Ahn N G, Campbell J S, Seger R, Jensen A L, Graves L M, Krebs E G
Department of Biochemistry, University of Washington, Seattle 98195.
Proc Natl Acad Sci U S A. 1993 Jun 1;90(11):5143-7. doi: 10.1073/pnas.90.11.5143.
Mitogen-activated protein (MAP) kinase kinase is an enzyme that activates the growth factor-regulated MAP kinase in vitro by a mechanism that involves direct phosphorylation of MAP kinase on tyrosine and threonine residues. MAP kinase kinase is stimulated by growth factor treatment of cells and has been shown to be inactivated with protein phosphatases, suggesting that it is regulated by protein phosphorylation. Analysis of two epidermal growth factor-stimulated forms of MAP kinase kinase, purified from 32P-labeled A431 cells, shows that the kinase is phosphorylated on serine and threonine residues and that treatment with protein phosphatases leads to serine dephosphorylation. Under conditions that lead to complete inactivation, only partial dephosphorylation of MAP kinase kinase is observed. Consistent with this finding, inactive forms of MAP kinase kinase, which separate from active forms during the course of purification, are also observed to be phosphorylated in intact cells.
丝裂原活化蛋白（MAP）激酶激酶是一种酶，它在体外通过一种机制激活生长因子调节的MAP激酶，该机制涉及对MAP激酶的酪氨酸和苏氨酸残基进行直接磷酸化。MAP激酶激酶受到细胞生长因子处理的刺激，并且已被证明可被蛋白磷酸酶灭活，这表明它受蛋白质磷酸化调节。对从32P标记的A431细胞中纯化的两种表皮生长因子刺激形式的MAP激酶激酶进行分析，结果表明该激酶在丝氨酸和苏氨酸残基上被磷酸化，并且用蛋白磷酸酶处理会导致丝氨酸去磷酸化。在导致完全失活的条件下，仅观察到MAP激酶激酶的部分去磷酸化。与这一发现一致，在纯化过程中与活性形式分离的MAP激酶激酶的无活性形式，在完整细胞中也被观察到被磷酸化。
