Maturation, internalization, and turnover of soluble and membrane proteins associated with atrial myocyte secretory granules.

Primary cultures of neonatal atrial myocytes were used to study the biosynthesis of a prominent secretory granule enzyme that occurs naturally in soluble and integral membrane forms. The two most prominent forms of peptidylglycine alpha-amidating monooxygenase PAM) in atrial myocytes are type I integral membrane proteins (PAM-1 and -2); smaller amounts of a soluble form, PAM-3, are also found. All three PAM proteins are N-glycosylated, and PAM-1 also has sialylated O-linked oligosaccharide. Two hours after their biosynthesis, approximately half of the newly synthesized PAM-1 and PAM-2 proteins have acquired N-linked oligosaccharide chains resistant to digestion with endoglycosidase-H. Secretion of newly synthesized PAM-3 is detectable within 90 min after biosynthesis and is largely complete within 4 h. Release of the catalytic domains of PAM-1 and PAM-2, which requires endoproteolytic cleavage, occurs at a slow rate for many hours after biosynthesis. Release of PAM-3 and the soluble PAM proteins derived from PAM-1 and PAM-2 can be stimulated by secretagogue. Integral membrane PAM proteins that reach the surface of atrial myocytes are internalized and enter the endocytic pathway. The turnover of newly synthesized PAM-1 and PAM-2 is only partially accounted for by the release of soluble PAM protein into the medium and may involve a significant contribution from intracellular degradation.