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Silk fibroin ofBombyx moriis secreted from the posterior silkgland (PSG) as a2.3-MDa elementaryunit, consistingof six sets of a disulfide-linked heavy chain (H-chain)–light chain (L-chain) heterodimer and one molecule of fibrohex-amerin (fhx)/P25. Fhx/P25, a glycoprotein, associates non-covalently with the H–L heterodimers. The elementary unit was found and purified from the endoplasmic reticulum (ER) extract of PSG cells. | Eur. J. Biochem. 271 356-366 2004 FEBS 2003 doi 10.1046 j.1432-1033.2003.03934.x Assembly of the silk fibroin elementary unit in endoplasmic reticulum and a role of L-chain for protection of a1 2-mannose residues in N-linked oligosaccharide chains of fibrohexamerin P25 Satoshi inoue1 2 Kazunori Tanaka1 Hiromitsu Tanaka2 Kohei Ohtomo1 Toshio Kanda2 Morikazu imamura2 Guo-Xing Quan2 Katsura Kojima2 Tetsuro Yamashita3 Tasuku Nakajima4 Hideharu Taira3 Toshiki Tamura2 and Shigeki Mizuno1 5 1 Laboratory of Molecular Biology Department of Molecular and Cell Biology Graduate School of Agricultural Science Tohoku University Sendai 2Insect Biotechnology and Sericology Department National Institute of Agrobiological Sciences Tsukuba 3Faculty of Agriculture Iwate University Ueda Morioka 4Laboratory of Molecular Enzymology Department of Molecular and Cell Biology Graduate School of Agricultural Science Tohoku University Sendai 5Department of Agricultural and Biological Chemistry College of Bioresource Sciences Nihon University Fujisawa Japan 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 fibrohex-amerin fhx P25. Fhx P25 a glycoprotein associates non-covalently 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 a1 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
