Unravelling the complexity of silk sericins: P150/sericin 6 is a new silk gene in Bombyx mori

Sericins are a small family of highly divergent proteins that serve as adhesives and coatings for silk fibers and are produced in the middle part of the silk gland. So far, five genes encoding sericin proteins have been found in Bombyx mori. Sericins 1 and 3 are responsible for silk adhesion in the cocoon, while sericins 2, 4, and 5 are present in non-cocoon spun silk of younger larvae (including the early last instar). We found a new gene, which we named P150/sericin 6, which appears to be an ortholog of the sericin-like protein previously found in Galleria mellonella. The B. mori sequence of the P150/sericin 6 ORF was previously incorrectly predicted and assigned to two smaller, uncharacterized genes. We present a new P150/sericin 6 gene model and show that it encodes a large protein of 467 kDa. It is characterized by repeats with a high proportion of threonine residues and a short conserved region with a cysteine knot motif (CXCXCX) at the C-terminus. Expression analysis has shown that B. mori P150/ser6 has low transcriptional level in contrast to its G. mellonella homolog. We also discuss the synteny of homologous genes on corresponding chromosomes between moth species and possible phylogenetic relationships between P150/ser6 and cysteine knot mucins. Our results improve our understanding of the evolutionary relationships between adhesion proteins in different lepidopteran species.