About: Silk fibroin of Bombyx mori is secreted from the posterior silk gland (PSG) as a 2.3‐MDa elementary unit, consisting of six sets of a disulfide‐linked heavy chain (H‐chain)–light chain (L‐chain) heterodimer and one molecule of fibrohexamerin (fhx)/P25. Fhx/P25, a glycoprotein, associates noncovalently with the H–L heterodimers. The elementary unit was found and purified from the endoplasmic reticulum (ER) extract of PSG cells. A substantial amount of fhx/P25 unassembled into the elementary unit was also present in ER. In normal‐level fibroin‐producing breeds (J‐139 and C108), the elementary unit contained fhx/P25 of either 30 kDa (major) or 27 kDa (minor). The 27‐kDa fhx/P25 was produced from the 30‐kDa form by digestion with the bacterial α1,2‐mannosidase in vitro. The elementary unit in the ER extract contained only the 30‐kDa fhx/P25, whereas both 30‐ and 27‐kDa forms of fhx/P25 were present in the ER plus Golgi mixed extracts. In naked‐pupa mutants [Nd(2), Nd‐s and Nd‐s (D)], extremely small amounts of fibroin were produced and they consisted of one molecule of 27‐kDa fhx/P25 and six molecules of H‐chain but no L‐chain. When the Nd‐s (D) mutant was subjected to transgenesis with the normal L‐chain gene, the (H‐L)(6)fhx(1)‐type elementary unit containing the 30‐kDa fhx/P25, was produced. These results suggest that fhx/P25 in the elementary unit is largely protected from digestion with Golgi α1,2‐mannosidases when L‐chains are present in the unit. Models suggesting a role of L‐chain for the protection of α1,2‐mannose residues of fhx/P25 are presented.   Goto Sponge  NotDistinct  Permalink

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  • Silk fibroin of Bombyx mori is secreted from the posterior silk gland (PSG) as a 2.3‐MDa elementary unit, consisting of six sets of a disulfide‐linked heavy chain (H‐chain)–light chain (L‐chain) heterodimer and one molecule of fibrohexamerin (fhx)/P25. Fhx/P25, a glycoprotein, associates noncovalently with the H–L heterodimers. The elementary unit was found and purified from the endoplasmic reticulum (ER) extract of PSG cells. A substantial amount of fhx/P25 unassembled into the elementary unit was also present in ER. In normal‐level fibroin‐producing breeds (J‐139 and C108), the elementary unit contained fhx/P25 of either 30 kDa (major) or 27 kDa (minor). The 27‐kDa fhx/P25 was produced from the 30‐kDa form by digestion with the bacterial α1,2‐mannosidase in vitro. The elementary unit in the ER extract contained only the 30‐kDa fhx/P25, whereas both 30‐ and 27‐kDa forms of fhx/P25 were present in the ER plus Golgi mixed extracts. In naked‐pupa mutants [Nd(2), Nd‐s and Nd‐s (D)], extremely small amounts of fibroin were produced and they consisted of one molecule of 27‐kDa fhx/P25 and six molecules of H‐chain but no L‐chain. When the Nd‐s (D) mutant was subjected to transgenesis with the normal L‐chain gene, the (H‐L)(6)fhx(1)‐type elementary unit containing the 30‐kDa fhx/P25, was produced. These results suggest that fhx/P25 in the elementary unit is largely protected from digestion with Golgi α1,2‐mannosidases when L‐chains are present in the unit. Models suggesting a role of L‐chain for the protection of α1,2‐mannose residues of fhx/P25 are presented.
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