Dynamic Rearrangement Within the Antheraea pernyi Silk Fibroin Gene Is Associated with Four Types of Repetitive Units

Abstract

We characterized a full-length gene encoding wild silkmoth Antheraea pernyi fibroin (Ap-fibroin) to clarify the conformation of repetitive sequences. The gene consisted of a first exon encoding 14 amino acid residues, a short intron (120 bp), and a long second exon encoding 2,625 amino acid residues. Three amino acids, alanine, glycine, and serine, amounted to 81% of the Ap-fibroin sequence. The Ap-fibroin, except for 155 residues of the amino terminus, was composed of 80 tandemly arranged polyalanine-containing units (motifs). A motif was a doublet of a polyalanine block (PAB) and a nonpolyalanine block (NPAB). Seventy-eight of the 80 motifs were classified into four types based on differences in the NPAB sequences. Although respective motifs were significantly conserved, many rearrangements were observed within the second exon, i.e., the triplication of a 558-bp-long sequence and other duplication events of shorter sequences. Chi-like sequences, GCTGGAG, might contribute to the rearrangement within the gene as described in human minisatellite loci, because they were found at specific sites of NPAB-encoding sequences in three of four types of motifs. The present results support the idea that the Ap-fibroin gene is unstable like minisatellite sequences and that the evolution of this gene is strongly associated with its instability.

Elastomeric gradients: a hedge against stress concentration in marine holdfasts?

“…These collagens are referred to as 'prepepsinized collagens' or preCols (Qin & Waite 1995;Waite et al 1998) and have been described as naturally occurring molecular 'chimaeras' on account of their linear array of structural motifs (figure 4): (i) a central, kinked collagen domain representing 52-58% of the sequence; (ii) an acidic patch (C-side of collagen); (iii) variable flanking domains (both N-and C-side), and (iv) aminoand carboxy-termini rich in histidine and 3, 4-DOPA residues. The variable flanking domains in preCol D have sequence motifs reminiscent of spider dragline and Antheraea cocoon silk (Guerette et al 1996;Sezutsu & Yukuhiro 2000). As demonstrated in table 3, there are six slightly substituted poly-alanine blocks that, in spun silks, give rise to crystalline β-sheets (Simmons et al 1996).…”

Correlation between Fibroin Amino Acid Sequence and Physical Silk Properties

“…The overall gene structure and the nucleotide sequences at the 5Ј and 3Ј gene ends were similar to the H-fibroin genes of B. mori (6), A. pernyi (7), and Antheraea yamamai (8), but the major internal sequence was unique. About 95% of the gene encoded amino acid repeats arranged hierarchically into 10 -12 large assemblies.…”

“…The absence of concatenations of simple motifs and the short length of polyalanine chains distinguish H-fibroin of pyralid moths from the H-fibroins of Saturniidae (7,8) and Bombycidae (24), the only other lepidopteran families for which data are available. Fibroins of many spider silks also contain polyalanine chains (25)(26)(27) or reiteration of simple Gly-rich motifs (28).…”

“…Most of Antheraea H-fibroin is built of four types of alternating repeats, which are 20 -40 residues long and include chains of 13 alanine residues (7,8). The repeats in B. mori H-fibroin consist of GlyX iterations in which X is Ala in 65% of cases, Ser in 23%, and Tyr in 9% (24).…”

“…Most of the physical fiber properties appear to be determined by the amino acid sequence of the H-fibroin, which is for the most part composed from repeats. The repeats in B. mori H-fibroin are dominated by iterations of a GAGAGS motif (5,6), and a string of alanines occurs at the end of each of the four types of repeats in Antheraea H-fibroin (7,8).…”