Patent Publication Number: US-2022235100-A1

Title: Water-Absorbing and Quick-Drying Property-Imparting Agent, and Method for Imparting Water-Absorbing and Quick-Drying Properties

Description:
TECHNICAL FIELD 
     The present invention relates to a water-absorbing and quick-drying property-imparting agent, and a method for imparting water-absorbing and quick-drying properties. 
     BACKGROUND ART 
     In general, clothing such as underwear, innerwear, and sportswear, and bedclothes and the like are required to have so-called water-absorbing and quick-drying properties such as absorbing sweat well and drying quickly. Therefore, many underwear, bedclothes, and the like are produced using cotton spun yarns having relatively high hygroscopic properties. 
     Although cotton has high water-absorbing properties, it is difficult to say that cotton has sufficient quick-drying properties because absorbed moisture evaporates slowly. In addition, in sportswear and the like, polyurethane fibers and polyester fibers are often used because followability to movement of a human body and stretchability are emphasized, but these synthetic fibers are inferior in water-absorbing properties. 
     Under such circumstances, various techniques for improving water-absorbing and quick-drying properties of various fibers, fabrics, and the like have been proposed. For example, Patent Literature 1 discloses a water-absorptive quick-drying spun yarn composed of a false-twisted spun yarn including a core made of polyester fibers having a heteromorphic cross section and a sheath made of short fibers including cotton fibers, the core being in a non-twisted state and the sheath being in a bound state. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: JU 3213540 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     For example, as in the water-absorptive quick-drying spun yarn described in Patent Literature 1, the water-absorbing and quick-drying properties of conventional fibers, fabrics, and the like are achieved by using fibers or the like artificially imparted with a special structure. Therefore, the conventional water-absorptive quick-drying fibers and water-absorptive quick-drying fabrics inevitably require complicated production processes. 
     An object of the present invention is to provide a water-absorbing and quick-drying property-imparting agent capable of easily imparting water-absorbing and quick-drying properties to various materials or articles in a simple process, and a method capable of easily imparting water-absorbing and quick-drying properties to predetermined materials or articles. 
     Solution to Problem 
     The present inventors have found that modified fibroin has excellent water-absorbing and quick-drying properties. The present invention is based on this novel finding. 
     The present invention relates to, for example, the following inventions. 
     [1] 
     A water-absorbing and quick-drying property-imparting agent containing modified fibroin as an active ingredient. 
     [2] 
     The water-absorbing and quick-drying property-imparting agent according to [1], wherein the modified fibroin contains modified fibroin having an average value of hydropathy indices (average hydropathy index (HI)) of 0 or less. 
     [3] 
     The water-absorbing and quick-drying property-imparting agent according to [1] or [2], wherein the modified fibroin contains modified spider silk fibroin. 
     [4] 
     The water-absorbing and quick-drying property-imparting agent according to any one of [1] to [3], wherein the water-absorbing and quick-drying property-imparting agent is in the form of a fiber. 
     [5] 
     A method of imparting water-absorbing and quick-drying properties to an article, the method including: 
     a step of incorporating modified fibroin into the article. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide a water-absorbing and quick-drying property-imparting agent capable of easily imparting water-absorbing and quick-drying properties to various materials or articles in a simple process, and a method capable of easily imparting water-absorbing and quick-drying properties to predetermined materials or articles. 
     According to the present invention, for example, by mixing the water-absorbing and quick-drying property-imparting agent according to the present invention with a material having biodegradability, water-absorbing and quick-drying properties can be imparted without impairing biodegradability. According to the present invention, by adjusting the amount of modified fibroin (active ingredient) contained in a predetermined article, the degree of water-absorbing and quick-drying properties of the article can also be adjusted. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram illustrating an example of a domain sequence of modified fibroin. 
         FIG. 2  is a graph illustrating a distribution of values of z/w (%) in naturally derived fibroin. 
         FIG. 3  is a graph illustrating a distribution of values of x/y (%) in naturally derived fibroin. 
         FIG. 4  is a schematic diagram illustrating an example of a domain sequence of modified fibroin. 
         FIG. 5  is a schematic diagram illustrating an example of a domain sequence of modified fibroin. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments. 
     [Water-Absorbing and Quick-Drying Property-Imparting Agent] 
     The water-absorbing and quick-drying property-imparting agent according to the present embodiment contains modified fibroin as an active ingredient. The water-absorbing and quick-drying properties refer to a property of absorbing moisture such as sweat and quickly drying. In the present specification, the moisture may be liquid water or gaseous water. That is, in the present specification, water absorption includes moisture absorption. The water-absorbing and quick-drying property-imparting agent according to the present embodiment utilizes the property of modified fibroin excellent in water-absorbing and quick-drying properties. 
     (Modified Fibroin) 
     The modified fibroin according to the present embodiment is a protein including a domain sequence represented by Formula 1: [(A) n  motif-REP] m  or Formula 2: [(A) n  motif-REP] m -(A) n  motif. An amino acid sequence (N-terminal sequence and C-terminal sequence) may be further added to either or both of the N-terminal side and the C-terminal side of the domain sequence of the modified fibroin. The N-terminal sequence and the C-terminal sequence, although not limited thereto, are typically regions that do not have repetitions of amino acid motifs characteristic of fibroin and consist of amino acids of about 100 residues. 
     The term “modified fibroin” in the present specification refers to artificially produced fibroin (artificial fibroin). The modified fibroin may be fibroin in which a domain sequence is different from an amino acid sequence of naturally derived fibroin or may be fibroin in which a domain sequence is the same as an amino acid sequence of naturally derived fibroin. The term “naturally derived fibroin” as used herein is also a protein including a domain sequence represented by Formula 1: [(A) n  motif-REP] m  or Formula 2: [(A) n  motif-REP] m -(A) n  motif. 
     The “modified fibroin” may be fibroin obtained by using an amino acid sequence of naturally derived fibroin as it is, fibroin in which an amino acid sequence is modified based on an amino acid sequence of naturally derived fibroin (for example, fibroin in which an amino acid sequence is modified by modifying a cloned gene sequence of naturally derived fibroin), or fibroin artificially designed and synthesized independently of naturally derived fibroin (for example, fibroin having a desired amino acid sequence by chemically synthesizing a nucleic acid encoding a designed amino acid sequence). 
     The term “domain sequence” in the present specification is an amino acid sequence that produces a crystal region (typically, corresponding to the (A) n  motif of the amino acid sequence) and an amorphous region (typically, corresponding to REP of the amino acid sequence) specific to fibroin, and means an amino acid sequence represented by Formula 1: [(A) n  motif-REP] m  or Formula 2: [(A) n  motif-REP] m -(A) n  motif. Here, the (A) n  motif represents an amino acid sequence mainly composed of alanine residues, and the number of amino acid residues therein is 2 to 27. The number of the amino acid residues in the (A) n  motif may be an integer of 2 to 20, 4 to 27, 4 to 20, 8 to 20, 10 to 20, 4 to 16, 8 to 16, or 10 to 16. In addition, the proportion of the number of alanine residues in the total number of amino acid residues in the (A) n  motif may be 40% or more, or may also be 60% or more, 70% or more, 80% or more, 83% or more, 85% or more, 86% or more, 90% or more, 95% or more, or 100% (meaning that the (A) n  motif only consists of alanine residues). At least seven of a plurality of (A) n  motifs in the domain sequence may consist of only alanine residues. The REP represents an amino acid sequence consisting of 2 to 200 amino acid residues. The REP may be an amino acid sequence consisting of 10 to 200 amino acid residues. m represents an integer of 2 to 300, and may be an integer of 10 to 300. A plurality of (A) n  motifs may be the same amino acid sequences or different amino acid sequences. A plurality of REPs may be the same amino acid sequences or different amino acid sequences. 
     The modified fibroin according to the present embodiment can be obtained by, for example, performing modification of an amino acid sequence corresponding to substitution, deletion, insertion, and/or addition of one or a plurality of amino acid residues with respect to a cloned gene sequence of naturally derived fibroin. Substitution, deletion, insertion, and/or addition of the amino acid residues can be performed by methods well known to those skilled in the art, such as site-directed mutagenesis. Specifically, the modification may be performed in accordance with a method described in literatures such as Nucleic Acid Res. 10, 6487 (1982), and Methods in Enzymology, 100, 448 (1983). 
     The naturally derived fibroin is a protein including a domain sequence represented by Formula 1: [(A) n  motif-REP] m  or Formula 2: [(A) n  motif-REP] m -(A) n  motif, and a specific example thereof can include fibroin produced by insects or spiders. 
     Examples of the fibroin produced by insects can include silk proteins produced by silkworms such as  Bombyx mori, Bombyx mandarina, Antheraea yamamai, Anteraea pernyi, Eriogyna pyretorum, Pilosamia Cynthia ricini, Samia cynthia, Caligura japonica, Antheraea mylitta , and  Antheraea assama  and a hornet silk protein secreted by larvae of  Vespasimillima xanthoptera.    
     More specific examples of the fibroin produced by insects can include the silkworm fibroin L chain (GenBank Accession Nos. M76430 (nucleotide sequence) and AAA27840.1 (amino acid sequence)). 
     Examples of the fibroin produced by spiders can include spider silk proteins produced by spiders belonging to the order Araneae. More specific examples thereof can include spider silk proteins produced by spiders belonging to the genus  Araneus , such as  Araneus ventricosus, Araneus diadematus, Araneus pinguis, Araneus pentagrammicus , and  Araneus nojimai , spiders belonging to the genus  Neoscona , such as  Neoscona scylla, Neoscona nautica, Neoscona adianta , and  Neoscona scylloides , spiders belonging to the genus  Pronus , such as  Pronous minutus , spiders belonging to the genus  Cyrtarachne , such as  Cyrtarachne bufo  and  Cyrtarachne inaequalis , spiders belonging to the genus  Gasteracantha , such as  Gasteracantha kuhlii  and  Gasteracantha mammosa , spiders belonging to the genus  Ordgarius , such as  Ordgarius hobsoni  and  Ordgarius sexspinosus , spiders belonging to the genus  Argiope , such as  Argiope amoena, Argiope minuta , and  Argiope bruennichi , spiders belonging to the genus  Arachnura , such as  Arachnura logio , spiders belonging to the genus  Acusilas , such as  Acusilas coccineus , spiders belonging to the genus  Cytophora , such as  Cyrtophora moluccensis, Cyrtophora exanthematica , and  Cyrtophora unicolor , spiders belonging to the genus  Poltys , such as  Poltys illepidus , spiders belonging to the genus  Cyclosa , such as  Cyclosa octotuberculata, Cyclosa sedeculata, Cyclosa vallata , and  Cyclosa atrata , and spiders belonging to the genus  Chorizopes , such as  Chorizopes nipponicus , and spider silk proteins produced by spiders belonging to the family Tetragnathidae, such as spiders belonging to the genus  Tetragnatha , such as  Tetragnatha praedonia, Tetragnatha maxillosa, Tetragnatha extensa , and  Tetragnatha squamata , spiders belonging to the genus  Leucauge , such as  Leucauge magnifica, Leucauge blanda , and  Leucauge subblanda , spiders belonging to the genus  Nephila , such as  Nephila clavata  and  Nephila pilipes , spiders belonging to the genus  Menosira , such as  Menosira ornata , spiders belonging to the genus  Dyschiriognatha , such as  Dyschiriognatha tenera , spiders belonging to the genus  Latrodectus , such as  Latrodectus mactans, Latrodectus hasseltii, Latrodectus geometricus , and  Latrodectus tredecimguttatus , and spiders belonging to the genus  Euprosthenops . Examples of the spider silk protein can include dragline silk proteins such as MaSps (MaSp1 and MaSp2) and ADFs (ADF3 and ADF4), MiSps (MiSp1 and MiSp2), AcSp, PySp, and Flag. 
     More specific examples of the spider silk protein produced by spiders include fibroin-3 (adf-3) [derived from  Araneus diadematus ] (GenBank Accession No. AAC47010 (amino acid sequence), U47855 (nucleotide sequence)), fibroin-4 (adf-4) [derived from  Araneus diadematus ] (GenBank Accession No. AAC47011 (amino acid sequence), U47856 (nucleotide sequence)), dragline silk protein spidroin 1 [derived from  Nephila clavipes ] (GenBank Accession No. AAC04504 (amino acid sequence), U37520 (nucleotide sequence)), major ampullate spidroin 1 [derived from  Latrodectus hesperus ] (GenBank Accession No. ABR68856 (amino acid sequence), EF595246 (nucleotide sequence)), dragline silk protein spidroin 2 [derived from  Nephila clavata ] (GenBank Accession No. AAL32472 (amino acid sequence), AF441245 (nucleotide sequence)), major ampullate spidroin 1 [derived from  Euprosthenops australis ] (GenBank Accession No. CAJ00428 (amino acid sequence), AJ973155 (nucleotide sequence)), and major ampullate spidroin 2 [ Euprosthenops australis ] (GenBank Accession No. CAM32249.1 (amino acid sequence), AM490169 (nucleotide sequence)), minor ampullate silk protein 1 [ Nephila clavipes ] (GenBank Accession No. AAC14589.1 (amino acid sequence)), minor ampullate silk protein 2 [ Nephila clavipes ] (GenBank Accession No. AAC14591.1 (amino acid sequence)), and minor ampullate spidroin-like protein [ Nephilengys cruentata ] (GenBank Accession No. ABR37278.1 (amino acid sequence). 
     More specific examples of the naturally derived fibroin can include fibroin whose sequence information is registered in NCBI GenBank. For example, sequences thereof may be confirmed by extracting sequences in which spidroin, ampullate, fibroin, “silk and polypeptide”, or “silk and protein” is described as a keyword in DEFINITION among sequences containing INV as DIVISION among sequence information registered in NCBI GenBank, sequences in which a specific character string of products is described from CDS, or sequences in which a specific character string is described from SOURCE to TISSUE TYPE. 
     The modified fibroin according to the present embodiment may be modified silk fibroin (in which an amino acid sequence of a silk protein produced by silkworm is modified), or may be modified spider silk fibroin (in which an amino acid sequence of a spider silk protein produced by spiders is modified). 
     Specific examples of the modified fibroin can include modified fibroin derived from a major dragline silk protein produced in a major ampullate gland of a spider (first modified fibroin), modified fibroin having a domain sequence in which the content of glycine residues is reduced (second modified fibroin), modified fibroin having a domain sequence in which the content of an (A) n  motif is reduced (third modified fibroin), modified fibroin in which the content of glycine residues and the content of an (A) n  motif are reduced (fourth modified fibroin), modified fibroin having a domain sequence including a region locally having a high hydropathy index (fifth modified fibroin), and modified fibroin having a domain sequence in which the content of glutamine residues is reduced (sixth modified fibroin). 
     An example of the first modified fibroin can include a protein including a domain sequence represented by Formula 1: [(A) n  motif-REP] m . In the first modified fibroin, the number of amino acid residues in the (A) n  motif is preferably an integer of 3 to 20, more preferably an integer of 4 to 20, still more preferably an integer of 8 to 20, even still more preferably an integer of 10 to 20, still further preferably an integer of 4 to 16, particularly preferably an integer of 8 to 16, and most preferably an integer of 10 to 16. In the first modified fibroin, the number of amino acid residues constituting REP in Formula 1 is preferably 10 to 200 residues, more preferably 10 to 150 residues, and still more preferably 20 to 100 residues, and still even more preferably 20 to 75 residues. In the first modified fibroin, the total number of glycine residues, serine residues, and alanine residues included in the amino acid sequence represented by Formula 1: [(A) n  motif-REP], is preferably 40% or more, more preferably 60% or more, and still more preferably 70% or more, relative to the total number of amino acid residues. 
     The first modified fibroin may be a polypeptide including an amino acid sequence unit represented by Formula 1: [(A) n  motif-REP] m , and including a C-terminal sequence which is an amino acid sequence set forth in any one of SEQ ID NO: 1 to 3 or a C-terminal sequence which is an amino acid sequence having 90% or more homology with the amino acid sequence set forth in any one of SEQ ID NO: 1 to 3. 
     The amino acid sequence set forth in SEQ ID NO: 1 is identical to an amino acid sequence consisting of 50 amino acid residues of the C-terminal of an amino acid sequence of ADF3 (GI: 1263287, NCBI). The amino acid sequence set forth in SEQ ID NO: 2 is identical to an amino acid sequence set forth in SEQ ID NO: 1 in which 20 amino acid residues have been removed from the C-terminal. The amino acid sequence set forth in SEQ ID NO: 3 is identical to an amino acid sequence set forth in SEQ ID NO: 1 in which 29 amino acid residues have been removed from the C-terminal. 
     A specific example of the first modified fibroin can include modified fibroin including (1-i) the amino acid sequence set forth in SEQ ID NO: 4 (recombinant spider silk protein ADF3KaiLargeNRSH1), or (1-ii) an amino acid sequence having 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 4. The sequence identity is preferably 95% or more. 
     The amino acid sequence set forth in SEQ ID NO: 4 is obtained by the following mutation: in an amino acid sequence of ADF3 in which an amino acid sequence (SEQ ID NO: 5) consisting of a start codon, a His 10-tag, and an HRV3C protease (Human rhinovirus 3C protease) recognition site is added to the N-terminal, the 1st to 13th repetitive regions are about doubled and the translation ends at the 1154th amino acid residue. The C-terminal amino acid sequence of the amino acid sequence set forth in SEQ ID NO: 4 is identical to the amino acid sequence set forth in SEQ ID NO: 3. 
     The modified fibroin of (1-i) may consist of the amino acid sequence set forth in SEQ ID NO: 4. 
     The domain sequence of the second modified fibroin has an amino acid sequence in which the content of glycine residues is reduced, as compared with naturally derived fibroin. It can be said that the second modified fibroin has an amino acid sequence corresponding to an amino acid sequence in which at least one or a plurality of glycine residues in REP are substituted with another amino acid residue, as compared with naturally derived fibroin. 
     The domain sequence of the second modified fibroin may have an amino acid sequence corresponding to an amino acid sequence in which one glycine residue in at least one or the plurality of motif sequences is substituted with another amino acid residue, in at least one motif sequence selected from GGX and GPGXX (where G represents a glycine residue, P represents a proline residue, and X represents an amino acid residue other than glycine) in REP, as compared with naturally derived fibroin. 
     In the second modified fibroin, the proportion of the motif sequence in which the glycine residue has been substituted with another amino acid residue may be 10% or more relative to the entire motif sequence. 
     The second modified fibroin includes a domain sequence represented by Formula 1: [(A) n  motif-REP] m , and may have an amino acid sequence in which z/w is 30% or more, 40% or more, 50% or more, or 50.9% or more in a case where the total number of amino acid residues in the amino acid sequence consisting of XGX (where X represents an amino acid residue other than glycine) included in all REPs in a sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence is defined as z, and the total number of amino acid residues in the sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence is defined as w. The number of alanine residues with respect to the total number of amino acid residues in the (A) n  motif is 83% or more, preferably 86% or more, more preferably 90% or more, still more preferably 95% or more, and even still more preferably 100% (meaning that the (A) n  motif consists of only alanine residues). 
     The second modified fibroin is preferably one in which the content ratio of the amino acid sequence consisting of XGX is increased by substituting one glycine residue of the GGX motif with another amino acid residue. In the second modified fibroin, the content ratio of the amino acid sequence consisting of GGX in the domain sequence is preferably 30% or less, more preferably 20% or less, still more preferably 10% or less, even still more preferably 6% or less, still further preferably 4% or less, and particularly preferably 2% or less. The content ratio of the amino acid sequence consisting of GGX in the domain sequence can be calculated by the same method as the calculation method of the content ratio (z/w) of the amino acid sequence consisting of XGX described below. 
     The method of calculating z/w will be described in more detail. First, the amino acid sequence consisting of XGX is extracted from all REPs included in a sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence in the fibroin including a domain sequence represented by Formula 1: [(A) n  motif-REP] m  (modified fibroin or naturally derived fibroin). The total number of amino acid residues constituting XGX is z. For example, in a case where 50 amino acid sequences consisting of XGX are extracted (there is no overlap), z is 50×3=150. Also, for example, in a case where X (central X) included in two XGXs exists as in a case of the amino acid sequence consisting of XGXGX, z is calculated by subtracting the overlapping portion (in a case of XGXGX, it is 5 amino acid residues). w is the total number of amino acid residues included in a sequence excluding the sequence from the (A)n motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence. For example, in a case of the domain sequence shown in  FIG. 1 , w is 4+50+4+100+4+10+4+20+4+30=230 (excluding the (A) n  motif located at the most C-terminal side). Next, z/w (%) can be calculated by dividing z by w. 
     Here, z/w in naturally derived fibroin will be described. First, as described above, 663 types of fibroins (415 types of fibroins derived from spiders among them) were extracted by confirming fibroins with amino acid sequence information registered in NCBI GenBank by an exemplified method. The values of z/w were calculated by the calculation method described above, from amino acid sequences of naturally derived fibroins which include a domain sequence represented by Formula 1: [(A) n  motif-REP]m and in which the content ratio of the amino acid sequence consisting of GGX in the fibroin is 6% or less, among all the extracted fibroins. The results are shown in  FIG. 2 . In  FIG. 2 , the horizontal axis represents z/w (%), and the vertical axis represents a frequency. As is clear from  FIG. 2 , the values of z/w in naturally derived fibroin are all smaller than 50.9% (the largest value is 50.86%). 
     In the second modified fibroin, z/w is preferably 50.9% or more, more preferably 56.1% or more, still more preferably 58.7% or more, even still more preferably 70% or more, and still further preferably 80% or more. The upper limit of z/w is not particularly limited, but may be 95% or less, for example. 
     The second modified fibroin can be obtained by, for example, substituting and modifying at least a part of a nucleotide sequence encoding a glycine residue from a cloned gene sequence of naturally derived fibroin so as to encode another amino acid residue. In this case, one glycine residue in a GGX motif or a GPGXX motif may be selected as the glycine residue to be modified, and substitution may be performed so that z/w is 50.9% or more. In addition, the second modified fibroin can also be obtained by, for example, designing an amino acid sequence satisfying each of the above aspects from the amino acid sequence of naturally derived fibroin, and chemically synthesizing a nucleic acid encoding the designed amino acid sequence. In any case, in addition to the modification corresponding to substitution of a glycine residue in REP with another amino acid residue from the amino acid sequence of naturally derived fibroin, modification of the amino acid sequence corresponding to substitution, deletion, insertion, and/or addition of one or a plurality of amino acid residues may be performed. 
     The above-described another amino acid residue is not particularly limited as long as it is an amino acid residue other than a glycine residue, but it is preferably a hydrophobic amino acid residue such as a valine (V) residue, a leucine (L) residue, an isoleucine (I) residue, a methionine (M) residue, a proline (P) residue, a phenylalanine (F) residue, or a tryptophan (W) residue, or a hydrophilic amino acid residue such as a glutamine (Q) residue, an asparagine (N) residue, a serine (S) residue, a lysine (K) residue, or a glutamic acid (E) residue, more preferably a valine (V) residue, a leucine (L) residue, an isoleucine (I) residue, a phenylalanine (F) residue, or a glutamine (Q) residue, and still more preferably a glutamine (Q) residue. 
     A more specific example of the second modified fibroin can include modified fibroin including (2-i) the amino acid sequence set forth in SEQ ID NO: 6 (Met-PRT380), SEQ ID NO: 7 (Met-PRT410), SEQ ID NO: 8 (Met-PRT525), or SEQ ID NO: 9 (Met-PRT799), or (2-ii) an amino acid sequence having 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, or SEQ ID NO: 9. 
     The modified fibroin of (2-i) will be described. The amino acid sequence set forth in SEQ ID NO: 6 is obtained by substituting all GGXs with GQX in REP of the amino acid sequence set forth in SEQ ID NO: 10 (Met-PRT313) corresponding to naturally derived fibroin. The amino acid sequence set forth in SEQ ID NO: 7 is obtained by deleting every other two (A) n  motifs from the N-terminal side to the C-terminal side from the amino acid sequence set forth in SEQ ID NO: 6 and further inserting one [(A) n  motif-REP] before the C-terminal sequence. The amino acid sequence set forth in SEQ ID NO: 8 is obtained by inserting two alanine residues on the C-terminal side of each (A) n  motif of the amino acid sequence set forth in SEQ ID NO: 7 and further substituting a part of glutamine (Q) residues with a serine (S) residue to delete a part of amino acids on the C-terminal side so as to be almost the same as the molecular weight of SEQ ID NO: 7. The amino acid sequence set forth in SEQ ID NO: 9 is obtained by adding a predetermined hinge sequence and a His tag sequence to the C-terminal of a sequence obtained by repeating a region of 20 domain sequences (where several amino acid residues on the C-terminal side of the region are substituted) present in the amino acid sequence set forth in SEQ ID NO: 7 four times. 
     The value of z/w in the amino acid sequence set forth in SEQ ID NO: 10 (corresponding to naturally derived fibroin) is 46.8%. The values of z/w in the amino acid sequence set forth in SEQ ID NO: 6, the amino acid sequence set forth in SEQ ID NO: 7, the amino acid sequence set forth in SEQ ID NO: 8, and the amino acid sequence set forth in SEQ ID NO: 9 are 58.7%, 70.1%, 66.1%, and 70.0%, respectively. In addition, the values of x/y in the amino acid sequences set forth in SEQ ID NO: 10, SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, and SEQ ID NO: 9 at a Giza ratio (described below) of 1:1.8 to 11.3 are 15.0%, 15.0%, 93.4%, 92.7%, and 89.8%, respectively. 
     The modified fibroin of (2-i) may consist of the amino acid sequence set forth in SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, or SEQ ID NO: 9. 
     The modified fibroin of (2-ii) includes an amino acid sequence having 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, or SEQ ID NO: 9. The modified fibroin of (2-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. 
     It is preferred that the modified fibroin of (2-ii) preferably has 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, or SEQ ID NO: 9, and z/w is 50.9% or more in a case where the total number of amino acid residues in the amino acid sequence consisting of XGX (where X represents an amino acid residue other than glycine) included in REP is defined as z, and the total number of amino acid residues of REP in the domain sequence is defined as w. 
     The second modified fibroin may have a tag sequence at either or both of the N-terminal and the C-terminal. This enables the modified fibroin to be isolated, immobilized, detected, and visualized. 
     The tag sequence may be, for example, an affinity tag utilizing specific affinity (binding property, affinity) with another molecule. A specific example of the affinity tag includes a histidine tag (His tag). The His tag is a short peptide in which about 4 to 10 histidine residues are arranged and has a property of specifically binding to a metal ion such as nickel. Thus, the His tag can be used for isolation of modified fibroin by chelating metal chromatography. A specific example of the tag sequence can include the amino acid sequence set forth in SEQ ID NO: 11 (amino acid sequence including a His tag sequence and a hinge sequence). 
     Also, a tag sequence such as glutathione-S-transferase (GST) that specifically binds to glutathione, and a maltose binding protein (MBP) that specifically binds to maltose can also be utilized. 
     Further, an “epitope tag” utilizing an antigen-antibody reaction can also be utilized. Adding a peptide (epitope) exhibiting antigenicity as a tag sequence allows an antibody against the epitope to be bound. Examples of the epitope tag include an HA (peptide sequence of hemagglutinin of influenza virus) tag, a myc tag, and a FLAG tag. The modified fibroin can easily be purified with high specificity by utilizing an epitope tag. 
     Moreover, it is possible to use a tag sequence which can be cleaved with a specific protease. The modified fibroin from which the tag sequence has been cleaved can be recovered by treating a protein adsorbed through the tag sequence with protease. 
     A more specific example of the modified fibroin including a tag sequence can include modified fibroin including (2-iii) the amino acid sequence set forth in SEQ ID NO: 12 (PRT380), SEQ ID NO: 13 (PRT410), SEQ ID NO: 14 (PRT525), or SEQ ID NO: 15 (PRT799), or (2-iv) an amino acid sequence having 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, or SEQ ID NO: 15. 
     Each of the amino acid sequences set forth in SEQ ID NO: 16 (PRT313), SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, and SEQ ID NO: 15 is obtained by adding the amino acid sequence set forth in SEQ ID NO: 11 (including a His tag sequence and a hinge sequence) to the N-terminal of each of the amino acid sequences set forth in SEQ ID NO: 10, SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, and SEQ ID NO: 9. 
     The modified fibroin of (2-iii) may consist of the amino acid sequence set forth in SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, or SEQ ID NO: 15. 
     The modified fibroin of (2-iv) includes an amino acid sequence having 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, or SEQ ID NO: 15. The modified fibroin of (2-iv) 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. 
     It is preferred that the modified fibroin of (2-iv) has 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, or SEQ ID NO: 15, and z/w is 50.9% or more in a case where the total number of amino acid residues in the amino acid sequence consisting of XGX (where X represents the amino acid residue other than glycine) in REP is defined as z, and the total number of amino acid residues in REP in the domain sequence is defined as w. 
     The second modified fibroin may include a secretory signal for releasing the protein produced in the recombinant protein production system to the outside of a host. The sequence of the secretory signal can be appropriately set depending on the type of the host. 
     The domain sequence of the third modified fibroin has an amino acid sequence in which the content of the (A) n  motif is reduced, as compared with naturally derived fibroin. It can be said that the domain sequence of the third modified fibroin has an amino acid sequence corresponding to an amino acid sequence in which at least one or a plurality of (A) n  motifs are deleted, as compared with naturally derived fibroin. 
     The third modified fibroin may have an amino acid sequence corresponding to an amino acid sequence in which 10 to 40% of the (A) n  motifs are deleted from naturally derived fibroin. 
     The domain sequence of the third modified fibroin may have an amino acid sequence corresponding to an amino acid sequence in which at least one (A) n  motif of every one to three (A) n  motifs is deleted from the N-terminal side to the C-terminal side, as compared with naturally derived fibroin. 
     The third modified fibroin may have an amino acid sequence corresponding to an amino acid sequence in which deletion of at least two consecutive (A) n  motifs and deletion of one (A) n  motif are repeated in this order from the N-terminal side to the C-terminal side, as compared with naturally derived fibroin. 
     The third modified fibroin may have a domain sequence having an amino acid sequence corresponding to an amino acid sequence in which at least (A) n  motif every other two positions is deleted from the N-terminal side to the C-terminal side. 
     The third modified fibroin includes a domain sequence represented by Formula 1: [(A) n  motif-REP] m , and may have an amino acid sequence in which x/y is 20% or more, 30% or more, 40% or more, or 50% or more in a case where the numbers of amino acid residues in REPs of two adjacent [(A) n  motif-REP] units are sequentially compared from the N-terminal side to the C-terminal side, and the number of amino acid residues in one REP having a smaller number of amino acid residues is defined as 1, the maximum value of the total value of the number of amino acid residues in the two adjacent [(A) n  motif-REP] units where the ratio of the number of amino acid residues in the other REP is 1.8 to 11.3 is defined as x, and the total number of amino acid residues in the domain sequence is defined as y. The number of alanine residues with respect to the total number of amino acid residues in the (A) n  motif is 83% or more, preferably 86% or more, more preferably 90% or more, still more preferably 95% or more, and even still more preferably 100% (meaning that the (A) n  motif consists of only alanine residues). 
     The method of calculating x/y will be described in more detail with reference to  FIG. 1 .  FIG. 1  shows a domain sequence excluding the N-terminal sequence and the C-terminal sequence from the modified fibroin. The domain sequence has a sequence of (A) n  motif-first REP (50 amino acid residues)-(A) n  motif-second REP (100 amino acid residues)-(A) n  motif-third REP (10 amino acid residues)-(A) n  motif-fourth REP (20 amino acid residues)-(A) n  motif-fifth REP (30 amino acid residues)-(A) n  motif from the N-terminal side (left side). 
     The two adjacent [(A) n  motif-REP] units are sequentially selected from the N-terminal side to the C-terminal side so as not to overlap. At this time, an unselected [(A) n  motif-REP] unit may exist.  FIG. 1  shows a pattern 1 (a comparison between the first REP and the second REP, and a comparison between the third REP and the fourth REP), a pattern 2 (a comparison between the first REP and the second REP, and a comparison between the fourth REP and the fifth REP), a pattern 3 (a comparison between the second REP and the third REP, and a comparison between the fourth REP and the fifth REP), and a pattern 4 (a comparison between the first REP and the second REP). There are other selection methods besides this. 
     Subsequently, the number of amino acid residues of each REP in the selected two adjacent [(A) n  motif-REP] units is compared for each pattern. The comparison is performed by determining the ratio of the number of amino acid residues of the other REP in a case where one REP having a smaller number of amino acid residues is defined as 1. For example, in a case of comparing the first REP (50 amino acid residues) and the second REP (100 amino acid residues), the ratio of the number of amino acid residues of the second REP is 100/50=2 in a case where the first REP having a smaller number of amino acid residues is defined as 1. Similarly, in a case of comparing the fourth REP (20 amino acid residues) and the fifth REP (30 amino acid residues), the ratio of the number of amino acid residues of the fifth REP is 30/20=1.5 in a case where the fourth REP having a smaller number of amino acid residues is defined as 1. 
     In  FIG. 1 , a set of [(A) n  motif-REP] units in which the ratio of the number of amino acid residues of the other REP is 1.8 to 11.3 in a case where one REP having a smaller number of amino acid residues is defined as 1 is indicated by a solid line. In the present specification, the ratio is referred to as a Giza ratio. A set of [(A) n  motif-REP] units in which the ratio of the number of amino acid residues of the other REP is less than 1.8 or more than 11.3 in a case where one REP having a smaller number of amino acid residues is defined as 1 is indicated by a broken line. 
     In each pattern, the number of all amino acid residues of two adjacent [(A) n  motif-REP] units indicated by solid lines (including not only the number of amino acid residues of REP but also the number of amino acid residues of the (A)n motif) is combined. Then, the total values combined are compared, and the total value of the pattern whose total value is the maximum (the maximum value of the total value) is defined as x. In the example shown in  FIG. 1 , the total value of the pattern 1 is the maximum. 
     Then, x/y (%) can be calculated by dividing x by the total number of amino acid residues y of the domain sequence. 
     In the third modified fibroin, x/y is preferably 50% or more, more preferably 60% or more, still more preferably 65% or more, even still more preferably 70% or more, still further preferably 75% or more, and particularly preferably 80% or more. The upper limit of x/y is not particularly limited, but may be, for example, 100% or less. In a case where the Giza ratio is 1:1.9 to 11.3, x/y is preferably 89.6% or more; in a case where the Giza ratio is 1:1.8 to 3.4, x/y is preferably 77.1% or more; in a case where the Giza ratio is 1:1.9 to 8.4, x/y is preferably 75.9% or more; and in a case where the Giza ratio is 1:1.9 to 4.1, x/y is preferably 64.2% or more. 
     In a case where the third modified fibroin is modified fibroin in which at least seven of a plurality of (A) n  motifs in the domain sequence consist of only alanine residues, x/y is preferably 46.4% or more, more preferably 50% or more, still more preferably 55% or more, even still more preferably 60% or more, still further preferably 70% or more, and particularly preferably 80% or more. The upper limit of x/y is not particularly limited, but is only required to be 100% or less. 
     Here, x/y in naturally derived fibroin will be described. First, as described above, 663 types of fibroins (415 types of fibroins derived from spiders among them) were extracted by confirming fibroins with amino acid sequence information registered in NCBI GenBank by an exemplified method. The values of x/y were calculated by the calculation method described above, from amino acid sequences of naturally derived fibroins consisting of a domain sequence represented by Formula 1: [(A) n  motif-REP] m , among all the extracted fibroins. The results in a case where the Giza ratio is 1:1.9 to 4.1 are shown in  FIG. 3 . 
     The horizontal axis in  FIG. 3  represents x/y (%), and the vertical axis represents a frequency. As is clear from  FIG. 3 , the values of x/y in naturally derived fibroin are all smaller than 64.2% (the largest value is 64.14%). 
     The third modified fibroin can be obtained from, for example, a cloned gene sequence of naturally derived fibroin, by deleting one or a plurality of sequences encoding an (A) n  motif so that x/y is 64.2% or more. In addition, for example, the third modified fibroin can also be obtained, from the amino acid sequence of naturally derived fibroin, by designing an amino acid sequence corresponding to an amino acid sequence in which one or a plurality of (A) n  motifs are deleted so that x/y is 64.2% or more, and chemically synthesizing a nucleic acid encoding the designed amino acid sequence. In any case, in addition to the modification corresponding to deletion of the (A) n  motif from the amino acid sequence of naturally derived fibroin, modification of the amino acid sequence corresponding to substitution, deletion, insertion, and/or addition of one or a plurality of amino acid residues may be performed. 
     A more specific example of the third modified fibroin can include modified fibroin including (3-i) the amino acid sequence set forth in SEQ ID NO: 17 (Met-PRT399), SEQ ID NO: 7 (Met-PRT410), SEQ ID NO: 8 (Met-PRT525), or SEQ ID NO: 9 (Met-PRT799), or (3-ii) an amino acid sequence having 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 17, SEQ ID NO: 7, SEQ ID NO: 8, or SEQ ID NO: 9. 
     The modified fibroin of (3-i) will be described. The amino acid sequence set forth in SEQ ID NO: 17 is obtained by deleting every other two (A) n  motifs from the N-terminal side to the C-terminal side from the amino acid sequence set forth in SEQ ID NO: 10 (Met-PRT313) corresponding to naturally derived fibroin and further inserting one [(A) n  motif-REP] before the C-terminal sequence. The amino acid sequence set forth in SEQ ID NO: 7, SEQ ID NO: 8, or SEQ ID NO: 9 is as described in the second modified fibroin. 
     The value of x/y in the amino acid sequence set forth in SEQ ID NO: 10 (corresponding to naturally derived fibroin) at a Giza ratio of 1:1.8 to 11.3 is 15.0%. Both the value of x/y in the amino acid sequence set forth in SEQ ID NO: 17 and the value of x/y in the amino acid sequence set forth in SEQ ID NO: 7 are 93.4%. The value of x/y in the amino acid sequence set forth in SEQ ID NO: 8 is 92.7%. The value of x/y in the amino acid sequence set forth in SEQ ID NO: 9 is 89.8%. The values of z/w in the amino acid sequences set forth in SEQ ID NO: 10, SEQ ID NO: 17, SEQ ID NO: 7, SEQ ID NO: 8, and SEQ ID NO: 9 are 46.8%, 56.2%, 70.1%, 66.1%, and 70.0%, respectively. 
     The modified fibroin of (3-i) may consist of the amino acid sequence set forth in SEQ ID NO: 17, SEQ ID NO: 7, SEQ ID NO: 8, or SEQ ID NO: 9. 
     The modified fibroin of (3-ii) includes an amino acid sequence having 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 17, SEQ ID NO: 7, SEQ ID NO: 8, or SEQ ID NO: 9. The modified fibroin of (3-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. 
     It is preferred that the modified fibroin of (3-ii) has 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 17, SEQ ID NO: 7, SEQ ID NO: 8, or SEQ ID NO: 9, and x/y is 64.2% or more in a case where the numbers of amino acid residues in REPs of two adjacent [(A) n  motif-REP] units are sequentially compared from the N-terminal side to the C-terminal side, and the number of amino acid residues in one REP having a small number of amino acid residues is defined as 1, the maximum value of the total value of the number of amino acid residues in the two adjacent [(A) n  motif-REP] units where the ratio of the number of amino acid residues in the other REP is 1.8 to 11.3 (the Giza ratio is 1:1.8 to 11.3) is defined as x, and the total number of amino acid residues in the domain sequence is defined as y. 
     The third modified fibroin may include the above-described tag sequence at either or both of the N-terminal and the C-terminal. 
     A more specific example of the modified fibroin including a tag sequence can include modified fibroin including (3-iii) the amino acid sequence set forth in SEQ ID NO: 18 (PRT399), SEQ ID NO: 13 (PRT410), SEQ ID NO: 14 (PRT525), or SEQ ID NO: 15 (PRT799), or (3-iv) an amino acid sequence having 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 18, SEQ ID NO: 13, SEQ ID NO: 14, or SEQ ID NO: 15. 
     Each of the amino acid sequences set forth in SEQ ID NO: 18, SEQ ID NO: 13, SEQ ID NO: 14, and SEQ ID NO: 15 is obtained by adding the amino acid sequence set forth in SEQ ID NO: 11 (including a His tag sequence and a hinge sequence) to the N-terminal of each of the amino acid sequences set forth in SEQ ID NO: 17, SEQ ID NO: 7, SEQ ID NO: 8, and SEQ ID NO: 9. 
     The modified fibroin of (3-iii) may consist of the amino acid sequence set forth in SEQ ID NO: 18, SEQ ID NO: 13, SEQ ID NO: 14, or SEQ ID NO: 15. 
     The modified fibroin of (3-iv) includes an amino acid sequence having 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 18, SEQ ID NO: 13, SEQ ID NO: 14, or SEQ ID NO: 15. The modified fibroin of (3-iv) 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. 
     It is preferred that the modified fibroin of (3-iv) has 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 18, SEQ ID NO: 13, SEQ ID NO: 14, or SEQ ID NO: 15, and x/y is 64.2% or more in a case where the number of amino acid residues in REPs in two adjacent [(A) n  motif-REP] units are sequentially compared from the N-terminal side to the C-terminal side, and the number of amino acid residues in one REP having a small number of amino acid residues is defined as 1, the maximum value of the total value of the number of amino acid residues in the two adjacent [(A) n  motif-REP] units where the ratio of the number of amino acid residues in the other REP is 1.8 to 11.3 is defined as x, and the total number of amino acid residues in the domain sequence is defined as y. 
     The third modified fibroin may include a secretory signal for releasing the protein produced in the recombinant protein production system to the outside of a host. The sequence of the secretory signal can be appropriately set depending on the type of the host. 
     The domain sequence of the fourth modified fibroin has an amino acid sequence in which the content of an (A) n  motif and the content of glycine residues are reduced, as compared with naturally derived fibroin. It can be said that the domain sequence of the fourth modified fibroin has an amino acid sequence corresponding to an amino acid sequence in which at least one or a plurality of (A) n  motifs are deleted and at least one or a plurality of glycine residues in REP are substituted with another amino acid residue, as compared with naturally derived fibroin. That is, the fourth modified fibroin is modified fibroin having the characteristics of the above-described second modified fibroin and third modified fibroin. Specific aspects thereof, and the like are as in the descriptions for the second modified fibroin and the third modified fibroin. 
     A more specific example of the fourth modified fibroin can include modified fibroin including (4-i) the amino acid sequence set forth in SEQ ID NO: 7 (Met-PRT410), SEQ ID NO: 8 (Met-PRT525), SEQ ID NO: 9 (Met-PRT799), SEQ ID NO: 13 (PRT410), SEQ ID NO: 14 (PRT525), or SEQ ID NO: 15 (PRT799), or (4-ii) an amino acid sequence having 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9, SEQ ID NO: 13, SEQ ID NO: 14, or SEQ ID NO: 15. Specific aspects of the modified fibroin including the amino acid sequence set forth in SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9, SEQ ID NO: 13, SEQ ID NO: 14, or SEQ ID NO: 15 are as described above. 
     The domain sequence of the fifth modified fibroin may have an amino acid sequence including a region locally having a high hydropathy index corresponding to an amino acid sequence in which one or a plurality of amino acid residues in REP are substituted with amino acid residues having a high hydropathy index and/or one or a plurality of amino acid residues having a high hydropathy index are inserted into REP, as compared with naturally derived fibroin. 
     The region locally having a high hydropathy index preferably consists of consecutive two to four amino acid residues. 
     The above-described amino acid residue having a high hydropathy index is more preferably an amino acid residue selected from isoleucine (I), valine (V), leucine (L), phenylalanine (F), cysteine (C), methionine (M), and alanine (A). 
     The fifth modified fibroin may be further subjected to modification of an amino acid sequence corresponding to substitution, deletion, insertion, and/or addition of one or a plurality of amino acid residues as compared with naturally derived fibroin, in addition to modification corresponding to substitution of one or a plurality of amino acid residues in REP with amino acid residues having a high hydropathy index and/or insertion of one or a plurality of amino acid residues having a high hydropathy index into REP, as compared with naturally derived fibroin. 
     The fifth modified fibroin can be obtained by, for example, substituting one or a plurality of hydrophilic amino acid residues in REP (for example, amino acid residues having a negative hydropathy index) with hydrophobic amino acid residues (for example, amino acid residues having a positive hydropathy index) from a cloned gene sequence of naturally derived fibroin, and/or inserting one or a plurality of hydrophobic amino acid residues into REP. In addition, the fifth modified fibroin can be obtained by, for example, designing an amino acid sequence corresponding to an amino acid sequence in which one or a plurality of hydrophilic amino acid residues in REP are substituted with hydrophobic amino acid residues from an amino acid sequence of naturally derived fibroin, and/or one or a plurality of hydrophobic amino acid residues are inserted into REP, and chemically synthesizing a nucleic acid encoding the designed amino acid sequence. In any case, in addition to modification corresponding to substitution of one or a plurality of hydrophilic amino acid residues in REP with hydrophobic amino acid residues from an amino acid sequence of naturally derived fibroin, and/or insertion of one or a plurality of hydrophobic amino acid residues into REP, modification of an amino acid sequence corresponding to substitution, deletion, insertion, and/or addition of one or a plurality of amino acid residues may be further performed. 
     The fifth modified fibroin includes a domain sequence represented by Formula 1: [(A) n  motif-REP] m , and may have an amino acid sequence in which p/q is 6.2% or more in a case where in all REPs included in a sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence, the total number of amino acid residues included in a region where the average value of hydropathy indices of four consecutive amino acid residues is 2.6 or more is defined as p, and the total number of amino acid residues included in the sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence is defined as q. 
     A known index (Hydropathy index: Kyte J, &amp; Doolittle R (1982), “A simple method for displaying the hydropathic character of a protein”, J. Mol. Biol., 157, pp. 105-132) is used as the hydropathy index of the amino acid residue. Specifically, the hydropathy index (hereinafter, also referred to as “HI”) of each amino acid is as shown in Table 1. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Amino acid 
                 HI 
                 Amino acid 
                 HI 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Isoleucine (Ile) 
                 4.5 
                 Tryptophan (Trp) 
                 −0.9 
               
               
                   
                 Valine (Val) 
                 4.2 
                 Tyrosine (Tyr) 
                 −1.3 
               
               
                   
                 Leucine (Leu) 
                 3.8 
                 Proline (Pro) 
                 −1.6 
               
               
                   
                 Phenylalanine (Phe) 
                 2.8 
                 Histidine (His) 
                 −3.2 
               
               
                   
                 Cysteine (Cys) 
                 2.5 
                 Asparagine (Asn) 
                 −3.5 
               
               
                   
                 Methionine (Met) 
                 1.9 
                 Aspartic acid (Asp) 
                 −3.5 
               
               
                   
                 Alanine (Ala) 
                 1.8 
                 Glutamine (Gln) 
                 −3.5 
               
               
                   
                 Glycine (Gly) 
                 −0.4 
                 Glutamine acid (Glu) 
                 −3.5 
               
               
                   
                 Threonine (Thr) 
                 −0.7 
                 Lysine (Lys) 
                 −3.9 
               
               
                   
                 Serine (Ser) 
                 −0.8 
                 Arginine (Arg) 
                 −4.5 
               
               
                   
                   
               
            
           
         
       
     
     The method of calculating p/q will be described in more detail. In the calculation, a sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence represented by Formula 1 [(A) n  motif-REP] m  (hereinafter also referred to as “sequence A”) is used. First, in all REPs included in the sequence A, the average values of hydropathy indices of four consecutive amino acid residues are calculated. The average value of hydropathy indices is determined by dividing the sum of HIs of respective amino acid residues included in the four consecutive amino acid residues by 4 (number of amino acid residues). The average value of hydropathy indices is determined for all of the four consecutive amino acid residues (each of the amino acid residues is used for calculating the average value 1 to 4 times). Then, a region where the average value of hydropathy indices of the four consecutive amino acid residues is 2.6 or more is specified. Even in a case where a certain amino acid residue corresponds to the “four consecutive amino acid residues having an average value of hydropathy indices of 2.6 or more” multiple times, the amino acid residue is included as one amino acid residue in the region. The total number of amino acid residues included in the region is p. Also, the total number of amino acid residues included in the sequence A is q. 
     For example, in a case where the “four consecutive amino acid residues having an average value of hydropathy indices of 2.6 or more” are extracted from 20 places (no overlap), in the region where the average value of hydropathy indices of four consecutive amino acid residues is 2.6 or more, 20 of the four consecutive amino acid residues (no overlap) are included, and thus p is 20×4=80. Further, for example, in a case where two of the “four consecutive amino acid residues having an average value of hydropathy indices of 2.6 or more” overlap by one amino acid residue, in the region where the average value of hydropathy indices of the four consecutive amino acid residues is 2.6 or more, seven amino acid residues are included (p=2×4−1=7. “−1” is the deduction of the overlapping portion). For example, in a case of the domain sequence shown in  FIG. 4 , seven sets of “four consecutive amino acid residues having an average value of hydropathy indices of 2.6 or more” are present without overlaps, and thus, p is 7×4=28. Furthermore, for example, in the case of the domain sequence shown in  FIG. 4 , q is 4+50+4+40+4+10+4+20+4+30=170 (the (A) n  motif located at the end of the C-terminal side is excluded). Next, p/q (%) can be calculated by dividing p by q. In the case of  FIG. 4 , p/q is 28/170=16.47%. 
     In the fifth modified fibroin, p/q is preferably 6.2% or more, more preferably 7% or more, still more preferably 10% or more, even still more preferably 20% or more, and still further preferably 30% or more. The upper limit of p/q is not particularly limited, but may be 45% or less, for example. 
     The fifth modified fibroin can be obtained by, for example, substituting one or a plurality of hydrophilic amino acid residues in REP (for example, amino acid residues having a negative hydropathy index) with hydrophobic amino acid residues (for example, amino acid residues having a positive hydropathy index) so that a cloned amino acid sequence of naturally derived fibroin satisfies the condition of p/q, and/or modifying the cloned amino acid sequence of naturally derived fibroin into an amino acid sequence including a region locally having a high hydropathy index by inserting one or a plurality of hydrophobic amino acid residues into REP. In addition, the fifth modified fibroin can also be obtained by, for example, designing an amino acid sequence satisfying the condition of p/q from the amino acid sequence of naturally derived fibroin, and chemically synthesizing a nucleic acid encoding the designed amino acid sequence. In any case, modification corresponding to substitution, deletion, insertion, and/or addition of one or a plurality of amino acid residues may also be performed, in addition to modification corresponding to substitution of one or a plurality of amino acid residues in REP with amino acid residues having a high hydropathy index, and/or insertion of one or a plurality of amino acid residues having a high hydropathy index into REP, as compared with naturally derived fibroin. 
     The amino acid residue having a high hydropathy index is not particularly limited, but is preferably isoleucine (I), valine (V), leucine (L), phenylalanine (F), cysteine (C), methionine (M), and alanine (A), and more preferably valine (V), leucine (L), and isoleucine (I). 
     A more specific example of the fifth modified fibroin can include modified fibroin including (5-i) the amino acid sequence set forth in SEQ ID NO: 19 (Met-PRT720), SEQ ID NO: 20 (Met-PRT665), or SEQ ID NO: 21 (Met-PRT666), or (5-ii) an amino acid sequence having 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 19, SEQ ID NO: 20, or SEQ ID NO: 21. 
     The modified fibroin of (5-i) will be described. The amino acid sequence set forth in SEQ ID NO: 19 is obtained by inserting an amino acid sequence consisting of three amino acid residues (VLI) at two sites for each REP into the amino acid sequence set forth in SEQ ID NO: 7 (Met-PRT410), except for the domain sequence at the end on the C-terminal side, and further substituting a part of glutamine (Q) residues with serine (S) residues, and deleting a part of amino acids on the C-terminal side. The amino acid sequence set forth in SEQ ID NO: 20 is obtained by inserting the amino acid sequence consisting of three amino acid residues (VLI) at one site for each REP into the amino acid sequence set forth in SEQ ID NO: 8 (Met-PRT525). The amino acid sequence set forth in SEQ ID NO: 21 is obtained by inserting the amino acid sequence consisting of three amino acid residues (VLI) at two sites for each REP into the amino acid sequence set forth in SEQ ID NO: 8. 
     The modified fibroin of (5-i) may consist of the amino acid sequence set forth in SEQ ID NO: 19, SEQ ID NO: 20, or SEQ ID NO: 21. 
     The modified fibroin of (5-ii) includes an amino acid sequence having 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 19, SEQ ID NO: 20, or SEQ ID NO: 21. The modified fibroin of (5-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. 
     It is preferred that the modified fibroin of (5-ii) has 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 19, SEQ ID NO: 20, or SEQ ID NO: 21, and p/q is 6.2% or more in a case where in all REPs included in a sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence, the total number of amino acid residues included in a region where the average value of hydropathy indices of four consecutive amino acid residues is 2.6 or more is defined as p, and the total number of amino acid residues included in the sequence excluding the sequence from the (A) n  motif located at the most the C-terminal side to the C-terminal of the domain sequence from the domain sequence is defined as q. 
     The fifth modified fibroin may include a tag sequence at either or both of the N-terminal and the C-terminal. 
     A more specific example of the modified fibroin including a tag sequence can include modified fibroin including (5-iii) the amino acid sequence set forth in SEQ ID NO: 22 (PRT720), SEQ ID NO: 23 (PRT665), or SEQ ID NO: 24 (PRT666), or (5-iv) an amino acid sequence having 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 22, SEQ ID NO: 23, or SEQ ID NO: 24. 
     Each of the amino acid sequences set forth in SEQ ID NO: 22, SEQ ID NO: 23, and SEQ ID NO: 24 is obtained by adding the amino acid sequence set forth in SEQ ID NO: 11 (including a His tag sequence and a hinge sequence) to the N-terminal of each of the amino acid sequences set forth in SEQ ID NO: 19, SEQ ID NO: 20, and SEQ ID NO: 21. 
     The modified fibroin of (5-iii) may consist of the amino acid sequence set forth in SEQ ID NO: 22, SEQ ID NO: 23, or SEQ ID NO: 24. 
     The modified fibroin of (5-iv) includes an amino acid sequence having 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 22, SEQ ID NO: 23, or SEQ ID NO: 24. The modified fibroin of (5-iv) 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. 
     It is preferred that the modified fibroin of (5-iv) has 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 22, SEQ ID NO: 23, or SEQ ID NO: 24, and p/q is 6.2% or more in a case where in all REPs included in a sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence, the total number of amino acid residues included in a region where the average value of hydropathy indices of four consecutive amino acid residues is 2.6 or more is defined as p, and the total number of amino acid residues included in the sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence is defined as q. 
     The fifth modified fibroin may include a secretory signal for releasing the protein produced in the recombinant protein production system to the outside of a host. The sequence of the secretory signal can be appropriately set depending on the type of the host. 
     The sixth modified fibroin has an amino acid sequence in which the content of glutamine residues is reduced, as compared with naturally derived fibroin. 
     In the sixth modified fibroin, at least one motif selected from a GGX motif and a GPGXX motif is preferably included in the amino acid sequence of REP. 
     In a case where the sixth modified fibroin has the GPGXX motif in REP, the GPGXX motif content is usually 1% or more, may also be 5% or more, and preferably 10% or more. The upper limit of the GPGXX motif content is not particularly limited, and may be 50% or less, or may also be 30% or less. 
     In the present specification, the “GPGXX motif content” is a value calculated by the following method. 
     The content rate of the GPGXX motif in fibroin (modified fibroin or naturally derived fibroin) including a domain sequence represented by Formula 1: [(A) n  motif-REP] m  or Formula 2: [(A) n  motif-REP] m -(A) n  motif is calculated as s/t, in a case where the number obtained by tripling the total number of GPGXX motifs in regions of all REPs included in a sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence (that is, corresponding to the total number of G and P in the GPGXX motifs) is defined as s, and the total number of amino acid residues in all REPs excluding a sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence and further excluding the (A) n  motifs is defined as t. 
     In the calculation of the content rate of the GPGXX motif, the “sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence” is used to exclude the effect occurring due to the fact that the “sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence” (a sequence corresponding to REP) may include a sequence having a low correlation with the sequence characteristic of fibroin, which influences the calculation result of the content rate of the GPGXX motif in a case where m is small (that is, in a case where the domain sequence is short). Incidentally, in a case where the “GPGXX motif” is located at the C-terminal of REP, even when “XX” is “AA”, for example, it is treated as the “GPGXX motif”. 
       FIG. 5  is a schematic diagram illustrating a domain sequence of modified fibroin. The method for calculating the content rate of the GPGXX motif will be specifically described with reference to  FIG. 5 . First, in the domain sequence of the modified fibroin shown in  FIG. 5  (which is the “[(A) n  motif-REP] m -(A) n  motif” type), all REPs are included in the “sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence” (in  FIG. 5 , the sequence indicated as a “region A”), and therefore, the number of the GPGXX motifs for calculating s is 7, and s is 7×3=21. Similarly, since all REPs are included in the “sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence” (in  FIG. 5 , the sequence indicated as the “region A”), the total number t of the amino acid residues in all REPs when the (A) n  motifs are further excluded from the sequence is 50+40+10+20+30=150. Next, s/t (%) can be calculated by dividing s by t, and in the case of the modified fibroin of  FIG. 5 , s/t is 21/150=14.0%. 
     In the sixth modified fibroin, the glutamine residue content is preferably 9% or less, more preferably 7% or less, still more preferably 4% or less, and particularly preferably 0%. 
     In the present specification, the “glutamine residue content” is a value calculated by the following method. 
     The glutamine residue content in fibroin (modified fibroin or naturally derived fibroin) including a domain sequence represented by Formula 1: [(A) n  motif-REP] m  or Formula 2: [(A) n  motif-REP] m -(A) n  motif is calculated as u/t, in a case where the total number of glutamine residues included in regions of all REPs included in a sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence (a sequence corresponding to the “region A” in  FIG. 5 ) is defined as u, and the total number of amino acid residues in all REPs in the sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence and further excluding the (A) n  motifs is defined as t. In the calculation of the glutamine residue content, the reason for targeting the “sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence” is the same as the reason descried above. 
     The domain sequence of the sixth modified fibroin may have an amino acid sequence corresponding to an amino acid sequence in which one or a plurality of glutamine residues in REP are deleted, or one or a plurality of glutamine residues are substituted with another amino acid residue, as compared with naturally derived fibroin. 
     The “another amino acid residue” may be an amino acid residue other than the glutamine residue, but is preferably an amino acid residue having a higher hydropathy index than that of the glutamine residue. The hydropathy index of the amino acid residue is as shown in Table 1. 
     As shown in Table 1, examples of the amino acid residue having a higher hydropathy index than that of the glutamine residue include amino acid residues selected from isoleucine (I), valine (V), leucine (L), phenylalanine (F), cysteine (C), methionine (M), alanine (A), glycine (G), threonine (T), serine (S), tryptophan (W), tyrosine (Y), proline (P), and histidine (H). Among them, the amino acid residue is more preferably an amino acid residue selected from isoleucine (I), valine (V), leucine (L), phenylalanine (F), cysteine (C), methionine (M), and alanine (A), and still more preferably an amino acid residue selected from isoleucine (I), valine (V), leucine (L), and phenylalanine (F). 
     In the sixth modified fibroin, the hydrophobicity of REP is preferably −0.8 or more, more preferably −0.7 or more, still more preferably 0 or more, even still more preferably 0.3 or more, and particularly preferably 0.4 or more. The upper limit of the hydrophobicity of REP is not particularly limited, but may be 1.0 or less or 0.7 or less. 
     In the present specification, the “hydrophobicity of REP” is a value calculated by the following method. 
     The hydrophobicity of REP in fibroin including a domain sequence represented by Formula 1: [(A) n  motif-REP] m  or Formula 2: [(A) n  motif-REP] m -(A) n  motif (modified fibroin or naturally derived fibroin) is calculated as v/t, in a case where the sum of hydropathy indices of amino acid residues in regions of all REPs included in a sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence (a sequence corresponding to the “region A” in  FIG. 5 ) is defined as v, and the total number of amino acid residues in all REPs in the sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence and further excluding the (A) n  motifs is defined as t. In the calculation of the hydrophobicity of REP, the reason for targeting the “sequence excluding the sequence from the (A) n  motif located at the most C-terminal side to the C-terminal of the domain sequence from the domain sequence” is the same as the reason descried above. 
     The domain sequence of the sixth modified fibroin may be further subjected to modification of an amino acid sequence corresponding to substitution, deletion, insertion, and/or addition of one or a plurality of amino acid residues, in addition to modification corresponding to deletion of one or a plurality of glutamine residues in REP, and/or substitution of one or a plurality of glutamine residues in REP with another amino acid residue, as compared with naturally derived fibroin. 
     The sixth modified fibroin can be obtained by, for example, deleting one or a plurality of glutamine residues in REP from a cloned gene sequence of naturally derived fibroin, and/or substituting one or a plurality of glutamine residues in REP with another amino acid residue. In addition, the sixth modified fibroin can be obtained by, for example, designing an amino acid sequence corresponding to an amino acid sequence in which one or a plurality of glutamine residues in REP are deleted from an amino acid sequence of naturally derived fibroin, and/or one or a plurality of glutamine residues in REP are substituted with another amino acid residue, and chemically synthesizing a nucleic acid encoding the designed amino acid sequence. 
     A more specific example of the sixth modified fibroin can include modified fibroin including (6-i) the amino acid sequence set forth in SEQ ID NO: 25 (Met-PRT888), SEQ ID NO: 26 (Met-PRT965), SEQ ID NO: 27 (Met-PRT889), SEQ ID NO: 28 (Met-PRT916), SEQ ID NO: 29 (Met-PRT918), SEQ ID NO: 30 (Met-PRT699), SEQ ID NO: 31 (Met-PRT698), SEQ ID NO: 32 (Met-PRT966), SEQ ID NO: 41 (Met-PRT917), or SEQ ID NO: 42 (Met-PRT1028), and modified fibroin including (6-ii) an amino acid sequence having 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 25, SEQ ID NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31, SEQ ID NO: 32, SEQ ID NO: 41, or SEQ ID NO: 42. 
     The modified fibroin of (6-i) will be described. The amino acid sequence set forth in SEQ ID NO: 25 is obtained by substituting all QQs in the amino acid sequence set forth in SEQ ID NO: 7 (Met-PRT410) with VL. The amino acid sequence set forth in SEQ ID NO: 26 is obtained by substituting all QQs in the amino acid sequence set forth in SEQ ID NO: 7 with TS and substituting the remaining Q with A. The amino acid sequence set forth in SEQ ID NO: 27 is obtained by substituting all QQs in the amino acid sequence set forth in SEQ ID NO: 7 with VL and substituting the remaining Q with I. The amino acid sequence set forth in SEQ ID NO: 28 is obtained by substituting all QQs in the amino acid sequence set forth in SEQ ID NO: 7 with VI and substituting the remaining Q with L. The amino acid sequence set forth in SEQ ID NO: 29 is obtained by substituting all QQs in the amino acid sequence set forth in SEQ ID NO: 7 with VF and substituting the remaining Q with I. 
     The amino acid sequence set forth in SEQ ID NO: 30 is obtained by substituting all QQs in the amino acid sequence set forth in SEQ ID NO: 8 (Met-PRT525) with VL. The amino acid sequence set forth in SEQ ID NO: 31 is obtained by substituting all QQs in the amino acid sequence set forth in SEQ ID NO: 8 with VL and substituting the remaining Q with I. 
     The amino acid sequence set forth in SEQ ID NO: 32 is obtained by substituting, with VF, all QQs in a sequence obtained by repeating a region of 20 domain sequences present in the amino acid sequence set forth in SEQ ID NO: 7 (Met-PRT410) two times and substituting the remaining Q with I. 
     The amino acid sequence set forth in SEQ ID NO: 41 (Met-PRT917) is obtained by substituting all QQs in the amino acid sequence set forth in SEQ ID NO: 7 with LI and substituting the remaining Q with V. The amino acid sequence set forth in SEQ ID NO: 42 (Met-PRT1028) is obtained by substituting all QQs in the amino acid sequence set forth in SEQ ID NO: 7 with IF and substituting the remaining Q with T. 
     The glutamine residue content in each of the amino acid sequences set forth in SEQ ID NO: 25, SEQ ID NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31, SEQ ID NO: 32, SEQ ID NO: 41, and SEQ ID NO: 42 is 9% or less (Table 2). 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Glutamine 
                 GPGXX 
                 Hydro- 
               
               
                   
                 residue 
                 motif 
                 phobicity 
               
               
                 Modified Fibroin 
                 content 
                 content 
                 of REP 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Met-PRT410 (SEQ ID NO: 7) 
                 17.7% 
                 27.9% 
                 −1.52 
               
               
                 Met-PRT888 (SEQ ID NO: 25) 
                 6.3% 
                 27.9% 
                 −0.07 
               
               
                 Met-PRT965 (SEQ ID NO: 26) 
                 0.0% 
                 27.9% 
                 −0.65 
               
               
                 Met-PRT889 (SEQ ID NO: 27) 
                 0.0% 
                 27.9% 
                 0.35 
               
               
                 Met-PRT916 (SEQ ID NO: 28) 
                 0.0% 
                 27.9% 
                 0.47 
               
               
                 Met-PRT918 (SEQ ID NO: 29) 
                 0.0% 
                 27.9% 
                 0.45 
               
               
                 Met-PRT699 (SEQ ID NO: 30) 
                 3.6% 
                 26.4% 
                 −0.78 
               
               
                 Met-PRT698 (SEQ ID NO: 31) 
                 0.0% 
                 26.4% 
                 −0.03 
               
               
                 Met-PRT966 (SEQ ID NO: 32) 
                 0.0% 
                 28.0% 
                 0.35 
               
               
                 Met-PRT917 (SEQ ID NO: 41) 
                 0.0% 
                 27.9% 
                 0.46 
               
               
                 Met-PRT1028 (SEQ ID NO: 42) 
                 0.0% 
                 28.1% 
                 0.05 
               
               
                   
               
            
           
         
       
     
     The modified fibroin of (6-i) may consist of the amino acid sequence set forth in SEQ ID NO: 25, SEQ ID NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31, SEQ ID NO: 32, SEQ ID NO: 41, or SEQ ID NO: 42. 
     The modified fibroin of (6-ii) includes an amino acid sequence having 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 25, SEQ ID NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31, SEQ ID NO: 32, SEQ ID NO: 41, or SEQ ID NO: 42. The modified fibroin of (6-ii) is also a protein including a domain sequence represented by Formula 1: [(A) n  motif-REP], or Formula 2: [(A) n  motif-REP] m -(A) n  motif. The sequence identity is preferably 95% or more. 
     In the modified fibroin of (6-ii), the glutamine residue content is preferably 9% or less. In the modified fibroin of (6-ii), the GPGXX motif content is preferably 10% or more. 
     The sixth modified fibroin may have a tag sequence at either or both of the N-terminal and the C-terminal. This enables the modified fibroin to be isolated, immobilized, detected, and visualized. 
     A more specific example of the modified fibroin including a tag sequence can include modified fibroin including (6-iii) the amino acid sequence set forth in SEQ ID NO: 33 (PRT888), SEQ ID NO: 34 (PRT965), SEQ ID NO: 35 (PRT889), SEQ ID NO: 36 (PRT916), SEQ ID NO: 37 (PRT918), SEQ ID NO: 38 (PRT699), SEQ ID NO: 39 (PRT698), SEQ ID NO: 40 (PRT966), SEQ ID NO: 43 (PRT917), or SEQ ID NO: 44 (PRT1028), or modified fibroin including (6-iv) an amino acid sequence having 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 33, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39, SEQ ID NO: 40, SEQ ID NO: 43, or SEQ ID NO: 44. 
     Each of the amino acid sequences set forth in SEQ ID NO: 33, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39, SEQ ID NO: 40, SEQ ID NO: 43, and SEQ ID NO: 44 is obtained by adding the amino acid sequence set forth in SEQ ID NO: 11 (including a His tag sequence and a hinge sequence) to the N-terminal of each of the amino acid sequences set forth in SEQ ID NO: 25, SEQ ID NO: 26, SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31, SEQ ID NO: 32, SEQ ID NO: 41, and SEQ ID NO: 42. Since only the tag sequence is added to the N-terminal, the glutamine residue content is not changed, and the glutamine residue content in each of the amino acid sequences set forth in SEQ ID NO: 33, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39, SEQ ID NO: 40, SEQ ID NO: 43, or SEQ ID NO: 44 is 9% or less (Table 3). 
     
       
         
           
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                 Glutamine 
                 GPGXX 
                 Hydro- 
               
               
                   
                 residue 
                 motif 
                 phobicity 
               
               
                 Modified Fibroin 
                 content 
                 content 
                 of REP 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 PRT888 (SEQ ID NO: 33) 
                 6.3% 
                 27.9% 
                 −0.07 
               
               
                 PRT965 (SEQ ID NO: 34) 
                 0.0% 
                 27.9% 
                 −0.65 
               
               
                 PRT889 (SEQ ID NO: 35) 
                 0.0% 
                 27.9% 
                 0.35 
               
               
                 PRT916 (SEQ ID NO: 36) 
                 0.0% 
                 27.9% 
                 0.47 
               
               
                 PRT918 (SEQ ID NO: 37) 
                 0.0% 
                 27.9% 
                 0.45 
               
               
                 PRT699 (SEQ ID NO: 38) 
                 3.6% 
                 26.4% 
                 −0.78 
               
               
                 PRT698 (SEQ ID NO: 39) 
                 0.0% 
                 26.4% 
                 −0.03 
               
               
                 PRT966 (SEQ ID NO: 40) 
                 0.0% 
                 28.0% 
                 0.35 
               
               
                 PRT917 (SEQ ID NO: 43) 
                 0.0% 
                 27.9% 
                 0.46 
               
               
                 PRT1028 (SEQ ID NO: 44) 
                 0.0% 
                 28.1% 
                 0.05 
               
               
                   
               
            
           
         
       
     
     The modified fibroin of (6-iii) may consist of the amino acid sequence set forth in SEQ ID NO: 33, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39, SEQ ID NO: 40, SEQ ID NO: 43, or SEQ ID NO: 44. 
     The modified fibroin of (6-iv) includes an amino acid sequence having 90% or more sequence identity with the amino acid sequence set forth in SEQ ID NO: 33, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39, SEQ ID NO: 40, SEQ ID NO: 43, or SEQ ID NO: 44. The modified fibroin of (6-iv) is also a protein including a domain sequence represented by Formula 1: [(A) n  motif-REP] m  or Formula 2: [(A) n  motif-REP] m -(A) n  motif. The sequence identity is preferably 95% or more. 
     In the modified fibroin of (6-iv), the glutamine residue content is preferably 9% or less. In the modified fibroin of (6-iv), the GPGXX motif content is preferably 10% or more. 
     The sixth modified fibroin may include a secretory signal for releasing the protein produced in the recombinant protein production system to the outside of a host. The sequence of the secretory signal can be appropriately set depending on the type of the host. 
     The modified fibroin may also be modified fibroin having at least two or more characteristics among the characteristics of the first modified fibroin, the second modified fibroin, the third modified fibroin, the fourth modified fibroin, the fifth modified fibroin, and the sixth modified fibroin. 
     The modified fibroin is preferably a hydrophilic modified fibroin because it is more excellent in water-absorbing and quick-drying properties. In the present specification, the “hydrophilic modified fibroin” is modified fibroin of which a value calculated by obtaining the sum of hydropathy indices (HIs) of all amino acid residues constituting the modified fibroin and then dividing the sum by the total number of amino acid residues (average HI) is 0 or less. The hydropathy index is as shown in Table 1. Modified fibroin having an average HI of more than 0 may also be referred to as hydrophobic modified fibroin. 
     Examples of the hydrophilic modified fibroin can include modified fibroin including the amino acid sequence set forth in SEQ ID NO: 4, the amino acid sequence set forth in SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, or SEQ ID NO: 9, the amino acid sequence set forth in SEQ ID NO: 13, SEQ ID NO: 11, SEQ ID NO: 14, or SEQ ID NO: 15, the amino acid sequence set forth in SEQ ID NO: 18, SEQ ID NO: 7, SEQ ID NO: 8, or SEQ ID NO: 9, the amino acid sequence set forth in SEQ ID NO: 17, SEQ ID NO: 11, SEQ ID NO: 14, or SEQ ID NO: 15, or the amino acid sequence set forth in SEQ ID NO: 19, SEQ ID NO: 20, or SEQ ID NO: 21. 
     Examples of the hydrophobic modified fibroin can include modified fibroin including the amino acid sequence set forth in SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31, SEQ ID NO: 32, SEQ ID NO: 33, or SEQ ID NO: 43, or the amino acid sequence set forth in SEQ ID NO: 35, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39, SEQ ID NO: 40, SEQ ID NO: 41, or SEQ ID NO: 44. 
     The modified fibroin according to the present embodiment can be produced by an ordinary method using a nucleic acid encoding the modified fibroin. The nucleic acid encoding the modified fibroin may be chemically synthesized based on nucleotide sequence information, or may be synthesized using a PCR method or the like. Isolation and purification of the produced modified fibroin can be performed by a commonly used method. 
     (Water-Absorbing and Quick-Drying Property-Imparting Agent) 
     The water-absorbing and quick-drying property-imparting agent according to the present embodiment may contain one type of modified fibroin alone or a combination of two or more types thereof. 
     The water-absorbing properties of the water-absorbing and quick-drying property-imparting agent according to the present embodiment as evaluated in accordance with JIS L 1907 may be 60 seconds or less, 30 seconds or less, 20 seconds or less, 10 seconds or less, or 5 seconds or less. Water-absorbing properties can be evaluated, for example, by a method described in Examples described later. 
     The quick-drying properties of the water-absorbing and quick-drying property-imparting agent according to the present embodiment as evaluated through measurement of the diffusible residual moisture content may be 100 minutes or less, 90 minutes or less, 80 minutes or less, or 70 minutes or less. The diffusible residual moisture content (%) is a value calculated by the following equation. 
       Diffusible residual moisture content (%)=weight (g) of water at each time/weight (g) of water at the start of measurement×100
 
     The quick-drying properties means a time required until the diffusible residual moisture content reaches 10% or less. The quick-drying properties can be evaluated, for example, by a method described in Examples described later. 
     The water-absorbing and quick-drying property-imparting agent according to the present embodiment may further contain other additives (components other than the active ingredient) according to its form, application, and the like. Examples of the additive include a plasticizer, a leveling agent, a crosslinking agent, a nucleating agent, an antioxidant, an ultraviolet absorber, a coloring agent, a filler, and a synthetic resin. The content of the additive may be 50 parts by mass or less with respect to 100 parts by mass of the total amount of the water-absorbing and quick-drying property-imparting agent. 
     The water-absorbing and quick-drying property-imparting agent according to the present embodiment may be in any form of, for example, a powder, a paste, and a liquid (for example, a suspension or solution). The water-absorbing and quick-drying property-imparting agent according to the present embodiment may also be in the form of, for example, a fiber, a film, a gel, a porous body, a particle, or the like. The form of the water-absorbing and quick-drying property-imparting agent according to the present embodiment may be appropriately set according to the object to which water-absorbing and quick-drying properties are imparted (water-absorbing and quick-drying property-imparted article) and the application thereof. 
     Since the water-absorbing and quick-drying property-imparting agent according to the present embodiment contains modified fibroin as a main component, the water-absorbing and quick-drying property-imparting agent can be formed into any form described above. The formed body may be formed of modified fibroin itself, or may be formed of a combination of modified fibroin and another material. 
     When the water-absorbing and quick-drying property-imparting agent according to the present embodiment is prepared in the form of a powder, for example, a protein obtained by the above-described method for producing modified fibroin may be dried into a powder. The protein powder may contain other additives as necessary. 
     When the water-absorbing and quick-drying property-imparting agent according to the present embodiment is prepared in the form of a liquid (for example, a solution), for example, a protein obtained by the above-described method for producing modified fibroin may be dissolved in a solvent that can dissolve modified fibroin to obtain a liquid (modified fibroin solution). The modified fibroin solution may contain other additives as necessary. Examples of the solvent that can dissolve modified fibroin can include dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), formic acid, and hexafluoroisopropanol (HFIP). An inorganic salt may be added to the solvent as a dissolution promoter. 
     When the water-absorbing and quick-drying property-imparting agent according to the present embodiment is prepared in the form of a fiber, for example, the modified fibroin solution may be used as a dope solution, and the dope solution is spun into fibers (modified fibroin fibers) by a known spinning method such as wet spinning, dry spinning, dry-wet spinning, or melt spinning. The form of the fiber may be a single yarn, may be a composite yarn such as a blended yarn, a combined filament yarn, a union yarn, a mix-weaved yarn, a piled yarn, and a covering yarn, or may be a nonwoven fabric or the like. 
     The modified fibroin fiber may be a short fiber or a long fiber. The modified fibroin fiber may also be a modified fibroin fiber alone or may be combined with other fibers. That is, a single yarn composed of only modified fibroin fibers, or a composite yarn composed of a combination of modified fibroin fibers and other fibers may be used singly or in combination. The single yarn and the composite yarn may be spun yarns in which short fibers are twisted, or may be filament yarns in which long fibers are twisted or not twisted. The modified fibroin fiber may be either a short fiber or a long fiber, and may be used as a fiber alone or in combination with another fiber without being processed into a yarn. Examples of the other fiber include synthetic fibers such as nylon and polyester, regenerated fibers such as cupra and rayon, and natural fibers such as cotton and hemp. When the modified fibroin fiber is used in combination with the other fiber, the content of the modified fibroin fiber is preferably 20 mass % or more, more preferably 30 mass % or more, still more preferably 40 mass % or more, and even still more preferably 50 mass % or more, based on the total amount of fibers. 
     When the water-absorbing and quick-drying property-imparting agent according to the present embodiment is prepared in the form of a film, a gel, a porous body, a particle, or the like, the water-absorbing and quick-drying property-imparting agent can be produced, for example, in accordance with the methods described in JP 2009-505668 A, JP 2009-505668 A, JP 5678283 B2, JP 4638735 B2, and the like. 
     [Method for Imparting Water-Absorbing and Quick-Drying Properties to Article] 
     A method for imparting water-absorbing and quick-drying properties to an article according to the present embodiment includes a step of incorporating modified fibroin into the article. Since the modified fibroin according to the present invention is excellent in water-absorbing and quick-drying properties, it is possible to impart water-absorbing and quick-drying properties to an article by incorporating the modified fibroin into the article. 
     The article is not particularly limited as long as it should impart water-absorbing and quick-drying properties. Specific examples thereof include fibers, woven fabrics, knitted fabrics, nonwoven fabrics, cotton, sponges, films, resins, composite materials (the form of the water-absorbing and quick-drying property-imparting agent according to the present embodiment is not limited), and various articles produced using these materials. 
     The step of incorporating the modified fibroin may be a step of incorporating the modified fibroin by blending the water-absorbing and quick-drying property-imparting agent according to the present invention described above. The method for incorporating the modified fibroin is not particularly limited, and may be a method of mixing the modified fibroin with the material (raw material), or a method of forming an article by combining the water-absorbing and quick-drying property-imparting agent prepared in the form of the above-described formed body with another material (formed body or the like). Alternatively, a method of forming an article (formed body) by forming modified fibroin itself (other additives may be included as necessary) may be used. 
     The content of the modified fibroin in the article is preferably 20 mass % or more, more preferably 30 mass % or more, still more preferably 40 mass % or more, and even still more preferably 50 mass % or more, based on the total amount of the article. The upper limit of the content of the modified fibroin may be 100 mass % or 90 mass % or less, based on the total amount of the article. By adjusting the content of the modified fibroin in the article, the water-absorbing and quick-drying properties of the article can be controlled. That is, since the modified fibroin according to the present invention is excellent in water-absorbing and quick-drying properties, the water-absorbing and quick-drying properties of the article can be further enhanced as the content of the modified fibroin in the article is increased. 
     Examples 
     Hereinafter, the present invention will be described more specifically based on Examples and the like. However, the present invention is not limited to the following Examples. 
     [Production of Modified Fibroin] 
     (1) Production of Expression Vector 
     Modified fibroin having the amino acid sequence set forth in SEQ ID NO: 37 (PRT918) and modified fibroin having the amino acid sequence shown in SEQ ID NO: 15 (PRT799) were designed. A nucleic acid encoding the designed modified fibroin was synthesized. In the nucleic acid, an NdeI site was added to the 5′ end and an EcoRI site was added downstream of the stop codon. The nucleic acid was cloned into a cloning vector (pUC118). Thereafter, the nucleic acid was enzymatically cleaved by treatment with NdeI and EcoRI, and then recombined into a protein expression vector pET-22b(+) to obtain an expression vector. 
     (2) Expression of Protein 
       Escherichia coli  BLR (DE3) was transformed with the obtained expression vector. The transformed  Escherichia coli  was cultured in 2 mL of an LB culture medium containing ampicillin for 15 hours. The culture solution was added to a 100 mL seed culture medium containing ampicillin (Table 4) so that OD 600  was 0.005. The temperature of the culture solution was maintained at 30° C., and the flask culture was performed (for about 15 hours) until the OD 600  reached 5, thus obtaining a seed culture solution. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Seed culture medium 
               
            
           
           
               
               
               
            
               
                   
                 Reagent 
                 Concentration (g/L) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Glucose 
                 5.0 
               
               
                   
                 KH 2 PO 4   
                 4.0 
               
               
                   
                 K 2 HPO 4   
                 9.3 
               
               
                   
                 Yeast Extract 
                 6.0 
               
               
                   
                 Ampicillin 
                 0.4 
               
               
                   
                   
               
            
           
         
       
     
     The seed culture solution was added to a jar fermenter to which a 500 mL production culture medium (Table 5) was added so that OD 600  was 0.05. The culture was performed while maintaining the culture solution temperature at 37° C. and constantly controlling the pH to 6.9. Further, the dissolved oxygen concentration in the culture solution was maintained at 20% of the dissolved oxygen saturation concentration. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Production culture medium 
               
            
           
           
               
               
               
            
               
                   
                 Reagent 
                 Concentration (g/L) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Glucose 
                 12.0 
               
               
                   
                 KH 2 PO 4   
                 9.0 
               
               
                   
                 MgSO 4 •7H 2 O 
                 2.4 
               
               
                   
                 Yeast Extract 
                 15 
               
               
                   
                 FeSO 4 •7H 2 O 
                 0.04 
               
               
                   
                 MnSO 4 •5H 2 O 
                 0.04 
               
               
                   
                 CaCl 2 •2H 2 O 
                 0.04 
               
               
                   
                 ADEKANOL (Adeka, LG-295S) 
                 0.1 (mL/L) 
               
               
                   
                   
               
            
           
         
       
     
     Immediately after glucose in the production culture medium was completely consumed, a feed solution (455 g/1 L of glucose, 120 g/1 L of Yeast Extract) was added at a rate of 1 mL/min. The culture was performed while maintaining the culture solution temperature at 37° C. and constantly controlling the pH to 6.9. Further, the dissolved oxygen concentration in the culture solution was maintained at 20% of the dissolved oxygen saturation concentration, and the culture was performed for 20 hours. Thereafter, 1 M isopropyl-β-thiogalactopyranoside (IPTG) was added to the culture solution to a final concentration of 1 mM to induce the expression of the modified fibroin. The culture solution was centrifuged 20 hours after addition of IPTG, and bacterial cells were recovered. SDS-PAGE was conducted using the bacterial cells prepared from the culture solutions obtained before the addition of IPTG and after the addition of IPTG. The expression of the target modified fibroin which depended on the addition of IPTG was confirmed by the appearance of a band of the size of the target modified fibroin. 
     (3) Purification of Protein 
     The bacterial cells recovered 2 hours after the addition of IPTG were washed with a 20 mM Tris-HCl buffer (pH 7.4). 
     The bacterial cells after washing were suspended in a 20 mM Tris-HCl buffer (pH 7.4) containing about 1 mM PMSF and the cells were disrupted with a high-pressure homogenizer (manufactured by GEA Niro Soavi). 
     The disrupted cells were centrifuged to obtain a precipitate. The obtained precipitate was washed with a 20 mM Tris-HCl buffer (pH 7.4) until the purity of the precipitate became high. The precipitate after washing was suspended in a 8 M guanidine buffer (8 M guanidine hydrochloride, 10 mM sodium dihydrogen phosphate, 20 mM NaCl, 1 mM Tris-HCl, pH 7.0) so that the concentration of the precipitate was 100 mg/mL, and dissolved by stirring with a stirrer for 30 minutes at 60° C. After dissolution, dialysis was performed with water using a dialysis tube (cellulose tube 36/32, manufactured by Sanko Junyaku Co., Ltd.). A white aggregated protein obtained after dialysis was recovered by centrifugation, and moisture was removed in a lyophilizer to recover lyophilized powder, thereby obtaining modified fibroins (PRT918 and PRT799). 
     PRT918 is hydrophobic modified fibroin having an average HI of more than 0. PRT799 is hydrophilic modified fibroin having an average HI of 0 or less. 
     [Production of Protein Fiber] 
     Dimethyl sulfoxide (DMSO) in which LiCl was dissolved so as to be 4.0 mass % was prepared as a solvent, and a lyophilized powder of the modified fibroin was added thereto so as to have a concentration of 24 mass %, and dissolved for 3 hours using a shaker. Thereafter, insoluble matters and foams were removed to obtain a modified fibroin solution (spinning dope). 
     The prepared spinning dope at 60° C. was filtrated with a metal filter having an opening of 5 μm. Thereafter, the filtrate was allowed to stand in a 30 mL-stainless steel syringe to remove foams. The resulting spinning dope was discharged from a solid nozzle having a needle diameter of 0.2 mm into a 100 mass % methanol coagulation bath. The discharge temperature was 60° C. After completion of the coagulation, the obtained original yarn was wound up, and naturally dried to obtain a modified fibroin fiber (raw material fiber). 
     For comparison, commercially available silk fibers, cotton fibers, and polyester fibers were prepared as raw material fibers. 
     [Production of Knitted Fabric] 
     Knitted fabrics were produced by weft-knitting respective raw material fibers by using a weft-knitting machine. The knitted fabric using PRT918 fibers as raw material fibers had a thickness of 1/30N (metric count, single yarn) and a gauge number of 18. The knitted fabric using PRT799 fibers as raw material fibers had a thickness of 1/30N (metric count, single yarn) and a gauge number of 16. The thickness and gauge number of each of the knitted fabrics formed by using other raw material fibers were adjusted so as to have a cover factor approximately the same as those of the knitted fabrics using PRT918 fibers and PRT799 fibers. Details are as follows. 
     Silk thickness: 2/60N (single yarn), gauge number: 14 
     Cotton thickness: 2/34 N (two ply yarn), gauge number: 14 
     Polyester thickness: 1/60N (single yarn), gauge number: 14 
     [Evaluation of Water-Absorbing and Quick-Drying Properties] 
     (Evaluation of Water-Absorbing Properties) 
     The water-absorbing properties were evaluated by a test in accordance with JIS L 1907 (Testing methods for water-absorbing properties of textiles/dropping method). Specifically, under a standard environment (temperature 20±2° C./humidity 65±4% RH), one drop of water was dropped from the burette onto the surface of the knitted fabric prepared above, and the time until the dropped water droplet was absorbed by the knitted fabric (specular reflection disappeared) was measured (maximum measurement time was 60 seconds). The results are shown in Table 6. 
     (Evaluation of Quick-Drying Properties) 
     The quick-drying properties were evaluated by measuring the diffusible residual moisture content. Specifically, under a standard environment (temperature 20±2° C./humidity 65±4% RH), 0.6 mL of tap water was dropped onto the back side of the knitted fabric, and the weight of water was determined by measuring the weight of the knitted fabric at every lapse of a certain time (every 5 minutes), and the diffusible residual moisture content was calculated by the following equation. 
       Diffusible residual moisture content (%)=weight (g) of water at each time/weight (g) of water at the start of measurement×100
 
     The measurement was performed until the diffusible residual moisture content reached 10% or less (that is, the weight of water reached 10% of 0.6 mL=60 μL (60 mg)), and the time required until the diffusible residual moisture content reached 10% was determined. The results are shown in Table 6. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                 Water-absorbing 
                 Rapid-drying 
               
               
                   
                 properties (sec) 
                 properties (min) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Modified fibroin 
                 &gt;60 
                 54 
               
               
                   
                 (PRT918) 
               
               
                   
                 Modified fibroin 
                 3 
                 66 
               
               
                   
                 (PRT799) 
               
               
                   
                 Silk 
                 &gt;60 
                 134.3 
               
               
                   
                 Cotton 
                 1 
                 125.7 
               
               
                   
                 Polyester 
                 &gt;60 
                 300.8 
               
               
                   
                   
               
            
           
         
       
     
     It can be understood that the modified fibroins (PRT799 and PRT918) are excellent in water-absorbing and quick-drying properties. In particular, it can be understood that the hydrophilic modified fibroin (PRT799) is excellent in both water-absorbing properties and quick-drying properties.