Date of Award
Doctor of Philosophy (PhD)
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 dog pancreas microsomal vesicles with an 11-15 kilodalton domain remaining in the cytoplasm. We studied the biosynthesis and processing of human SC in a cell-free translations and pulse labelling of cells, SC is made as a single larger precursor with an approximately 15 kilodalton cytoplasmic domain. In pulse-chase experiments, the carbohydrate moieties of the precursor are first converted to the complex type and the precursor is then proteolytically cleaved to a form slightly larger than SC isolated from colostrum. This cleaved form is slowly released from the cell. Partial NH2-terminal sequencing indicates that the cleaved form of SC is derived from the NH2- terminal, ectoplasmic (non-cytoplasmic) domain of the precursor. To determine the structure of the cytoplasmic and membrane spanning portions of the SC precursor, we cloned and sequenced DNA complementary to 1563 nucleotides at the 3' end of rabbit SC mRNA, and deduced the corresponding sequence of the COOH-terminal 163 amino acid residues of the SC precursor. The SC precursor has a putative membrane spanning segment of 23 non-charged amino acid residues, followed by a cytoplasmic tail of 103 amino acid residues with a preponderance of charged and hydrophilic residues.
Mostov, Keith Elliot, "A Transmembrane Precursor of Secretory Component" (1983). Student Theses and Dissertations. 464.