Moran M F, Polakis P, McCormick F, Pawson T, Ellis C
Department of Molecular and Medical Genetics, University of Toronto, Ontario, Canada.
Mol Cell Biol. 1991 Apr;11(4):1804-12. doi: 10.1128/mcb.11.4.1804-1812.1991.
The p21ras GTPase-activating protein (GAP) down-regulates p21ras by stimulating its intrinsic GTPase activity. GAP is found predominantly as a monomer in the cytosol of normal cells. However, in cells expressing an activated cytoplasmic protein-tyrosine kinase, p60v-src, or stimulated with epidermal growth factor, GAP becomes phosphorylated on tyrosine and serine and forms distinct complexes with two phosphoproteins of 62 and 190 kDa (p62 and p190). In v-src-transformed Rat-2 cells, a minor fraction of GAP associates with the highly tyrosine phosphorylated p62 to form a complex that is localized at the plasma membrane and in the cytosol. In contrast, the majority of GAP enters a distinct complex with p190 that is exclusively cytosolic and contains predominantly phosphoserine. Epidermal growth factor stimulation also induces a marked conversion of monomeric GAP to higher-molecular-weight species in rat fibroblasts. The GAP-p190 complex is dependent on phosphorylation and shows reduced GAP activity. These results indicate that protein-tyrosine kinases induce GAP to form multiple heteromeric complexes, which are strong candidates for regulators or targets of p21ras.
p21ras GTP酶激活蛋白（GAP）通过刺激其内在的GTP酶活性来下调p21ras。在正常细胞的胞质溶胶中，GAP主要以单体形式存在。然而，在表达活化的细胞质蛋白酪氨酸激酶p60v-src的细胞中，或在用表皮生长因子刺激的细胞中，GAP在酪氨酸和丝氨酸上发生磷酸化，并与62 kDa和190 kDa的两种磷蛋白（p62和p190）形成不同的复合物。在v-src转化的大鼠2细胞中，一小部分GAP与高度酪氨酸磷酸化的p62结合形成一种复合物，该复合物定位于质膜和胞质溶胶中。相比之下，大多数GAP与p190形成一种独特的复合物，该复合物仅存在于胞质溶胶中，且主要含有磷酸丝氨酸。表皮生长因子刺激也会在大鼠成纤维细胞中诱导单体GAP显著转化为更高分子量的物种。GAP-p190复合物依赖于磷酸化，且GAP活性降低。这些结果表明，蛋白酪氨酸激酶诱导GAP形成多种异源复合物，这些复合物极有可能是p21ras的调节剂或作用靶点。
