Regulation of calpactin I phospholipid binding by calpactin I light-chain binding and phosphorylation by p60v-src.
Calpactins I and II are proteins that bind Ca2+, phospholipids, actin and spectrin; they are also major substrates of oncogene and growth-factor-receptor tyrosine kinases. Since calpactins have been proposed to provide a link between membrane lipids and the cytoskeleton, we examined in detail the interactions between purified calpactin I and phospholipid liposomes. We focused on the Ca2+-dependence, the effects of phosphorylation of calpactin I by p60v-src (the protein kinase coded for by the Rous-sarcoma-virus oncogene), and the effects of the binding of calpactin I light chain to calpactin I heavy chain. Binding of the light chain to the heavy chain increased the affinity of calpactin I for phosphatidylserine (PS) liposomes. The opposite effect was observed for phosphorylation by p60v-src; phosphorylation decreased the affinity of calpactin I for PS liposomes. These two opposite effects appeared to be independent, since phosphorylation did not prevent light-chain binding to the heavy chain. Calpactin I was found, by the use of three different techniques, to bind to phospholipid liposomes at less than 10(-8) M free Ca2+. This result is in contrast with those of previous studies, which indicated that greater than 10(-6) M free Ca2+ was required. Our findings suggest that calpactin I may be bound to phospholipids in vivo at Ca2+ concentrations of about 1.5 x 10(-7) M, typical of resting unstimulated cells, and that this interaction may be modulated by light-chain binding and phosphorylation by p60v-src.
D017302:Annexins; D002118:Calcium / Q000494:pharmacology; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D002499:Centrifugation, Density Gradient; D008081:Liposomes / Q000378:metabolism; D015688:Oncogene Protein pp60(v-src); D010743:Phospholipids / Q000378:metabolism*; D010766:Phosphorylation; D011485:Protein Binding / Q000187:drug effects; D011494:Protein Kinases / Q000378:metabolism; D012191:Retroviridae Proteins / Q000378:metabolism*; D014443:Tyrosine / Q000378:metabolism
D017302:Annexins; D002135:Calcium-Binding Proteins; D008081:Liposomes; D010743:Phospholipids; D012191:Retroviridae Proteins; D014443:Tyrosine; D011494:Protein Kinases; D015688:Oncogene Protein pp60(v-src); D002118:Calcium