Takei F, Oyama F, Kimura K, Hyodo A, Mizuno S, Shimura K
J Cell Biol. 1984 Dec;99(6):2005-10. doi: 10.1083/jcb.99.6.2005.
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.
丝心蛋白通常由一条重链（350千道尔顿）和一条轻链（25千道尔顿）组成，二者通过二硫键相连。然而，尽管在Nd(2)（裸蛹）突变体的后部丝腺腔中存在轻链mRNA且轻链在该突变体的后部丝腺细胞中合成，但分泌到其中的少量丝心蛋白并不包含轻链。在一种杂交蚕Nd(2)/玉名尻缫中，玉名尻缫是丝心蛋白的正常产生者，来自两个等位基因的轻链在电泳中是可区分的。利用该系统证明，来自Nd(2)等位基因的轻链能与来自玉名尻缫的重链正常结合，且重链 - 轻链复合物能正常分泌。在另一个杂交系统Nd(2)/J - 131中，J - 131是丝心蛋白的正常产生者，由于重链的电泳迁移率不同，来自两个等位基因的丝心蛋白是可区分的。来自J - 131等位基因的丝心蛋白由重链和轻链组成，而来自Nd(2)等位基因的丝心蛋白不含轻链，其分泌也大大减少。我们提供的证据表明，来自Nd(2)等位基因的重链在结构上是异常的，并讨论了重链 - 轻链亚基结构在丝心蛋白分泌中的优势。
