de Vries-Smits A M, Pronk G J, Medema J P, Burgering B M, Bos J L
Laboratory for Physiological Chemistry, Utrecht University, The Netherlands.
Oncogene. 1995 Mar 2;10(5):919-25.
Association of the p21ras guanine nucleotide exchange factor mSOS with tyrosine-phosphorylated Shc has been implicated in the activation of p21ras. In addition, after growth factor stimulation mSOS becomes phosphorylated as indicated by the appearance of a form of mSOS with reduced electrophoretic mobility. This phosphorylation is delayed with respect to Shc-Grb2-mSOS complex formation and activation of p21ras. To investigate the role of mSOS phosphorylation in further detail we have investigated the effect of phosphorylation on mSOS complex formation and p21ras activation. We found that Shc is associated with the unphosphorylated, faster migrating form of mSOS. Furthermore, although there is a correlation between the amount of complexes formed and the activation of p21ras, there is no such a correlation between mSOS phosphorylation and p21ras activation. In addition, inhibition of mSOS phosphorylation did not affect complex formation of mSOS with tyrosine phosphorylated Shc. Also, induction of mSOS phosphorylation prior to complex formation did not affect EGF-induced association of mSOS with Shc significantly, and Shc still associated predominantly with the faster migrating form of mSOS. From these results we conclude that the unphosphorylated form of mSOS is associated with Shc and that perhaps a phosphorylation-dephosphorylation step is part of the mSOS activation-inactivation cycle.
p21ras鸟嘌呤核苷酸交换因子mSOS与酪氨酸磷酸化的Shc的结合与p21ras的激活有关。此外，生长因子刺激后，mSOS会发生磷酸化，表现为一种电泳迁移率降低的mSOS形式的出现。这种磷酸化相对于Shc-Grb2-mSOS复合物的形成和p21ras的激活有所延迟。为了更详细地研究mSOS磷酸化的作用，我们研究了磷酸化对mSOS复合物形成和p21ras激活的影响。我们发现Shc与未磷酸化、迁移速度更快的mSOS形式相关。此外，虽然形成的复合物数量与p21ras的激活之间存在相关性，但mSOS磷酸化与p21ras激活之间不存在这种相关性。此外，抑制mSOS磷酸化并不影响mSOS与酪氨酸磷酸化的Shc的复合物形成。同样，在复合物形成之前诱导mSOS磷酸化对表皮生长因子（EGF）诱导的mSOS与Shc的结合没有显著影响，并且Shc仍然主要与迁移速度更快的mSOS形式结合。从这些结果我们得出结论，mSOS的未磷酸化形式与Shc相关，并且磷酸化-去磷酸化步骤可能是mSOS激活-失活循环的一部分。
