Yamashita R A, May G S
Department of Cell Biology, Baylor College of Medicine, Houston, Texas 77030, USA.
J Biol Chem. 1998 Jun 5;273(23):14644-8. doi: 10.1074/jbc.273.23.14644.
Class I myosins function in cell motility, intracellular vesicle trafficking and endocytosis. Recently, it was shown that class I myosins are phosphorylated by a member of the p21-activated kinase (PAK) family. PAK phosphorylates a conserved serine or threonine residue in the myosin heavy chain. Phosphorylation at this site is required for maximal activation of the actin-activated Mg2+-ATPase activity in vitro. This serine or threonine residue is conserved in all known class I myosins of microbial origin and in the human and mouse class VI myosins. We have investigated the in vivo significance of this phosphorylation by mutating serine 371 of the class I myosin heavy chain gene myoA of Aspergillus nidulans. Mutation to glutamic acid, which mimics phosphorylation and therefore activation of the myosin, results in an accumulation of membranes in growing hyphae. This accumulation of membranes results from an activation of endocytosis. In contrast, mutation of serine 371 to alanine had no discernible effect on endocytosis. These studies are the first to demonstrate the in vivo significance of a regulatory phosphorylation on a class I myosin. Furthermore, our results suggest that MYOA has two functions, one dependent and one independent of phosphorylation.
I 类肌球蛋白在细胞运动、细胞内囊泡运输和内吞作用中发挥作用。最近的研究表明，I 类肌球蛋白可被 p21 激活激酶（PAK）家族的一个成员磷酸化。PAK 使肌球蛋白重链中的一个保守丝氨酸或苏氨酸残基磷酸化。体外肌动蛋白激活的 Mg2+ -ATP 酶活性的最大激活需要该位点的磷酸化。这个丝氨酸或苏氨酸残基在所有已知的微生物来源的 I 类肌球蛋白以及人和小鼠的 VI 类肌球蛋白中都是保守的。我们通过突变构巢曲霉 I 类肌球蛋白重链基因 myoA 的丝氨酸 371 来研究这种磷酸化在体内的意义。突变为谷氨酸，模拟磷酸化从而激活肌球蛋白，导致生长中的菌丝中膜的积累。这种膜的积累是由内吞作用的激活引起的。相比之下, 将丝氨酸 371 突变为丙氨酸对内吞作用没有明显影响。这些研究首次证明了 I 类肌球蛋白上调节性磷酸化在体内的意义。此外，我们的结果表明 MYOA 有两种功能，一种依赖磷酸化，一种不依赖磷酸化。
