Abstract:
Described are silk proteins derived from spider mite, more specifically derived from  Tetranychus urticae . More specifically, described is the use of these proteins to make fibers, or fiber-composed material and the resulting fibers and materials.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This is a national phase entry under 35 U.S.C. §371 of International Patent Application PCT/EP2010/064632, filed Oct. 1, 2010, published in English as International Patent Publication WO 2011/039345 A1 on Apr. 7, 2011, which claims benefit under Article 8 of the Patent Cooperation Treaty to European Patent Application Serial No. 09172104.3, filed Oct. 2, 2009. 
     STATEMENT ACCORDING TO 37 C.F.R. §1.821(c) or (e)—SEQUENCE LISTING SUBMITTED AS ASCII TEXT FILE 
     Pursuant to 37 C.F.R. §1.821(c) or (e), a file containing an ASCII text version of the Sequence Listing has been submitted concomitant with this application, the contents of which are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The disclosure relates to silk proteins derived from spider mite, more specifically derived from  Tetranychus urticae . More specifically, the disclosure relates to the use of these proteins to make fibers or fiber-composed material. 
     BACKGROUND 
     Silk is a secreted, fibrous material that is deposited or spun by an organism. From a biochemical point of view, silk consists of protein threads composed of repeating arrays of polypeptides that contain both discrete crystalline and noncrystalline domains that are oriented around a fiber axis. 
     Several arthropods, such as spiders, caterpillars mites, mantids, moths, and beetles, produce silk, or silk-like fibers. Insects, as a group, as well as spiders, produce many different types of silks and fibrous proteins, such as fibroins and spidroins. An individual spider may produce as many as nine different types of silks and fibrous proteins, each of which may be composed of more than one type of protein (Kovoor 1987; Haupt &amp; Kovoor 1993). Different silks differ in number as well as in sequence of composing proteins. Although all fibroin and spidroin proteins do comprise several repeats, the repeat structures are species dependent and the amino acid composition, as well as the mechanical characteristics, may vary strongly from silk to silk (Zurovec and Sehnal 2002; Fedic et al. 2003). 
     Although the domesticated silkworm  Bombyx mori  is the mainstay of the silk industry, there is a considerable trade in some countries in silk produced by silkworms living “wild.” The most important of these wild silks are those that are known as Tussah. Tussah is the product of several species of silkworm of the genus  Antheraea , particularly  Antheraea mylitta , indigenous to India, and  Antheraea pernyi , which is native to China (Huber 1947; Cook 1984). Although Tussah silk is the most important wild silk in commercial use, there are still other varieties of caterpillars that produce silk. These silks are called wild, because these worms are not capable of being domesticated and artificially cultivated. Some examples are:  Antheraea yamamai, Attacus ricini , and  Attacus Atlas.    
     In recent years, spider silk was receiving more and more interest, mainly due to the excellent mechanical characteristics of this silk. For spiders, one species can make different silk fibers for different purposes, such as dragline silk or major ampullate silk, capture-spiral silk, tubuliform silk, aciniform silk and minor-ampullate silk. 
     The most investigated type of spider silk is the dragline or major ampullate (MA) silk that is secreted by the major ampullate glands of the spider. The dragline is used to support the spider when constructing a web and to prevent it from falling. This function results in mechanical properties combining a high Young&#39;s modulus with a high strength. Due to its size and accessibility, the major ampullate gland has been the focus of most studies. 
     A second important type of spider silk is the flagelliform, spiral or capture silk. This type of silk is composed of an acidific glycoprotein, secreted from the flagelliform gland, and coated with glue from the aggregate gland, which makes it sticky. The glue is not regarded as silk because it is composed of glycoproteins and other amino acids. The flagelliform silk is exclusively used for the construction of the spiral components of the web. This function results in a fiber that is highly extensible and capable of absorbing the energy of the flying prey without failure. The functional role of the glue is believed to allow for more effective capture of prey. 
     Minor ampullate (MI) silk is the spider silk that is secreted by the minor ampullate glands and is a strong, non-elastic, deformably stretchable silk used in web formation (Colgin &amp; Lewis 1998). 
     Another spider silk that is discussed in this text is the egg sac silk that is used to wrap eggs. Vollrath (1992, 2000) mentioned in his representation of the spinning glands associated to its function that the soft inner silk of the egg sac is produced by the aciniform glands (aciniform silk), whereas the tough outer silk of the egg sac is secreted by the cylindrical or tubuliform spinning glands (tubuliform silk). Viney et al. (2000) believes the opposite. The tubuliform glands are only found in female spiders, which makes it more probable that the inner silk is indeed secreted by the tubuliform glands. 
     Because of its attractive properties (high strength, flexible with good water-absorbing power, soft, good elastic recovery behavior, glossiness, etc.), silk has a wide variety of uses in the apparel, drapery, upholstery and military fields. Natural silk has a long history of use as a textile fiber, and has been used in recent years for medical sutures, blood vessels, artificial skin, tendons and for binding enzymes (Bunning et al. 1994; Kuzuhara et al. 1987). Interest in  Antheraea pernyi  silk for biomedical applications has recently grown because  A. pernyi  SF contains the tripeptide sequence of arg-gly-asp (RGD), known as cell adhesive site for mammalian cell culture (Minoura et al. 1995; Pierschbacher &amp; Ruoslahti 1984a, 1984b; Li et al. 2003). Therefore, it has been investigated as a potential biomaterial such as a matrix for the enzyme immobilization and mammalian fibroblast cell culture (Kweon et al. 2001a, 2001b). Silk of the spider  Nephila clavipes  has been used to help mammalian neural regeneration (Allmeling et al. 2006). 
     As each silk has its own composition and characteristics, there is a lot of interest in the identification of new silk proteins, opening the possibility for new applications. Surprisingly, we found that spider mites, and particularly  Tetranychus urticae , are making silk proteins of which the amino acid composition differs rather strongly from that of classical fibroins and spidroins, especially in the alanine, glycine and serine content. Those differences are found in the global protein composition, as well as in the composition of the repeats. 
     DISCLOSURE 
     A first aspect of the disclosure is a spider mite silk protein, comprising a sequence selected from the group consisting of SEQ ID NO:1 through SEQ ID NO:19, or a homologue thereof. “Homologues,” as used herein, means protein with at least 70%, preferably at least 80%, even more preferably at least 90% identities, as measured using BLASTp (Altschul et al. 1997). Preferably, the spider mite is  Tetranychus urticae . Preferably, the proteins have a composition comprising at least 40%, preferably at least 45%, even more preferably at least 50% serine and glycine (taking both amino acids together), whereby the individual composition of serine and glycine for each is at least 15%, preferably at least 18%, even more preferably at least 20%, calculated as percentage of the number of the specific amino acid on the total number of amino acids. Even more preferably, independent of the percentage of glycine, serine is present in at least 21%, preferably at least 26%, even more preferably at least 30%. Even more preferably, the proteins comprise, beside the serine and glycine content, also at least 15%, preferably at least 17%, even more preferably at least 20% of alanine. One preferred embodiment is a spider mite silk protein, whereby the protein is selected from the group consisting of SEQ ID NO:1, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15, SEQ ID NO:16, and SEQ ID NO:17. An even more preferred embodiment is a spider mite silk protein whereby the protein is selected from the group consisting of SEQ ID NO:8, SEQ ID NO:13 and SEQ ID NO:15. The most preferred embodiment is a spider mite silk protein selected from the group consisting of SEQ ID NO:3, SEQ ID NO:9, SEQ ID NO:12, SEQ ID NO:14, and SEQ ID NO:17. 
     Another aspect hereof is a nucleic acid molecule encoding a protein according to the invention, or the complement thereof, or a functional fragment thereof. “Nucleic acid molecule,” as used herein, refers to a polymeric form of nucleotides of any length, either ribonucleotides or deoxyribonucleotides. This term refers only to the primary structure of the molecule. Thus, this term includes double- and single-stranded DNA, and RNA under the forms known to the person skilled in the art, such as, but not limited to, genomic DNA, cDNA, mRNA, antisense RNA and RNAi. It also includes known types of modifications, for example, methylation, “caps” substitution of one or more of the naturally occurring nucleotides with an analog. One preferred embodiment of a functional fragment is a fragment that can be used as RNAi. 
     Still another aspect hereof is a recombinant host cell, comprising a nucleic acid molecule according to the invention. A “recombinant host cell,” as used here, is a cell that has been genetically modified, preferably by the introduction of a nucleic acid according to the invention. The recombinant host cell of the invention can be any prokaryotic or eukaryotic cell, including, but not limited to, bacterial cells such as  Escherichia coli , yeast cells, such as  Saccharomyces spp, Pichia  spp, or  Kluyveromyces  spp, insect cells, plant cells or mammalian cells. The recombinant host cells can be used to produce large quantities of the spider mite silk protein according to the invention. Methods for the production of recombinant silk proteins are known to the person skilled in the art and have been described, as a non-limiting example, in WO9116351 and WO9947661, hereby incorporated herein by this reference. 
     Another aspect of the invention is the use of a spider mite silk protein, according to the invention, to make a fiber. Methods to make artificial silk fibers using silk proteins are known to the person skilled in the art and have been disclosed, as a non-limited example, in WO0153333 and in Teulé et al. (2009), hereby incorporated herein by this reference. 
     Still another aspect of the invention is an artificially produced fiber, comprising one or more proteins of the invention. “Artificially produced,” as used here, means that the fiber and/or the composing proteins are not produced by a naturally occurring  Tetranychus urticae.    
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     EXAMPLES 
     Example 1 
     Sequencing of the  Tetranychus urticae  Genome 
     The London population of  T. urticae  developed from the isofemale line in London Ontario, following eight backcrosses (to generate maximum homozygote population) was mass produced on the bean plants in growth chambers at 27° C. and 16:8 photoperiod. Plants were washed in 0.1% TRITON X detergent solution in 2-liter beakers to release all spider mite life stages. Adult spider mites, nymphs, larvae and eggs were filtered through series of fine sieves to isolate pure egg fraction. Eggs were collected in the Eppendorf tube, treated with bleach solution (to remove plant tissue and possible microbial contaminants) and prepared for the DNA extraction. Embryos were ground in the glass tissue grinder and DNA extraction was performed using QUIAGEN Blood&amp;cell culture DNA kit (Midi column #13433) according to manufacturer&#39;s protocol. DNA for whole genome sequencing project was sequenced using Sanger sequencing protocol at the Joint Genome Institute (USA Department of Energy), Walnut Creek, Calif. 
     Example 2 
     Identification of the Genes 
     From fragments of fibroin genes available in the database, blastp and tblastn were run over the proteome and genome of  Tetranychus urticae . The obtained hits were all checked manually as due to the highly repeated nature of the sequence problems occurred with the prediction and even assembly of the original genomic sequence. About half of the gene models were originally wrongly predicted, involving incorrectly predicted reading frames. The corrections were iteratively evaluated and aligned using MUSCLE, including the existing fibroin genes from the public databases and the already found (and corrected) genes found in  Tetranychus urticae.    
     The originally found proteins all had in common a high percentage of G, A and P organized in repetitive patterns. This particular aspect was further used to identify more divergent proteins having similar patterns. To find them, tblastn was run again with the low-complexity filters turned off. From the multiple hits returned, six more genes were retained, based on similarity of patterns and coverage by Illumina transcript reads. All were manually annotated and added to the already found genes, as potentially involved in the fibers. In total, twelve genes were found having a similar repetitive domain. 
     Example 3 
     Analysis of the Spider Mite Silk 
     Mechanical and antimicrobial characteristics of the spider mite silk are investigated. Thread thickness and strength are measured using the standard techniques. 
     The FAVIMAT-ROBOT (Textechno) is used to analyze the tensile properties. It is a semi-automatic single-strength tester, working according to the principle of constant rate of extension (DIN 51221, DIN 53816, ISO 5079). The instrument is equipped with a balance allowing the mass to be measured at a high resolution of 0.1 mg. The instrument includes a ROBOT, which is a fiber storage, equipped with a computer-controlled transfer clamp for the transport of the single fiber to the testing position of the FAVIMAT. Moreover, this instrument is equipped with an integrated measuring unit for linear density (in dtex=0.1 g/km). This has the considerable advantage, certainly for natural fibers, that the fineness is determined simultaneously with the tensile properties. The linear density is measured according to the vibroscopic method (ASTM D 1577—BISFA 1985/1989 chapter F). The fiber is preloaded at a predefined speed. Further on, the fiber is subjected to an electro-acoustic sinusoidal vibration and the resonance frequency is detected with an opto-electronic sensor. The fiber linear density is calculated from the resonance condition, i.e., length, preload, and resonance frequency of the fiber. Suggesting a uniform mass distribution and a round cross-section, the linear density can be calculated as follows: 
               T   t     =         F   v     ·     10   11         4   ·     f   2     ·     L   2               
In this equation, Tt is the linear density in dtex, Fv is the preload in cN, f is the resonance frequency and L is the test length in mm.
 
     As spider mite silk is very resistant to degradation, possible antimicrobial activity of the silk is measured by measuring the inhibition circle around the silk on solid medium 
     Example 4 
     Confirmation of the Presence of the Proteins in Spider Mite Silk by Mass Spectrometry (MS) Analysis 
     Ten  T. urticae  adults were placed into capped and Parafilm-sealed 35 mm Petri plates for 24 hours at room temperature. Petri plate cap was removed and examined for signs of mites, eggs and debris, which were removed as necessary. After this, a cap was washed with 1 mL of 95% ethanol and silk threads suspended in ethanol were collected in Eppendorf tubes. Content of 10-15 tubes was pooled together and silk threads were transferred to a glass container for a wash with acetic acid. Silk threads were transferred back into 95% ethanol, pulled apart, and transferred into Eppendorf tubes with 95% ethanol for storage and subsequent analysis. Silk thread suspensions were initially evaporated using a SpeedVac system. The dried samples were re-suspended in 75% TFA (trifluoroacetic acid) in glass vials. Vials were then microwaved for 45 minutes at full power in a beaker filled with water. The contents of the vials were then dried using a SpeedVac system and, following this, reconstituted in 10% formic acid. Samples were then injected on a Q-ToF MS system using a 150 minute 0-40% ACN gradient acquiring data in a data-dependent fashion. Data analysis was performed using Peaks Studio 5.2 software. Peptides were matched against  T. urticae  proteome database. Analysis was performed both with and without consideration for possible variable post-translational modifications, such as deamidation and oxidation. 
     Protein ID matches from  T. urticae  proteome database that appeared in both types of analysis and were also predicted using computational approach were considered for subsequent amplification and cloning by means of PCR. SEQ ID NO:3, SEQ ID NO:14, and SEQ ID NO:17 have been confirmed as being part of the silk by MS. 
     Example 5 
     Use of the Polymerase Chain Reaction (PCR) to Confirm Gene Expression 
       T. urticae  RNA was extracted using Trizol reagent (Invitrogen). Samples for PCR were prepared by reverse transcribing 3 μg of total RNA using Superscript II Reverse Transcriptase (Invitrogen). Aliquots of this reaction were then used in PCR reactions. Primers for PCR were designed to amplify short (100-200 bp) fragments from the non-repetitive 5′ and 3′ regions of candidate genes predicted mRNA sequence. PCR was performed using Taq DNA Polymerase (Fermentas) according to manufacturer&#39;s recommendations and amplified fragments were cloned into pGEM-T Easy vector (Promega) for sequencing. SEQ ID NO:9, SEQ ID NO:12 and SEQ ID NO:17 were confirmed as being expressed as mRNA by PCR. 
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