A Transmembrane Precursor of Secretory Component

Secretory component (SC), a glycoprotein associated with polymeric IgA and IgM (pIg) in external secretions, is produced by certain epithelial cells and is thought to be the receptor mediating the transcellular transport of pIg. We studied the biosynthesis and processing of rabbit and human SC. Using translation of mRNA from rabbit mammary gland and liver in a cell-free system supplemented with dog pancreas microsomal vesicles, we discovered that the translation products of rabbit SC include at least four polypeptides. Moreover, we found that all four polypeptides are synthesized not as soluble secretory forms, but as larger transmembrane forms that are core glycosylated and asymmetrically integrated into the…