Reduced level of secretion and absence of subunit combination for the fibroin synthesized by a mutant silkworm, Nd(2).

Abstract

Fibroin is normally composed of one H chain (350 kd) and one L chain (25 kd) which are connected by disulfide bond(s). However, the small amount of fibroin secreted into the lumen of the posterior silk gland of the Nd(2) (naked pupa) mutant does not contain L chain, although L chain mRNA is present and L chain is synthesized in the posterior silk gland cells of the mutant. In a hybrid silkworm, Nd(2)/Tamanashikasuri, where Tamanashikasuri is a normal producer of fibroin, L chain from the two alleles are distinguishable electrophoretically. It is demonstrated using this system that the L chain from the Nd(2) allele can combine normally with the H chain from Tamanashikasuri and the H-L complex is secreted normally. In another hybrid system, Nd(2)[J-131, where J-131 is a normal producer of fibroin, fibroin derived from the two alleles are distinguishable due to the different electrophoretic mobility of H chain. The fibroin derived from the J-131 allele is composed of H chain and L chain, while the fibroin derived from the Nd(2) allele is devoid of L chain, and its secretion is greatly reduced. We present evidence suggesting that the H chain derived from the Nd(2) allele is structurally abnormal and discuss how the H-L subunit structure is advantageous in the secretion of fibroin.

A fibroin secretion-deficient silkworm mutant, , provides an efficient system for producing recombinant proteins

“…The quality control function of the ER (Hammond and Helenius, 1995) is critical to ensuring that only properly folded proteins enter secretory pathways. In the case of silk fibroin, a disulfide linkage between the H-and L-chains (Takei et al, 1984b(Takei et al, , 1987, the addition of three N-linked oligosaccharide chains onto fibrohexamerin (Tanaka et al, 1999a;Inoue et al, 2000), and the assembly of the elementary unit (Inoue et al, 2000) all take place in the ER (Inoue et al, 2004). These processes are important for the efficient secretion of fibroin.…”

Construction of a binary transgenic gene expression system for recombinant protein production in the middle silk gland of the silkworm Bombyx mori

“…Although the Nd-s D strain, which is fibroinsecretion deficient and naked-pupa mutant (Takei et al 1984;Mori et al 1995), contains the mutant gene Nd-s D , the mutant gene was removed in the cross with the w-1 strain for the establishment of the transgenic lines. The UAS-EGFP homozygous strain reported by Uchino et al (2006) was used to study the activity of the sericin gene promoters.…”

Functional Material Features of Bombyx mori Silk Light versus Heavy Chain Proteins

“…Interestingly, in a few species of silkworms (saturniidae family), the L-chain and P25 are missing and silk is composed of only the H-chain. 31 …”