Institute of Biotechnology and Genome-Scale Biology Program, University of Helsinki, PO Box 56 Viikinkaari 9, 00014 University of Helsinki, Finland.
J Cell Sci. 2011 Feb 1;124(Pt 3):384-93. doi: 10.1242/jcs.072660.
Actin stress fiber assembly and contractility in nonmuscle motile cells requires phosphorylation of myosin regulatory light chain (MLC). Dephosphorylation and disassembly are mediated by MLC phosphatase, which is targeted to actin fibers by the association of its regulatory subunit MYPT1 with myosin phosphatase Rho-interacting protein (MRIP). In the present study, we identify the kinase NUAK2 as a second protein targeted by MRIP to actin fibers. Association of NUAK2 with MRIP increases MLC phosphorylation and promotes formation of stress fibers. This activity does not require the kinase activity of NUAK2 but is dependent on both MRIP and MYPT1, indicating that the NUAK2-MRIP association inhibits fiber disassembly and MYPT1-mediated MLC dephosphorylation. NUAK2 levels are strongly induced by stimuli increasing actomyosin fiber formation, and NUAK2 is required for fiber maintenance in exponentially growing cells, implicating NUAK2 in a positive-feedback loop regulating actin stress fibers independently of the MLC kinase Rho-associated protein kinase (ROCK). The identified MRIP-NUAK2 association reveals a novel mechanism for the maintenance of actin stress fibers through counteracting MYPT1 and, together with recent results, implicates the NUAK proteins as important regulators of the MLC phosphatase acting in both a kinase-dependent and kinase-independent manner.
肌球蛋白轻链（MLC）的磷酸化作用是驱动非肌肉运动细胞中肌动蛋白应激纤维组装和收缩的关键。MLC 的去磷酸化和组装是由 MLC 磷酸酶介导的，其通过调节亚基 MYPT1 与肌球蛋白磷酸酶 Rho 相互作用蛋白（MRIP）的结合而靶向到肌动蛋白纤维上。在本研究中，我们鉴定出激酶 NUAK2 是第二个被 MRIP 靶向到肌动蛋白纤维的蛋白。NUAK2 与 MRIP 的结合增加了 MLC 的磷酸化，并促进了应激纤维的形成。这种活性不依赖于 NUAK2 的激酶活性，但依赖于 MRIP 和 MYPT1，表明 NUAK2-MRIP 复合物抑制纤维解组装和 MYPT1 介导的 MLC 去磷酸化。刺激肌动球蛋白纤维形成会强烈诱导 NUAK2 水平升高，并且在指数生长期的细胞中，NUAK2 是纤维维持所必需的，这表明 NUAK2 参与了一个独立于肌球蛋白轻链激酶 Rho 相关蛋白激酶（ROCK）的正反馈回路，调节肌动蛋白应激纤维。所鉴定出的 MRIP-NUAK2 复合物揭示了通过拮抗 MYPT1 来维持肌动蛋白应激纤维的新机制，并且与最近的结果一起，表明 NUAK 蛋白是 MLC 磷酸酶的重要调节因子，以激酶依赖和非依赖性方式发挥作用。
