Polypeptides of fibroin and sericin secreted from the different sections of the silk gland in Bombyx mori

Abstract

Secretory proteins, fibroin and sericin, extracted with disulfide cleavage from the lumen of several sections of the silk gland of the mature silkworm, Bombyx mori, were separated individually by gel electrophoresis at acid pH containing 4 M urea. Fibroin dissociated into two major polypeptides of light (f-2: 2.5·10 4) and heavy (f-1: 3.6·10 5) suggesting a multichain molecule cross-linked with disulfide bonds, while sericin is displayed as five polypeptides showing the molecular weight: 8.0·10 4 to 3.09·10 5. The electrophoresis clearly demonstrated the secretion of different polypeptides from the section of silk gland; the posterior silk gland secretes two major (f-1 and f-2) and one minor (f-3) polypeptides of fibroin, the posterior section of the middle gland one polypeptide (s-4) of sericin, the middle section s-1 and s-3, and the anterior section s-2 and s-5, respectively. These polypeptides were characterized by the analyses of amino acid composition and the staining with PAS. Every five polypeptides, s-1, s-2, s-3, s-4 and s-5, secreted from the middle silk gland were similarly rich in serine, glycine and aspartic acid; and two polypeptides, s-1 and s-2, were stained in high intensity with PAS, and recognized as sericin. Two light polypeptides, f-2 and f-3, secreted from the posterior silk gland showed different composition from the reference data of fibroin and sericin, while the composition in the heaviest polypeptide, f-1, was identical with the reference data of fibroin.