Regulation of the chromaffin granule aggregating activity of annexin I by phosphorylation.
Annexin I (lipocortin I) binds to secretory granule membranes and promotes their aggregation in a Ca(2+)-dependent manner [Creutz, C. E., et al. (1987) J. Biol. Chem. 262, 1860-1868; Drust, D. S., & Creutz, C. E. (1988) Nature 331, 88-91]. It is also phosphorylated on serine residues when bovine chromaffin cells are stimulated to secrete [Michener, M. L., et al. (1986) J. Biol. Chem. 261, 6548-6555], suggesting phosphorylation may be involved in modulating the function of annexin I. We report here that phosphorylation of the N-terminal tail by protein kinase C strongly inhibits the ability of annexin I to aggregate chromaffin granules by increasing the calcium requirement 4-fold. This inhibition was readily reversed when the protein was dephosphorylated by protein phosphatase 2A. The inhibition was not due to inability of phosphorylated annexin I to bind to chromaffin granules, since the phosphorylated form bound to the granule membrane at slightly lower levels of calcium than the native form. The phosphorylated annexin I also bound to 20% phosphatidylserine/80% phosphatidylcholine vesicles at lower Ca2+ levels than the native form. The inhibitory effect of phosphorylation on the granule aggregating activity of annexin I was found to be amplified by an unusual mechanism: The phosphorylated form inhibited the activity of the unphosphorylated form. The possible importance of the regulation of annexin I activity by phosphorylation in exocytosis is discussed.
Wang
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Department of Microbiology, University of Virginia, Charlottesville 22908.
Creutz
C E
CE
D000818:Animals; D017305:Annexin A1 / Q000378:metabolism* / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D002417:Cattle; D002837:Chromaffin Granules / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure*; D007425:Intracellular Membranes / Q000378:metabolism; D007700:Kinetics; D008081:Liposomes / Q000378:metabolism; D010713:Phosphatidylcholines / Q000378:metabolism; D010718:Phosphatidylserines / Q000378:metabolism; D010766:Phosphorylation; D011493:Protein Kinase C / Q000378:metabolism*
D017305:Annexin A1; D008081:Liposomes; D010713:Phosphatidylcholines; D010718:Phosphatidylserines; D011493:Protein Kinase C; D002118:Calcium