The sorting mechanism underlying the separation of salivary proteins into secretory granules in parotid glands

BackgroundBecause of their ability to synthesize and secrete large amounts of proteins, salivary glands are considered target organs for gene therapy. Acinar cells of salivary glands are polarized and secrete proteins via two routes, apical and basolateral pathways, i.e., saliva and blood. It is necessary to deliver exogenous proteins as drugs via appropriate routes for efficient supply. We analyzed the sorting mechanism using an expression system of a reporter protein, HaloTag, to control secretory pathways in salivary glands.HighlightWe found that the HaloTag protein that has only the signal peptide sequence of amylase (SS25H) was transported to secretory granules and secreted upon β-adrenergic stimulation. However, the basal secretion without stimulation was slightly higher than that of amylase, suggesting that SS25H was partially transported via a constitutive secretory pathway. Because the signal peptide sequence is deleted after translocation into the endoplasmic reticulum, SS25H contains only the HaloTag amino acid sequence and no specific sorting signal when sorted in the trans-Golgi network. In addition, SS25H did not form oligomers in secretory granules.ConclusionThese results suggest that the sorting of SS25H is completely passive. Thus, it can be used as standard protein that is not subject to any regulation for sorting. It will be useful to study the effect of fusion with other proteins on transport efficiency via regulated secretory pathways. Using the HaloTag technology provides useful information about granule biogenesis and sorting mechanisms.