Hydrophobic interaction of P25, containing Asn-linked oligosaccharide chains, with the H-L complex of silk fibroin produced by Bombyx mori.

Fibroin light (L-) chain and P25 are low molecular weight protein components of silk fibroin which are secreted from the posterior silk gland cells of the silkworm, Bombyx mori. The primary structure of L-chain was determined previously by cDNA cloning and peptide analysis, but that of P25 has only been deduced from its genomic sequence. Our previous studies with specific antibodies against L-chain and P25 have shown that L-chain and H-chain are linked by disulfide bond(s) but P25 is not covalently linked to H-chain. Here, we present evidence that P25 associates with the H-L complex primarily by hydrophobic interactions and that P25 is a glycoprotein containing Asn-linked oligosaccharide chains. From the analysis of three fibroin-secretion-deficient 'naked pupa' mutant breeds [Nd(2), Nd-s and Nd-sD], it is suggested that P25 interacts with H-chain in the absence of H-L linkage but its content of oligosaccharide is reduced when the H-L linkage is not formed. From these results, models are presented implying that the H-L complex and P25 are associated to form a higher-order complex of specific conformation during the processes of intracellular transport and secretion, and that the Asn-linked glycosylation of P25 is partially altered under such conditions.