even without using an additive material such as a reinforced fiber and a method for producing the same are provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a press molding machine. 
         FIG. 2  illustrates schematic cross-sectional views of the press molding machine (a) before the introduction of a composition, (b) immediately after the introduction of the composition, and (c) with the composition in a state of being hot pressed. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a preferred embodiment of the present invention will be described. However, the present invention is not limited to the following embodiment by any means. 
     A molded article according to the present embodiment is obtained by introducing a composition including a structural protein to a casting mold (mold) and molding the composition. 
     [Structural Protein] 
     The structural protein refers to a protein having a role of building a living structure and is different from a functional protein such as an enzyme, a hormone, or an antibody. Examples of the structural protein include natural structural proteins such as fibroin, collagen, resilin, elastin, and keratin which occur in nature. As fibroin occurring in nature, fibroin produced by insects and spiders are known. In the present embodiment, the structural protein preferably includes a spider silk protein. 
     Examples of the fibroin produced by insects include silk proteins produced by silkworms such as  Bombyx mori, Bombyx mandarina, Antheraea yamamai, Anteraea pernyi, Eriogyna pyretorum, Pilosamia Cynthia ricini, Sarnia cynthia, Caligura japonica, Antheraea mylitta , and  Antheraea assama  and hornet silk proteins discharged by the larvae of  Vespa simillima xanthoptera.    
     More specific examples of the fibroin produced by insects include silkworm fibroin L chain (GenBank accession number M76430 (base sequence), AAA27840.1 (amino-acid sequence)). 
     Spiders have a maximum of seven types of silk glands from which various types of fibroin (spider silk protein) having different properties are produced respectively. Spider silk protein is called with various names of major ampullate spider protein (MaSp) having a high toughness, minor ampullate spider protein (MiSp) having a high elongation power, flagelliform (Flag), tubuliform, aggregate, aciniform, and pyriform depending on the source organs. 
     Examples of the fibroin produced by spiders include spider silk proteins produced by spiders belonging to  Araneus  such as  Araneus ventricosus , diadem spiders,  Araneus pinguis, Araneus pentagrammicus , and  Araneus nojimai , spiders belonging to  Neoscona  such as  Neoscona Scylla, Neoscona nautica, Neoscona adianta , and  Neoscona scylloides , spiders belonging to  Pronus  such as  Pronous minutus , spiders belonging to  Cyrtarachne  such as  Cyrtarachne bufo  and  Gyrtarachne inaequalis , spiders belonging to  Gasteracantha  such as  Gasteracantha kuhlii  and  Gasteracantha mammosa , spiders belonging to  Ordgarius  such as  Ordgarius hobsoni  and  Ordgarius sexspinosus , spiders belonging to  Argiope  such as  Argiope amoena, Argiope minuta , and  Argiope bruennichii , spiders belonging to  Arachnura  such as  Arachnura logio Yaginuma , spiders belonging to  Acusilas  such as  Acusilas coccineus , spiders belonging to  Cytophora  such as  Cyrtophora moluccensis, Cyrtophora exanthematica , and  Cyrtophora unicolor , spiders belonging to  Poltys  such as  Poltys illepidus , spiders belonging to  Cyclosa  such as  Cyclosa octotuberculata, Cyclosa sedeculata Karsch, Cyclosa vallata , and  Cyclosa atrata , and spiders belonging to  Chorizopes  such as  Chorizopes nipponicus  and spider silk proteins produced by spiders belonging to  Tetragnatha  such as  Tetragnatha praedonia, Tetragnatha maxillosa, Araneus viridiventris Yaginuma , and  Tetragnatha squamata , spiders belonging to  Leucauge  such as  Leucauge magnifica, Leucauge blanda , and  Leucauge subblanda , spiders belonging to  Nephila  such as  Nephila clavata  and  Nephila pilipes , spiders belonging to  Menosira  such as  Menosira ornate , spiders belonging to  Dyschiriognatha  such as  Dyschiriognatha tenera , spiders belonging to  Latrodectus  such as  Latrodectus mactans, Latrodectus hasselti, Latrodectus geometricus , and  Latrodectus tredecimguttatus , and spiders belonging to  Tetragnathidae  such as spiders belonging to  Euprosthenops . Examples of the spider silk proteins include drag line proteins such as MaSp (MaSp1 and MaSp2) and ADF (ADF3 and ADF4), MiSp (MiSp1 and MiSp2), and the like. 
     More specific examples of the fibroin produced by spiders include fibroin-3 (adf-3) [derived from  Araneus diadematus ] (GenBank accession number AAC47010 (amino-acid sequence), U47855 (base sequence)), fibroin-4 (adf-4) [derived from  Araneus diadematus ] (GenBank accession number AAC47011 (amino-acid sequence), U47856 (base sequence)), dragline silk protein spidroin 1 [derived from  Nephila clavipes ] (GenBank accession number AAC04504 (amino-acid sequence), U37520 (base sequence)), major angullate spidroin 1 [derived from  Latrodectus hesperus ] (GenBank accession number ABR68856 (amino-acid sequence), EF595246 (base sequence)), dragline silk protein spidroin 2 [derived from  Nephila clavata ] (GenBank accession number AAL32472 (amino-acid sequence), AF441245 (base sequence)), major anpullate spidroin 1 [derived from  Euprosthenops australis ] (GenBank accession number CAJ00428 (amino-acid sequence), M973155 (base sequence)), and major ampullate spidroin 2 [derived from  Euprosthenops australis ] (GenBank accession number CAM32249.1 (amino-acid sequence), AM490169 (base sequence)), minor ampullate silk protein 1 [ Nephila clavipes ] (GenBank accession number AAC14589.1 (amino-acid sequence)), minor ampullate silk protein 2 [ Nephila clavipes ] (GenBank accession number AAC14591.1 (amino-acid sequence)), minor ampullate spidroin-like protein [ Nephilengys cruentata ] (GenBank accession number ABR37278.1 (amino-acid sequence)), and the like. 
     More specific examples of the naturally occurring fibroin further include fibroin having a SEQ ID No. registered in NCBI GenBank. The fibroin having a SEQ ID No. registered in NCBI GenBank can be confirmed by, for example, extracting, from sequences including INV as DIVISION among sequence information registered in NCBI GenBank, sequences for which spidroin, ampullate, fibroin, ‘silk and polypeptide’, or ‘silk and protein’ is described as the key word in DEFINITION, character strings of a specific product from CDS, and sequences for which a specific character string is described in TISSUE TYPE from SOURCE 
     The structural protein may be a polypeptide derived from the natural structural protein, that is, a recombinant polypeptide. For example, recombinant fibroin is produced in several foreign protein production systems, and, as a method for producing the recombinant fibroin, transgenic goats, transgenic silkworms, or recombinant plant or mammalian cells are used (refer to Non-Patent Literature 2). 
     The recombinant fibroin can be obtained by, for example, deleting one or a plurality of sequences that code an (A) n  motif from the genetic sequence of a cloned naturally occurring fibroin. In addition, the recombinant fibroin can also be obtained by, for example, designing an amino-acid sequence corresponding to an amino-acid sequence of naturally occurring fibroin from which one or a plurality of (A) n  motifs is deleted and chemically synthesizing a nucleic acid that codes the designed amino-acid sequence. In any cases, in addition to the modification corresponding to the deletion of the (A) n  motifs from the amino-acid sequence of naturally occurring fibroin, furthermore, the modification of the amino-acid sequence corresponding to the substitution, deletion, insertion, and/or addition of one or a plurality of amino-acid residues may be carried out. The substitution, deletion, insertion, and/or addition of amino-acid residues can be carried out using a method well known to a person skilled in the art such as site-directed mutagenesis. Specifically, these can be carried out according to the method described in Nucleic Acid Res. 10, 6487 (1982), Methods in Enzymology, 100, 448 (1983), or the like. 
     A recombinant polypeptide of a major dragline silk protein can be represented as, for example, a protein including a domain sequence represented by Formula 1: [(A) n  motif-REP] m  (here, in Formula 1, the (A) n  motif represents an amino-acid sequence constituted of 4 to 20 amino-acid residues, and the percentage of the number of alanine residues with respect to the number of all of amino-acid residues in the (A) n  motif is 80% or more. REP represents an amino-acid sequence constituted of 10 to 200 amino-acid residues. m represents an integer of 8 to 300. A plurality of (A) n  motifs may be identical amino-acid sequences or different amino-acid sequences. A plurality of REP&#39;s may be identical amino-acid sequences or different amino-acid sequences.). Specific examples thereof include a protein including an amino-acid sequence represented by a SEQ ID No. 12. 
     Examples of a recombinant polypeptide of collagen include a protein including a domain sequence represented by Formula 2: [REP2] o  (here, in Formula 2, o represents an integer of 5 to 300. REP2 represents an amino-acid sequence constituted of Gly-X-Y, and X and Y represent arbitrary amino-acid residues other than Gly. A plurality of REP2 may be identical amino-acid sequences or different amino-acid sequences.). Specific examples thereof include a protein including an amino-acid sequence represented by a SEQ ID No. 13. The amino-acid sequence represented by the SEQ ID No. 13 is an amino-acid sequence obtained by adding an amino-acid sequence represented by a SEQ ID No. 5 (tag sequence and hinge sequence) to an N terminal of an amino-acid sequence from the 301 st  residue through the 540 th  residue which corresponds to the repeat portion and the motif of a partial sequence (NCBI Genbank accession number: CAA56335.1, GI: 3702452) of a collagen type 4 of a human being procured from NCBI database. 
     Examples of a recombinant polypeptide of resilin include a protein including a domain sequence represented by Formula 3: [REP3] p  (here, in Formula 3, p represents an integer of 4 to 300. REP3 represents an amino-acid sequence constituted of Ser-J-J-Tyr-Gly-U-Pro. J represents an arbitrary amino-acid residue and is particularly preferably an amino-acid residue selected from the group consisting of Asp, Ser, and Thr. U represents an arbitrary amino-acid residue and is particularly preferably an amino-acid residue selected from the group consisting of Pro, Ala, Thr, and Ser. A plurality of REP3 may be identical amino-acid sequences or different amino-acid sequences.). Specific examples thereof include a protein including an amino-acid sequence represented by a SEQ ID No. 14. The amino-acid sequence represented by the SEQ ID No. 14 is an amino-acid sequence obtained by substituting Thr of the 87 th  residue into Ser in the amino-acid sequence of resilin (NCBI Genbank accession number: NP 611157, GI: 24654243) and adding the amino-acid sequence represented by the SEQ ID No. 5 (tag sequence and hinge sequence) to the N terminal of an amino-acid sequence from the 19 th  residue through the 320 residue of a sequence in which Asn of the 95 th  residue is substituted into Asp. 
     Examples of a recombinant polypeptide of elastin include a protein having an amino-acid sequence of NCBI Genbank accession number: AAC98395 (human), 147076 (sheep), NP786966 (cow), or the like. Specific examples thereof include a protein including an amino-acid sequence represented by a SEQ ID No. 15. The amino-acid sequence represented by the SEQ ID No. 15 is an amino-acid sequence obtained by adding the amino-acid sequence represented by the SEQ ID No. 5 (tag sequence and hinge sequence) to the N terminal of an amino-acid sequence from the 121 th  residue through the 390 st  residue of the amino-acid sequence of NCBI Genbank accession number: AAC98395. 
     Examples of a recombinant polypeptide of keratin include type I keratin of Capra hircus and the like. Specific examples thereof include a protein including an amino-acid sequence represented by a SEQ ID No. 16 (an amino-acid sequence of NCBI Genbank accession number of ACY30466). 
     The recombinant polypeptide may be recombinant fibroin including (i) an amino-acid sequence represented by a SEQ ID No. 2, a SEQ ID No. 4, or a SEQ ID No. 10 or (ii) an amino-acid sequence having a degree of identity of 90% or more with an amino-acid sequence represented by the SEQ ID No. 2, the SEQ ID No. 4, or the SEQ ID No. 10. 
     The recombinant fibroin including (i) the amino-acid sequence represented by the SEQ ID No. 2, the SEQ ID No. 4, or the SEQ ID No. 10 will be described. The amino-acid sequence represented by the SEQ ID No. 2 is an amino-acid sequence obtained by deleting (A) n  motifs every two motifs from an amino-acid sequence (corresponding to naturally occurring fibroin) represented by a SEQ ID No. 1 which corresponds to naturally occurring fibroin from the N terminal side toward the C terminal side and further inserting one [(A) n  motif-REP] to the front of a C terminal sequence. The amino-acid sequence represented by the SEQ ID No. 4 is an amino-acid sequence obtained by substituting all of GGX&#39;s in REP of the amino-acid sequence represented by the SEQ ID No. 2 into GQX&#39;s. The amino-acid sequence represented by the SEQ ID No. 10 is an amino-acid sequence obtained by inserting two alanine residues into the 0.0 terminal side of individual (A) n  motifs in the amino-acid sequence represented by the SEQ ID No. 4, furthermore, substituting some of glutamine (Q) residues into serin (S) residues, and deleting some of amino acids on the N terminal side so as to have a molecular weight that is almost the same as the molecular weight of the SEQ ID No. 4. Meanwhile, an amino-acid sequence represented by a SEQ ID No. 3 is an amino-acid sequence obtained by substituting all of GGX&#39;s in REP of the amino-acid sequence represented by the SEQ ID No. 1 into GQX&#39;s. 
     The recombinant fibroin including (ii) an amino-acid sequence having 90% or more of the sequence identity with the amino-acid sequence represented by the SEQ ID No. 2, the SEQ ID No. 4, or the SEQ ID No. 10 will be described. The recombinant fibroin (ii) includes an amino-acid sequence having 90% or more of the sequence identity with the amino-acid sequence represented by the SEQ ID No. 2, the SEQ ID No. 4, or the SEQ ID No. 10. The recombinant fibroin (ii) is also a protein including a domain sequence represented by Formula 1: [(A) n  motif-REP] m . The sequence identity is preferably 95% or more. 
     The above-described recombinant fibroin may include a tag sequence in any one or both of the N terminal and the C terminal. In such a case, the isolation, immobilization, detection, visualization, and the like of the recombinant fibroin become possible. 
     Examples of the tag sequence include an affinity tag which uses the specific affinity (binding property, affinity) to other molecules. Specific examples of the affinity tag include a histidine tag (His tag). The His tag is a short peptide in which approximately 4 to 10 histidine residues are arrayed and has a property of being specifically bound with a metal ion of nickel or the like and thus can be used for the isolation of recombinant fibroin by chelating metal chromatography. Specific examples of the tag sequence include the amino-acid sequence represented by the SEQ ID No. 5 (amino-acid sequence including the His tag). 
     In addition, it is also possible to use tag sequences such as glutathione-S-transferase (GST) which is specifically bound with glutathione and maltose-binding protein (MBP) which is specifically bound with maltose. 
     Furthermore, it is also possible to use an “epitope tag” which uses an antigen-antibody reaction. When a peptide exhibiting antigenicity (epitope) is added as a tag sequence, it is possible to bind an antibody to the epitope. As the epitope, an HA (a peptide sequence of hemagglutinin of an influenza virus) tag, a myc tag, a FLAG tag, and the like can be exemplified. When the epitope tag is used, it is possible to easily purify the recombinant fibroin with a high specificity. 
     Furthermore, it is also possible to use recombinant fibroin from which a tag sequence is cut away using a specific protease. It is also possible to collect recombinant fibroin from which a tag sequence is cut away by carrying out a protease treatment on a protein adsorbed through the tag sequence. 
     More specific examples of the recombinant fibroin including the tag sequence include recombinant fibroin including (iii) an amino-acid sequence represented by a SEQ ID No. 7, a SEQ ID No. 9, or a SEQ ID No. 11 or including (iv) an amino-acid sequence having a sequence identity of 90% or more with an amino-acid sequence represented by the SEQ ID No. 7, the SEQ ID No. 9, or the SEQ ID No. 11. 
     The recombinant polypeptide may be recombinant fibroin including (iii) an amino-acid sequence represented by the SEQ ID No. 7, the SEQ ID No. 9, or the SEQ ID No. II or including (iv) an amino-acid sequence having a sequence identity of 90% or more with an amino-acid sequence represented by the SEQ ID No. 7, the SEQ ID No. 9, or the SEQ ID No. 11. 
     The amino-acid sequences represented by the SEQ ID Nos. 6, 7, 8, 9, and 11 are respectively amino-acid sequences obtained by adding the amino-acid sequence represented by the SEQ ID No. 5 (including the His tag) to the N terminals of the amino-acid sequences represented by the SEQ ID Nos. 1, 2, 3, 4, and 10. The recombinant fibroin (iv) is also a protein including the domain sequence represented by Formula 1: [(A) n  motif-REP] m . The sequence identity is preferably 95% or more. 
     The structural protein preferably includes the recombinant polypeptide. When a molded article to be obtained includes the recombinant polypeptide as the structural protein, it is possible to adjust the bending elastic modulus, the bending strength, and the hardness of the molded article to desired numerical values. 
     [Recombinant Cell Expressing Structural Protein] 
     A method for producing the recombinant polypeptide will be described below in detail. A target recombinant polypeptide can be produced by, for example, expressing the gene using a host transformed by an expression vector having a gene sequence that codes the structural protein and one or a plurality of regulatory sequences operably coupled to the gene sequence. 
     A method for producing the gene that codes the target recombinant polypeptide is not particularly limited. For example, the gene can be produced using a method in which a gene that codes a natural structural protein is used, amplified by a polymerase chain reaction (PCR) or the like, and cloned or a chemical synthesis. A chemical synthesis method of the gene is also not particularly limited, and it is possible to chemically synthesize a gene using a method in which oligonucleotides automatically synthesized using AKTA oligopilot plus 10/100 (manufactured by General Electric Company, Japan) or the like are coupled together by PCR or the like on the basis of the amino-acid sequence information of a structural protein procured from the web database of NCBI or the like. At this time, in order to facilitate the purification or confirmation of the protein, a gene that codes a polypeptide obtained by adding an amino-acid sequence made up of an initiation codon and a His10 tag to the N terminal of the above-described amino-acid sequence may be synthesized. 
     The regulatory sequence is a sequence that suppresses the expression of a recombinant protein in the host (for example, a promoter, an enhancer, a ribosome-binding sequence, a transcription termination sequence, or the like) and can be appropriately selected depending on the kind of the host. As the promoter, an inducible promoter which functions in a host cell and is capable of inducing the expression of a target protein may be used. The inducible promoter is a promoter capable of controlling transcription using a physical factor such as the presence of an inducer (expression inducer), the absence of a repressor molecule, or an increase or decrease in the temperature, the osmotic pressure, or the pH value. 
     The kind of the expression vector can be appropriately selected depending on the kind of the host such as a plasmid vector, a virus vector, a cosmid vector, a fosmid vector, or an artificial chromosome vector. As the expression vector, an expression vector which is capable of autonomous replication in the host cell or capable of the integration of the host into a chrosome and contains a promotor at a location at which a gene that codes a target recombinant polypeptide can be transcribed is preferably used. 
     As the host, any of a prokaryote and a eukaryote such as an enzyme, a filamentous fungus, an insect cell, an animal cell, or a plant cell can be preferably used. 
     Preferred examples of the prokaryote include bacteria belonging to  Escherichia, Brevibacillus, Serratia, Bacillus, Microbacterium, Brevibacterium, Corynebacterium, Pseudomonas , and the like. 
     Examples of the vector that introduces the gene that codes the target recombinant polypeptide include pBTrp2 (manufactured by Boehringer Ingelheim GmbH), pGEX (manufactured by Pharmacia Corporation), pUC18, pBluescriptII, pSupex, pET22b, pCold, pUB110, pNCO2 (refer to JP 2002-238569 A), and the like. 
     Examples of the host of the eukaryote include an enzyme and a filamentous fungus (mold or the like). 
     Examples of the enzyme include enzymes belonging to  Saccharomyces, Pichia, Schizosaccharomyces , and the like. Examples of the filamentous fungus include filamentous fungi belonging to  Aspergillus, Penicillium, Trichoderma , and the like. 
     Examples of the vector include YEp13 (ATCC37115), YEp24 (ATCC37051), and the like. 
     As a method for introducing the expression vector to the host cell, any method can be used as long as the method is to introduce DNA to the host cell. Examples thereof include a method in which a calcium ion is used [refer to Proc. Natl. Acad. Sci. USA, 69, 2110 (1972)], an electroporation method, a spheroplast method, a protoplast method, a lithium acetate method, a competent method, and the like. 
     As a method for expressing a gene using a host transformed by the expression vector, it is possible to carry out not only direct expression but also secretory production, fused protein expression, and the like according to a method described in Molecular Cloning Vol. 2 or the like. 
     The target recombinant polypeptide can be produced by, for example, culturing a host transformed by the expression vector according to the present invention in an incubation medium, preparing and accumulating the protein in the incubation medium, and picking the protein from the incubation medium. Regarding a method for culturing the host according to the present invention in the incubation medium, the host can be cultured according to an ordinarily-used method. 
     In a case in which the host according to the present invention is a prokaryote such as  Bacillus coli  or a eukaryote such as an enzyme, as the incubation medium of the host according to the present invention, any of a natural culture medium or a synthetic culture medium may be used as long as the culture medium contains a carbon source, a nitrogen source, an inorganic salt, and the like which enable the utilization of the host and enables the efficient culture of the host. 
     The carbon source needs to be a source enabling the utilization of the transformed microorganism, and it is possible to use, for example, glucose, fructose, sucrose, molasses containing the above-described substance, carbohydrates such as starch and starch hydrolysate, organic acids such as acetic acid and propionic acid, and alcohols such as ethanol and propanol. 
     As the nitrogen source, it is possible to use, for example, inorganic acids such as ammonia, ammonium chloride, ammonium sulfate, ammonium acetate, and ammonium phosphate, ammonium salts of an organic acid, other nitrogen-containing compounds, peptone, meat extracts, yeast extracts, corn steep liquor, casein hydrolysate, soybean cake, soybean cake hydrolysate, a variety of fermented fungus bodies, and digested substances thereof. 
     As the inorganic salt, it is possible to use, for example, potassium dihydrogenphosphate, dipotassium phosphate, magnesium phosphate, magnesium sulfate, sodium chloride, ferrous sulfate, manganese sulfate, copper sulfate, and calcium carbonate. 
     The prokaryote such as  Bacillus coli  or the eukaryote such as an enzyme can be cultured, for example, under the aerobic conditions of shaking culture or deep aeration stirring culture. The culture temperature is, for example, 15° C. to 40° C. The culture time is generally 16 hours to 7 days. The pH of the incubation medium during culture is preferably maintained at 3.0 to 9.0. The pH of the incubation medium can be adjusted using an inorganic acid, an organic acid, an alkali solution, urea, calcium carbonate, ammonia, or the like. 
     In addition, during the culture, an antibiotic substance such as ampicillin or tetracycline may be added to the incubation medium if necessary. When a microorganism transformed using an expression vector for which an inducible promoter is used as a promoter is cultured, an inducer may be added to the culture medium if necessary. For example, when a microorganism transformed using an expression vector for which a lac promoter is used is cultured, isopropyl-β-D-thiogalactopyranoside or the like may be added to the culture medium, and, when a microorganism transformed using an expression vector for which a trp promoter is used is cultured, indoleacrylic acid may be added to the culture medium. 
     The recombinant polypeptide according to the present invention can be isolated and purified using a method that is ordinarily used to isolate and purify a protein. For example, in a case in which the recombinant polypeptide is expressed in a cell in a dissolved state, after the end of the culture, the host cell is collected by centrifugal separation, suspended in a water-based buffering solution, then, the host cell is crushed using an ultrasonic crusher, a French press, a Manton-Gaulin homogenizer, a DYNO MILL, or the like, and a cell-free extract is obtained. From the supernatant being obtained by centrifugally separating the cell-free extract, a purified preparation can be obtained using a method that is ordinarily used to isolate and purify a protein singly or the methods in combination. Examples of the above-described method include a solvent extraction method, a salting-out method using ammonium sulfate or the like, a desalination method, a precipitation method using an organic solvent, an anion-exchange chromatography method using a resin such as diethylaminoethyl SEPHAROSE (DEAE-SEPHAROSE) or DIAION HPA-75 (manufactured by Mitsubishi Kasei Kogyo Kabushiki Kaisha), a cation-exchange chromatography method using a resin such as S-Sepharose FF (manufactured by Pharmacia Corporation), a hydrophobic chromatography method using a resin such as BUTYL SEPHAROSE or PHENYL SEPHAROSE, a gel filtration method using a molecular sieve, an affinity chromatography method, a chromatofocusing method, an electrophoresis method such as isoelectric point electrophoresis, and the like. 
     In addition, in a case in which the recombinant polypeptide is expressed with forming an insoluble body in a cell, similarly, the host cell is collected, then, crushed, and centrifugally separated, thereby collecting an insoluble body of the recombinant polypeptide as a precipitation fraction. The collected insoluble body of the recombinant polypeptide can be solubilized using a protein denaturation agent. After the operation, a purified preparation of the recombinant polypeptide can be obtained using the same isolation and purification method as described above. 
     In a case in which the recombinant polypeptide is secreted to the outside of the cell, it is possible to collect the recombinant polypeptide from the culture supernatant. That is, the culture supernatant is acquired by treating a cultured substance using a method such as centrifugal separation, and a purified preparation can be obtained from the culture supernatant using the same isolation and purification method as described above. 
     [Method for Producing Molded Article] 
     A method for producing a molded article which is an embodiment includes a step 1 of adding water to a composition including a structural protein, a step 2 of introducing a water-containing composition obtained in the step 1 to a casting mold (mold) and molding the water-containing composition, and a step 3 of obtaining a molded article by drying the molded composition obtained in the step 2. 
     The step 1 is a step of adding water to a composition including a structural protein. The composition needs to comprise a structural protein. The composition may comprise only a structural protein or comprise a structural protein and an optional additive (for example, a plasticizer, a colorant, a filler, a synthetic resin, or the like). The content of the additive is preferably set to 50% by mass or less of the total amount of the structural protein. The composition typically has a shape of a powder shape (lyophilized powder or the like) or a fiber shape (a fiber being obtained by spinning or the like), and the molded article can be a fused body of a composition comprising a structural protein having the above-described shape. 
     The amount of water added in the step 1 is preferably 10 to 40 wt % and more preferably 20 to 30 wt % of the mass of the structural protein. 
     The step 2 is a step of introducing a water-containing composition obtained in the step 1 to a casting mold (mold) and molding the water-containing composition. In the molding, the composition may be heated and/or pressurized. 
     In the step 2, it is possible to mold the water-containing composition using, for example, a press molding machine.  FIG. 1  is a schematic cross-sectional view of a press molding machine that can be used to produce the molded article. A press molding machine  10  illustrated in  FIG. 1  includes a die  2  which has a through-hole formed therein and can be heated and an upper side pin  4  and a lower side pin  6  which are vertically movable in the through-hole of the die  2 . The water-containing composition being obtained in the step 1 is introduced to a space generated by inserting the upper side pin  4  and the lower side pin  6  into the through-hole of the die  2 , and the water-containing composition is compressed using the upper side pin  4  and the lower side pin  6  while heating the die  2 , whereby the molded article can be obtained. 
       FIG. 2  illustrates step views of obtaining the molded article and schematic cross-sectional views of the press molding machine (a) before the introduction of the water-containing composition, (b) immediately after the introduction of the water-containing composition, and (c) with the water-containing composition in a state of being hot pressed. As illustrated in  FIG. 2( a ) , the water-containing composition is introduced into the through-hole in a state in which only the lower side pin  6  is inserted into the through-hole of the die  2 , but moisture is added thereto before the introduction or after the introduction, and then the water-containing composition is stirred. Subsequently, as illustrated in  FIG. 2( b ) , the upper side pin  4  is inserted into the through-hole of the die  2  and lowered, and the die  2  begins to be heated, thereby hot pressing a water-containing composition  8   a  before hot pressing in the through-hole. The hot pressing is continued until the water-containing composition reaches a predetermined temperature in a state illustrated in  FIG. 2( c )  by lowering the upper side pin  4  until a previously-determined pressurization force is reached, thereby obtaining a water-containing composition  8   b  after hot pressing. After that, the temperature of the die  2  is lowered using a cooler (for example, a spot cooler), the upper side pin  4  or the lower side pin  6  is removed from the die  2  when the water-containing composition  8   b  reaches a predetermined temperature, and the content is removed. The pressurization may be carried out by lowering the upper side pin  4  in a state in which the lower side pin  6  is fixed, but may also be carried out by lowering the upper side pin  4  and the lifting the lower side pin  6  at the same time. The removed content is dried, thereby obtaining a molded article. The pressurization may be carried out by lowering the upper side pin  4  in a state in which the lower side pin  6  is fixed, but may also be carried out by lowering the upper side pin  4  and the lifting the lower side pin  6  at the same time. 
     The heating is carried out when the temperature of the die is preferably 80° C. to 300° C., more preferably 100° C. to 180° C., and still more preferably 100° C. to 130° C. The pressurization is preferably carried out at a pressure of 20 MPa or more. 
     The step 3 is a step of drying the molded composition obtained in the step 2 to obtain the molded article. The molded composition is preferably dried by being left to stand in a vacuum and/or in an environment of 100° C. for one minute to 10 days. 
     EXAMPLES 
     Hereinafter, the present invention will be more specifically described on the basis of examples. However, the present invention is not limited to the following examples. 
     (1) Production of Structural Protein Expression Line 
     The base sequence and the amino-acid sequence of fibroin derived from  Nephila clavipes  (GenBank accession number: P46804.1, GI: 1174415) were acquired from the web database of GenBank, the substitution, insertion, and deletion of amino-acid residues were carried out for the purpose of the improvement of the productivity, and furthermore, an amino-acid sequence (a tag sequence and a hinge sequence) represented by a SEQ ID No. 5 was added to an N terminal, thereby designing a recombinant fibroin having an amino-acid sequence represented by a SEQ ID No. 12 (hereinafter, also referred to as “PRT410”). 
     Next, a gene that coded PRT410 was synthesized and entrusted. As a result, a gene in which an NdeI site was added immediately upstream of a 5′ terminal of the gene and an EcoRI site was added immediately downstream of a 3′ terminal was obtained. The gene was cloned to a cloning vector (pUC118), then, subjected to a restriction enzyme treatment using NdeI and EcoRI, and recombined into a protein expression vector pET-22b(+). 
     (2) Expression of Protein 
       Bacillus coli  BLR (DE3) was transformed using a pET22b(+) expression vector including the gene that coded PRT410 obtained above. The transformed  Bacillus coli  was cultured in a 2 mL LB culture medium including ampicillin for 15 hours, and then the same culture fluid was added to a culture medium for seed cultivation (100 mL) shown in Table 1 so that OD 600  reached 0.005. The temperature of the culture fluid was maintained at 30° C., and the culture fluid was further cultured in a flask for 15 hours so that OD 600  reached 5, thereby obtaining a seed culture fluid. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Chemical 
                 Concentration 
               
               
                   
                   
               
             
            
               
                   
                 Glucose 
                 5.0 g/L 
               
               
                   
                 KH 2 PO 4   
                 4.0 g/L 
               
               
                   
                 K 2 HPO 4   
                 9.3 g/L 
               
               
                   
                 Yeast Extract 
                 6.0 g/L 
               
               
                   
                 Ampicillin 
                 0.1 g/L 
               
               
                   
                   
               
            
           
         
       
     
     The obtained seed culture fluid was added to a jar fermenter to which a production culture medium (500 mL) shown in Table 2 had been added so that OD 600  reached 0.05. The temperature of the culture fluid was maintained at 37° C., the pH was controlled so as to become constant at 6.9, and the culture fluid was cultured with the concentration of dissolved oxygen in the culture fluid set to be maintained at 20% of the saturation concentration of dissolved oxygen. Meanwhile, as a defoamer, ADEKA NOL LG-295S (manufactured by ADEKA Corporation) was used. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Chemical 
                 Concentration 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Glucose 
                 12.0 
                 g/L 
               
               
                   
                 KH 2 PO 4   
                 9.0 
                 g/L 
               
               
                   
                 MgSO 4 •7H 2 O 
                 2.4 
                 g/L 
               
               
                   
                 Yeast Extract 
                 15 
                 g/L 
               
               
                   
                 FeSO 4 •7H 2 O 
                 0.04 
                 g/L 
               
               
                   
                 MnSO 4 •5H 2 O 
                 0.04 
                 g/L 
               
               
                   
                 CaCl 2 •2H 2 O 
                 0.04 
                 g/L 
               
               
                   
                 LG-295S 
                 0.1 
                 mL/L 
               
               
                   
                   
               
            
           
         
       
     
     Immediately after the complete consumption of glucose in the production culture medium, a feed fluid (455 g/L of glucose and 120 g/L of yeast extract) was added at a rate of 1 mL/minute. The temperature of the culture fluid was maintained at 37° C., the pH was controlled to be constant at 6.9, and the culture fluid was cultured. In addition, the concentration of dissolved oxygen in the culture fluid was set to be maintained at 20% of the saturation concentration of dissolved oxygen, and the culture fluid was cultured for 20 hours. After that, a 1 M isopropyl-β-thiogalactopyranoside (IPTG) aqueous solution was added to the culture fluid so that the final concentration reached 1 mM, and the target protein was expressed and induced. At a point in time when 20 hours elapsed from the addition of IPTG, the culture fluid was centrifugally separated, and solid was collected. SDS-PAGE was carried out using fungus bodies prepared from the culture fluid before the addition of IPTG and after the addition of IPTG, and the expression of the target protein was confirmed from the appearance of a band of the target protein size which was dependent on the addition of IPTG 
     (3) Purification of Structural Protein 
     The fungus body collected after two hours from the addition of IPTG was washed with a 20 mM Tris-HCl buffer solution (pH: 7.4). The washed fungus body was suspended in a 20 mM Tris-HCl buffer solution (pH: 7.4) including phenylmethylsulfonyl fluoride (PMSF) (approximately 1 mM), and cells were broken using a high-pressure homogenizer (GEA Niro Soavi SpA). The broken cells were centrifugally separated, thereby obtaining a precipitate. The obtained precipitate was washed with a 20 mM Tris-HCl buffer solution (pH: 7.4) until the precipitate became highly pure. The washed precipitate was suspended in an 8 M guanidine buffer solution (8 M guanidinium hydrochloride, 10 mM sodium dihydrogen phosphate, 20 mM NaCl, 1 mM Tris-HCl, pH: 7.0) so as to be a concentration of 100 mg/mL, stirred using a stirrer at 60° C. for 30 minutes, and dissolved. After the dissolution, dialysis was carried out using a dialysis tube (cellulose tube 36/32 manufactured by Sanko Junyaku Co., Ltd.) and water. A white aggregate protein obtained after the dialysis was collected by centrifugal separation, moisture was removed using a freeze dryer, and lyophilized powder was collected. 
     Production of Molded Article 
     Example 1 
     A sample obtained by adding pure water (30 wt %) to the lyophilized powder (1.35 g) was introduced to a through-hole of a die  2  (the die  2  was a cylindrical die and had a circular through-hole having a diameter of 20 mm) of a press molding machine  10  illustrated in  FIG. 1 . After the entire sample was introduced to the through-hole, the die  2  began to be heated, and the sample was pressurized by inserting an upper side pin  4  and a lower side pin  6  into the through-hole using a hot press machine. At this time, the pressurization condition of the sample was controlled so that the pressure reached 31 MPa. The heating was stopped when the temperature of the sample reached 170° C., the sample was cooled, then, removed from the die, then, injected into a vacuum heating furnace, and dried at 100° C. for 16 hours in a vacuum, thereby obtaining a molded article having a disc shape having a diameter of 20 mm and a thickness of 2 mm. 
     Comparative Example 1 
     A molded article was obtained by hot pressing the above-described lyophilized powder in the same manner as in Example 1 except for the fact that the amount of the lyophilized powder was changed to 0.8 g in order to match the thickness of a compact to be obtained, and the achieving temperature during the hot pressing was set to 170° C. in order to control the temperature increase during molding. In Comparative Example 1, drying was not carried out after hot pressing. 
     &lt;Production of Test Specimen&gt; 
     The surface of the molded article was polished with sheets of polishing paper #600 and #1000 (based on JIS R6001: 1988 the grain size distribution of fine powder for precision polishing), flattened, and worked (cut) to a 4 mm-wide plate shape including a disc-shaped central line, thereby obtaining an approximately 20 mm×4 mm×2 mm test specimen. 
     &lt;Measurement of Bending Elastic Modulus and Bending Strength&gt; 
     On the obtained test specimen, a three-point bending test was carried out in AUTOGRAPH (manufactured by Shimadzu Corporation, trade name: AGS-X). At this time, the distance between supporting pins for three-point bending was fixed to 14 mm, and the measurement rate was set to 1 mm/minute. In addition, the dimensions of the test specimen were measured using a micrometer, then, the test specimen was installed on the supporting pins, and the bending elastic modulus and the bending strength were measured. 
     &lt;Measurement of Micro Vickers Hardness&gt; 
     The Vickers hardness of the obtained test specimen was measured using a Micro Vickers hardness tester (manufactured by Shimadzu Corporation, trade name: HMV-G). 
     &lt;Measurement of Moisture Percentage&gt; 
     The moisture percentage of the test specimen during the measurement of the bending strength was measured using a Karl Fischer-type moisture meter (manufactured by Metrohm AG, trade name: 860 KF Thermoprep, 851 Titrando, 801 Stirrer). At this time, the test specimen was heated to 180° C. 
     The respective measurement results are shown in Table 3. The molded articles of Example 1 and Comparative Example 1 had almost the same degree of moisture percentages. Example 1 improved more significantly than Comparative Example 1 in both of the bending elastic modulus and the bending strength. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
             
            
               
                   
                 Addition of 
                 Drying 
                 Moisture 
                 Vickers 
               
               
                   
                 moisture 
                 time 
                 percentage 
                 hardness 
               
               
                   
               
               
                 Example 1 
                 30 wt % 
                 16 h 
                 2.7% 
                 45 HV 
               
               
                 Comparative 
                 None 
                 None 
                 2.4% 
                 40 HV 
               
               
                 Example 1 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 Three-point 
               
               
                   
                   
                 Three-point 
                 bending elastic 
               
               
                   
                   
                 bending strength 
                 modulus 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Example 1 
                 141 
                 MPa 
                 7.3 GPa 
               
               
                   
                 Comparative 
                 98 
                 MPa 
                 6.8 GPa 
               
               
                   
                 Example 1 
               
               
                   
                   
               
            
           
         
       
     
     REFERENCE SIGNS LIST 
       2 : DIE,  4 : UPPER SIDE PIN,  6 : LOWER SIDE PIN,  8   a : WATER-CONTAINING COMPOSITION BEFORE HOT PRESSING,  8   b : WATER-CONTAINING COMPOSITION AFTER HOT PRESSING,  10 : PRESS MOLDING MACHINE 
     SEQUENCE LISTING