Evidence of Defect in Fibroin Secretion in the Posterior Silk Gland Cells of the flc Mutant of Bombyx mori

Abstract

The production and excretion of silk proteins in the silk gland cells are very marked processes at the final larval instar of the silkworm, Bombyx mori. Here, proteins in the posterior silk gland tissues and lumen contents in the mutant called flimsy cocoon (flc), wherein fibroin is hardly secreted into the lumen, were separately analyzed by two-dimensional gel electrophoresis. It was shown that the putative small subunit fibroin was accumulated in the cells, and that this protein did not appear in the lumen. In the normal silkworm, this protein was scarce in the tissues but rich in the lumen. Radioactivity counting in the hot-acid insoluble fractions after intraperitoneal injection of [³H]glycine also exhibited a rapid incorporation of label into the posterior silk gland tissues both in the normal and mutant larvae, but differential label was found in the lumen, being very small in the mutant. These results support the idea that the secretion machinery in the flc silk gland cells is deficient.