Sorting and secretion of salivary proteins.

Most salivary proteins are stored in secretion granules prior to export from acinar cells in response to neural stimuli. A small subset of these proteins undergo unstimulated secretion without apparent storage. This pathway probably comprises vesicles that bud from maturing storage granules and carries proteins that do not aggregate efficiently at the storage site. Expression of a parotid proline-rich protein (and deletion mutants) in pituitary AtT-20 cells has shown that an N-terminal domain is necessary for storage in secretion granules. Evidence suggests that self-aggregation of proline-rich protein mediated by this domain may function in both efficient intracellular transport and storage. Thus selective aggregation may be an important secretory sorting mechanism.