A transmembrane precursor of secretory component. The receptor for transcellular transport of polymeric immunoglobulins.

Secretory component (SC), a glycoprotein associated with polymeric IgA and IgM in external secretions, is produced by certain epithelial cells and is thought to be the receptor mediating the transepithelial transport of these immunoglobulins. We studied the biosynthesis of human SC in a cloned cell line (HT29.E10) derived from a colon adenocarcinoma. In both cell-free translations and pulse labeling of cells, SC is made as a larger precursor (Mr = 95,000). This precursor is a transmembrane protein, as a large fragment (Mr = 80,000), is protected by the membrane from proteolytic digestion. Both this membrane-protected fragment and the undigested precursor have the same NH2-terminal sequence as mature SC. These data indicate that SC is proteolytically cleaved from the NH2-terminal, ectoplasmic (noncytoplasmic) domain of the precursor. This conclusion is supported by pulse-chase experiments. The Mr = 95,000 form is first converted to a Mr = 100,000 form by addition of peripheral sugars. The Mr = 100,000 form is then slowly cleaved to a Mr = 80,000 form which is gradually released into the medium. We propose that the transmembrane precursor of SC is the receptor involved in transepithelial transport of polymeric immunoglobulins.