The physiological accumulation of mutant fibroin light chains induces an unfolded protein response in the posterior silk gland of the Sericin cocoon Nd-sD strain of silkworm Bombyx mori

Fibroin, a major component of silk fiber, is composed of light (L) chains, heavy chains, and fhx/P25 in the silkworm Bombyx mori. Sericin cocoon (Nd-sD) is a silkworm strain expressing a mutant fibroin L chain that is accumulated in the endoplasmic reticulum (ER) of posterior silk glands (PSGs). However, little is known about the effects of accumulation in PSGs in Nd-sD strain. We compared the PSG gene expression profiles of 5th-instar larvae of Nd-sD strain and the fibroin-producing normal strain. cDNA representational difference analysis identified candidate genes whose expression levels were higher in Nd-sD strain than in normal strain. We focused on heat shock proteins and cathepsin B, a major lysosomal protease. We confirmed upregulation of BiP(GRP78), Hsp20.8, and cathepsin B at the transcriptional and/or translational levels. These results suggest that the accumulation of mutant L chain induces the unfolded protein response in PSGs of Nd-sD strain, which will be a valuable tool for protein quality-control studies of silk grand.