title: Aggregation of chromaffin granules by calpactin at micromolar levels of calcium.
abstract: Several cytosolic proteins bind to secretory granule membranes in a Ca2+-dependent manner and thus may be involved in the mediation of membrane interactions during exocytosis. One of these proteins, calpactin, is a tetramer consisting of two heavy chains of relative molecular mass (Mr) 36K (p36) and two light chains of 10K (p10). We report here that calpactin promotes the Ca2+-dependent aggregation and fatty acid-dependent fusion of chromaffin granule membranes at a level of Ca2+ that is lower than that reported for other granule-aggregating proteins, and which parallels the Ca2+ requirement for secretion from permeabilized chromaffin cells. We found subunits of calpactin to be inactive in promoting granule aggregation. Two distinct 33K proteolytic fragments of p36, differing at their N termini, also promote granule aggregation but with different Ca2+ sensitivities from calpactin. These differences suggest that the N-terminal portion of p36 modulates the Ca2+/lipid binding sites in the core portion of p36 (ref.5).
mesh_terms: D000818:Animals; D017302:Annexins; D002118:Calcium / Q000494:pharmacology*; D002135:Calcium-Binding Proteins / Q000494:pharmacology*; D002417:Cattle; D002837:Chromaffin Granules / Q000187:drug effects*; D002838:Chromaffin System / Q000187:drug effects*; D005089:Exocytosis; D008561:Membrane Fusion / Q000187:drug effects*; D010446:Peptide Fragments / Q000494:pharmacology; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases
chemical_list: D017302:Annexins; D002135:Calcium-Binding Proteins; D010446:Peptide Fragments; D011494:Protein Kinases; D017346:Protein Serine-Threonine Kinases; D002118:Calcium