Endocrinology, Vol 133, 2579-2587, Copyright © 1993 by Endocrine Society

ARTICLES

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

JY Maltese and BA Eipper
Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.

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.

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