Biochemical and immunochemical characteristics of the major protein of the murine parotid-granule membrane.

The membrane fraction (ParB) of the secretory granules of mouse parotid gland was isolated and characterized. The major phospholipids were phosphatidylcholine and sphingomyelin. The membranes contained one major protein, PMC, constituting at least 30 per cent of the total protein. PMC was purified: it is a small acidic protein with molecular weight of 12,000, containing one residue of phosphate per molecule. Using anti-PMC serum, PMC was detected only in the mouse parotid and saliva. Immunochemical characterization of organelle fractions indicated that PMC was mainly present in the secretory granule fraction; it was in part tightly bound to granule membranes. PMC was also present in both the 100,000 g parotid-tissue supernatant and the water-extract of the ParB granules. This dual localization was corroborated by immunofluorescent studies with anti-PMC serum which demonstrated that PMC was distributed uniformly over the acinar cells. The major protein component of these membranes is absent from other exocrine organs, e.g. pancreas, submandibular and sublingual glands.