Dan Chuntao, Nath Niharika, Liberto Muriel, Minden Audrey
Department of Biological Sciences, Columbia University, New York, New York 10027, USA.
Mol Cell Biol. 2002 Jan;22(2):567-77. doi: 10.1128/MCB.22.2.567-577.2002.
We have characterized a new member of the mammalian PAK family of serine/threonine kinases, PAK5, which is a novel target of the Rho GTPases Cdc42 and Rac. The kinase domain and GTPase-binding domain (GBD) of PAK5 are most closely related in sequence to those of mammalian PAK4. Outside of these domains, however, PAK5 is completely different in sequence from any known mammalian proteins. PAK5 does share considerable sequence homology with the Drosophila MBT protein (for "mushroom body tiny"), however, which is thought to play a role in development of cells in Drosophila brain. Interestingly, PAK5 is highly expressed in mammalian brain and is not expressed in most other tissues. We have found that PAK5, like Cdc42, promotes the induction of filopodia. In N1E-115 neuroblastoma cells, expression of PAK5 also triggered the induction of neurite-like processes, and a dominant-negative PAK5 mutant inhibited neurite outgrowth. Expression of activated PAK1 caused no noticeable changes in these cells. An activated mutant of PAK5 had an even more dramatic effect than wild-type PAK5, indicating that the morphologic changes induced by PAK5 are directly related to its kinase activity. Although PAK5 activates the JNK pathway, dominant-negative JNK did not inhibit neurite outgrowth. In contrast, the induction of neurites by PAK5 was abolished by expression of activated RhoA. Previous work has shown that Cdc42 and Rac promote neurite outgrowth by a pathway that is antagonistic to Rho. Our results suggest, therefore, that PAK5 operates downstream to Cdc42 and Rac and antagonizes Rho in the pathway, leading to neurite development.
我们鉴定了丝氨酸/苏氨酸激酶哺乳动物PAK家族的一个新成员PAK5，它是Rho GTP酶Cdc42和Rac的一个新靶点。PAK5的激酶结构域和GTP酶结合结构域（GBD）在序列上与哺乳动物PAK4的最为密切相关。然而，在这些结构域之外，PAK5在序列上与任何已知的哺乳动物蛋白完全不同。不过，PAK5与果蝇的MBT蛋白（“蘑菇体微小”）确实有相当程度的序列同源性，据认为该蛋白在果蝇大脑细胞发育中起作用。有趣的是，PAK5在哺乳动物大脑中高度表达，而在大多数其他组织中不表达。我们发现，与Cdc42一样，PAK5促进丝状伪足的形成。在N1E-115神经母细胞瘤细胞中，PAK5的表达也触发了神经突样突起的形成，而一个显性负性PAK5突变体抑制了神经突的生长。活化的PAK1的表达在这些细胞中未引起明显变化。PAK5的一个活化突变体比野生型PAK5有更显著的作用，表明PAK5诱导的形态变化与其激酶活性直接相关。虽然PAK5激活JNK途径，但显性负性JNK并未抑制神经突的生长。相反，活化的RhoA的表达消除了PAK5对神经突的诱导作用。先前的研究表明，Cdc42和Rac通过一条与Rho拮抗的途径促进神经突的生长。因此，我们的结果表明，PAK5在Cdc42和Rac的下游起作用，并在该途径中拮抗Rho，从而导致神经突发育。
