Jaton J C, Frutiger S, Hughes G J
Département de Biochimie Médicale, Faculté de Médecine, Centre Médical Universitaire, Genève.
Ann Inst Pasteur Immunol. 1988 Jan-Feb;139(1):21-40. doi: 10.1016/0769-2625(88)90129-8.
Rabbit secretory components (SC) constitute a highly heterogeneous population of glycoprotein molecules that are present in secretions as free or bound forms to polymeric immunoglobulins (Ig). Two SC families are known, one of high molecular weight (approximately equal to 80 Kd) composed of five (perhaps six) domains related to Ig variable domains, and one of low molecular weight (approximately equal to 55 Kd). An account of our most recent experimental data is reviewed in this article. We have shown: 1) that both the high and low Mr SC families possess the same relative avidity for binding to dimeric IgA of the g-subclass; 2) that the first NH2-terminal domain of SC derived from the high and low Mr polypeptides is necessary and sufficient for efficient non-covalent binding to dimeric IgA of the g-subclass; 3) that the low Mr SC polypeptide derives from the high Mr SC by the internal deletion of the entire second and third domains, suggesting that these domains are not involved in the binding reaction with polymeric Ig; 4) that the heterogeneity of rabbit secretory components is, in large part, due to the expression of several polymorphic forms (allotypes) susceptible to be recognized by specific alloantisera; the biochemical characterization of the three known SC allotypes (t61, t62 and t63) reveals that t62 and t63 are structurally very similar to each other and markedly divergent from the t61 homologue; 5) that by using non-cross-reactive alloantisera, the major immunodominant allotopes are confined within the COOH-terminal domains 3, 4 and 5 of SC; 6) that the location of the residues involved in the attachment of the carbohydrate unit within domain 1 varies according to the allotype: t61 is N-linked glycosylated at position 70, whereas about 75% of t62 molecules are devoid of sugars; the remaining 25% of t62 molecules are glycosylated at residue position 90; these oligosaccharide chain units are linked to asparagine residues in the acceptor site consensus sequence, Asn-X-Thr/Ser; 7) that the presence of the carbohydrate unit in domain 1 is not required for efficient binding of this domain to polymeric Ig: indeed, after enzymatic deglycosylation, domain 1 exhibits a relative binding avidity which is indistinguishable from that of the native glycosylated domain 1.
兔分泌成分（SC）构成了一类高度异质性的糖蛋白分子群体，它们以游离形式或与聚合免疫球蛋白（Ig）结合的形式存在于分泌物中。已知有两个SC家族，一个是高分子量（约80kd）的，由五个（可能六个）与Ig可变区相关的结构域组成，另一个是低分子量（约55kd）的。本文综述了我们最新的实验数据。我们已经表明：1）高分子量和低分子量SC家族对g亚类二聚体IgA的结合具有相同的相对亲和力；2）源自高分子量和低分子量多肽的SC的第一个NH2末端结构域对于与g亚类二聚体IgA的有效非共价结合是必要且充分的；3）低分子量SC多肽是通过内部缺失整个第二和第三结构域而从高分子量SC衍生而来的，这表明这些结构域不参与与聚合Ig的结合反应；4）兔分泌成分的异质性在很大程度上归因于几种多态形式（同种异型）的表达，这些多态形式易被特异性同种抗血清识别；对三种已知的SC同种异型（t61、t62和t63）的生化特性分析表明，t62和t63在结构上彼此非常相似，与t61同源物明显不同；5）通过使用非交叉反应性同种抗血清，主要的免疫显性同种异型抗原决定簇局限于SC的COOH末端结构域3、4和5内；6）结构域1内参与碳水化合物单元连接的残基位置根据同种异型而变化：t61在第70位进行N-连接糖基化，而约75%的t62分子无糖；其余25%的t62分子在第90位残基处进行糖基化；这些寡糖链单元与受体位点共有序列Asn-X-Thr/Ser中的天冬酰胺残基相连；7）结构域1中碳水化合物单元的存在对于该结构域与聚合Ig的有效结合不是必需的：实际上，酶促去糖基化后，结构域1表现出与天然糖基化结构域1无法区分的相对结合亲和力。
