Receptor-mediated transcellular transport of immunoglobulin: synthesis of secretory component as multiple and larger transmembrane forms.

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Abstract
The early biosynthetic forms of secretory component (SC) were characterized. SC is synthesized in various glandular epithelial cells and functions in transepithelial transport of certain polymeric Ig. In rabbit, mature SC is heterogeneous, consisting of 2 or 3 immunologically related glycoproteins. Translation of mRNA from rabbit mammary gland or liver in a wheat germ cell-free system supplemented with dog pancreas microsomal vesicles showed that the translation products of SC mRNA include at least 4 distinct polypeptides. All 4 polypeptides are synthesized not as soluble secretory forms but as considerably larger transmembrane forms that are core-glycosylated and asymmetrically integrated into the dog pancreas microsomal vesicles in a translation-coupled manner. The mature secreted forms of SC apparently are endoproteolytic fragments derived from and comprising the ectoplasmic portion of the transmembrane precursor forms. The detection of transmembrane precursor forms for mature SC provides a protein structural basis for their function as receptors mediating transepithelial transport of Ig molecules.