LeClaire Lawrence L, Baumgartner Martin, Iwasa Janet H, Mullins R Dyche, Barber Diane L
Department of Cell and Tissue Biology, University of California, San Francisco, San Francisco, CA 94143, USA.
J Cell Biol. 2008 Aug 25;182(4):647-54. doi: 10.1083/jcb.200802145.
The actin-related protein 2/3 (Arp2/3) complex is the primary nucleator of new actin filaments in most crawling cells. Nucleation-promoting factors (NPFs) of the Wiskott-Aldrich syndrome protein (WASP)/Scar family are the currently recognized activators of the Arp2/3 complex. We now report that the Arp2/3 complex must be phosphorylated on either threonine or tyrosine residues to be activated by NPFs. Phosphorylation of the Arp2/3 complex is not necessary to bind NPFs or the sides of actin filaments but is critical for binding the pointed end of actin filaments and nucleating actin filaments. Mass spectrometry revealed phosphorylated Thr237 and Thr238 in Arp2, which are evolutionarily conserved residues. In cells, phosphorylation of only the Arp2 subunit increases in response to growth factors, and alanine substitutions of Arp2 T237 and T238 or Y202 inhibits membrane protrusion. These findings reveal an additional level of regulation of actin filament assembly independent of WASP proteins, and show that phosphorylation of the Arp2/3 complex provides a logical "or gate" capable integrating diverse upstream signals.
肌动蛋白相关蛋白2/3（Arp2/3）复合物是大多数爬行细胞中新肌动蛋白丝的主要成核因子。威斯科特-奥尔德里奇综合征蛋白（WASP）/伤疤家族的成核促进因子（NPFs）是目前公认的Arp2/3复合物激活剂。我们现在报告，Arp2/3复合物必须在苏氨酸或酪氨酸残基上磷酸化才能被NPFs激活。Arp2/3复合物的磷酸化对于结合NPFs或肌动蛋白丝的侧面不是必需的，但对于结合肌动蛋白丝的尖端和成核肌动蛋白丝至关重要。质谱分析揭示了Arp2中磷酸化的苏氨酸237和苏氨酸238，它们是进化上保守的残基。在细胞中，仅Arp2亚基的磷酸化会响应生长因子而增加，并且Arp2 T237和T238或Y202的丙氨酸替代会抑制膜突出。这些发现揭示了独立于WASP蛋白的肌动蛋白丝组装调控的额外水平，并表明Arp2/3复合物的磷酸化提供了一个合理的“或门”，能够整合多种上游信号。
