pCalpactins 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./p
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机译：Calpactins I和II是结合Ca2 +，磷脂，肌动蛋白和血影蛋白的蛋白;它们也是癌基因和生长因子受体酪氨酸激酶的主要底物。由于已提议钙调蛋白提供膜脂质和细胞骨架之间的联系，因此我们详细研究了纯化的钙调蛋白I与磷脂脂质体之间的相互作用。我们的研究集中在Ca2 +依赖性，p60v-src（劳斯肉瘤病毒致癌基因编码的蛋白激酶）对钙铁蛋白I磷酸化的影响以及钙铁蛋白I轻链与钙铁蛋白I重链结合的影响链。轻链与重链的结合增加了钙蛋白酶I对磷脂酰丝氨酸（PS）脂质体的亲和力。对于p60v-src的磷酸化，观察到相反的作用。磷酸化降低了钙蛋白酶I对PS脂质体的亲和力。这两个相反的作用似乎是独立的，因为磷酸化并不能阻止轻链与重链的结合。通过使用三种不同的技术，发现钙钙蛋白I在少于10（-8）M的游离Ca2 +下与磷脂脂质体结合。该结果与先前的研究相反，后者表明需要大于10（-6）M的游离Ca2 +。我们的发现表明钙蛋白酶I在Ca2 +浓度约为1.5 x 10（-7）M的情况下可能与体内磷脂结合，这是静止的未刺激细胞的典型特征，并且这种相互作用可能通过轻链结合和p60v-磷酸化来调节src。
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