Zikan J, Novotny J, Trapane T L, Koshland M E, Urry D W, Bennett J C, Mestecky J
Proc Natl Acad Sci U S A. 1985 Sep;82(17):5905-9. doi: 10.1073/pnas.82.17.5905.
J chain is a 137-residue polypeptide that is covalently linked to polymeric immunoglobulins and participates in their synthesis and transport to external secretions. To clarify these roles, the secondary structure of J chain was characterized by computer-assisted analyses of human and mouse sequences and by circular dichroism measurements of the isolated J chain. The secondary-structure profiles obtained were very similar to those of superoxide dismutase or immunoglobulin light chain variable domains, suggesting that the J chain folds into an eight-stranded antiparallel beta-barrel and should contain approximately 37% beta-sheet conformation, with the rest of the structure existing as reverse turns (random coil). The circular dichroism measurements indicated that the conformation of denatured, S-carboxymethylated or S-sulfonated J chain consists of 75% random coil and 25% beta-structure. Upon reformation of disulfide bonds the percentage of beta-structure in the air-oxidized J chain increased to 34%, a value that is in good agreement with the secondary-structure analysis. Two alternative models of J-chain structure, a two-domain model [Cann, G., Zaritsky, A. & Koshland, M.E. (1982) Proc. Natl. Acad. Sci. USA 79, 6656-6660] and a single-domain antiparallel beta-sheet bilayer model (proposed in this paper), are compared.
J链是一种由137个氨基酸残基组成的多肽，它与多聚免疫球蛋白共价连接，并参与其合成以及向外分泌液的转运过程。为了阐明这些作用，通过对人和小鼠序列进行计算机辅助分析以及对分离出的J链进行圆二色性测量，对J链的二级结构进行了表征。所获得的二级结构轮廓与超氧化物歧化酶或免疫球蛋白轻链可变区的二级结构轮廓非常相似，这表明J链折叠成一个八链反平行β桶结构，应该含有大约37%的β折叠构象，其余结构以反向转角（无规卷曲）形式存在。圆二色性测量表明，变性的、S-羧甲基化的或S-磺化的J链构象由75%的无规卷曲和25%的β结构组成。在二硫键重新形成后，空气氧化的J链中β结构的百分比增加到34%，这一数值与二级结构分析结果非常吻合。本文比较了J链结构的两种替代模型，即双结构域模型[Cann, G., Zaritsky, A. & Koshland, M.E. (1982) Proc. Natl. Acad. Sci. USA 79, 6656 - 6660]和单结构域反平行β折叠双层模型（本文提出）。
