Female spiders use the tubuliform gland to spin fibers that are constituents of egg sacs. The tubuliform gland has a cylindrical shape and extrudes fibers referred to as tubuliform silks (Fig. 1.1). The main component found in tubuliform silks repre­sents the spidroin family member Tubuliform Spidroin 1 (TuSpl). Northern blot and quantitative real-time PCR analyzes have demonstrated that TuSpl transcripts are highly expressed in the tubuliform gland.34 MS/MS analyzes of tryptic peptides gen­erated from enzymatic digestion of dissolved egg sacs have demonstrated the pres­ence of TuSpl, along with two other fibroins, Egg Case Protein 1 (ECP-1) and Egg Case Protein 2 (ECP-2) in the cob-weaver L. Hesperus.35 Immunoblot analysis has also confirmed that TuSp1 is specifically expressed in the tubuliform gland.36 Full — length cDNA sequences for CySp1 and CySp1 (equivalents of TuSp1 and TuSp2, respectively) from the orb-weaver, Argiope bruennichi, have been described.37 The basic architecture of TuSps resemble other spidroin family members, consisting of nonrepetitive N — and C-terminal domains and internal block repeats that are larger in size relative to repeats within MaSp1 and MaSp2. Repetitive regions from TuSp1 consist of approximately 180-200 amino acid residues. Relative to other spidroin family members, the TuSp1 C-terminal domain shows little similarity to other spi — droin family members, and it has been proposed to represent a silk protein that is spun into an evolutionary ancient silk.38 Inspection of the internal block repeats of TuSpl reveals little, if any, representation of the spider silk motifs commonly dis­cussed earlier, such as the GPGXX, GGX, poly A and/or GA couplets, and the spac­ers regions. Instead, different amino acid motifs are used, which include Sn, (SA) n, (SQ)n, and GX (X represents A, V, I, N, Q, Y, P or D).34b Interestingly, the ECP-1 and eCp-2 protein sequences lack recognizable internal block repeats as well as the nonrepetitive conserved N- and C-termini. These proteins also have predicted molecular masses that are approximately 80-kDa, which is considerably smaller relative to the spidroin family members. Protein alignments between ECP-1 and ECP-2 reveal a 52% identity at the amino acid level. Despite having well defined internal block repeats, the ECPs contain poly A/(GA) modules that are similar to sequences reported from dragline silk fibroins.35a Analysis of protein sequences of ECPs show cysteine-rich N-terminal domains, suggesting these molecules function as intermolecular cross linkers that interact with the TuSps to provide structural roles in tubuliform silks.

Mechanical studies performed using tubuliform silk collected from L. hesperus reveal that these fibers contain lower breaking stress than dragline silk, but higher breaking strain11b. When considering both properties, tubuliform silks are shown to be tougher relative to dragline silks. Synthetic fibers have been wet spun from truncated, purified TuSpl and ECP-2 recombinant proteins.39 Artificial fibers spun from TuSpl molecules that contain the C-terminal domain showed slightly lower breaking stress relative to truncated ECP-2-spun fibers, having values of 95.1 and 121.9 MPa, respectively (Table 1.2). Interestingly, reconstituted egg case silk fibers, which contain full-length fibroins, display lower tensile strength relative to natural tubuliform silks.11b40 This implies that the lower mechanical properties for the syn­thetic silks, is impart, due to imperfections in the artificial spinning process.

TABLE 1.2 (Continued)