Signaling, Regulation, and Specificity of the Type II p21-activated Kinases

Abstract

The p21-activated kinases (PAKs) are a family of six serine/ threonine kinases that act as key effectors of RHO family GTPases in mammalian cells. PAKs are subdivided into two groups: type I PAKs (PAK1, PAK2, and PAK3) and type II PAKs (PAK4, PAK5, and PAK6). Although these groups are involved in common signaling pathways, recent work indicates that the two groups have distinct modes of regulation and have both unique and common substrates. Here, we review recent insights into the molecular level details that govern regulation of type II PAK signaling. We also consider mechanisms by which signal transduction is regulated at the level of substrate specificity. Finally, we discuss the implications of these studies for clinical targeting of these kinases.The type II PAKs 2 are members of the Sterile 20 (Ste20) family of STE (homologs of yeast Sterile 7, Sterile 11, and Sterile 20) group serine/threonine kinases. They are important for signaling from small GTPases of the RHO family, particularly the CDC42 (cell division cycle 42) and to a lesser extent RAC (Rasrelated C3 botulinum toxin substrate) isoforms, to the actin cytoskeleton and to growth and survival pathways. Type II PAKs share significant sequence similarity with the well studied type I PAKs (PAK1, PAK2, and PAK3), which are extensively reviewed elsewhere (1-5), but the roles of the type I and type II PAKs in GTPase signaling pathways, and their mechanisms of regulation, differ considerably. In the sections below, we consider the function of the type II PAK serine/threonine kinases in small GTPase pathways, the structural and biochemical basis for their regulation, mechanisms of their targeting to substrates, and some of the current strategies for targeted small molecule inhibition of these enzymes. Type II PAKs in Signal TransductionThe type II PAKs are binding partners of RHO family small GTPases. Their dominant role in signal transduction is as serine/threonine kinases, where transfer of the ␥-phosphate from an ATP molecule to a substrate protein results in consequent regulation of downstream effector pathways. The activation of type II PAK kinase signaling is associated with small GTPase binding, but as discussed below, direct activation does not seem to occur upon small GTPase binding. Rather, GTPases are thought to primarily regulate type II PAKs by controlling their subcellular localization. Type II PAKs also have reported kinase-independent roles as scaffolding proteins, with functions that are still emerging.The type II PAKs are placed at critical nodal points in multiple signaling pathways that are associated with cell growth, cytoskeletal dynamics, cell polarity, survival, and development (6 -8). They are located directly downstream of RHO family GTPase activation. Numerous substrates of type II PAKs have been identified that are thought to act as downstream effectors in distinct cellular processes. For example, direct phosphorylation of LIM kinases (9, 10), p210/␤-catenin (11, 12), slingshot phosphatase (SSH) (13), 14), and PDZ-RHOGEF (15) has...

Group I Paks Promote Skeletal Myoblast Differentiation In Vivo and In Vitro

“…The essential role of group I Paks in Drosophila myoblast fusion is therefore not evolutionarily conserved in mammalian myogenesis in vivo. It is possible that one or more of the group II Paks (Pak4, Pak5, and Pak6) compensate for Pak1 and Pak2 in mammalian myogenesis, and there is some evidence of shared substrates between group I and II enzymes (48,50,51). However, the mechanisms of activation of group I versus group II Paks are distinct, and the two groups have divergent preferred phosphorylation consensus sites (17,48,51).…”

PAK6 targets to cell-cell adhesions via its N-terminus in a Cdc42-dependent manner to drive epithelial colony escape

“…PAK6 is one of six serine/threonine kinases that belong to the PAK family (Ha et al, 2015). In addition to a well-conserved C-terminal kinase domain, all PAK family kinases contain an N-terminal CRIB motif capable of binding Cdc42.…”

“…GTPase binding to the CRIB domain of type I PAKs triggers release of active PAK monomers through dissociation of an inactive PAK dimer, in which the catalytic domain is inhibited by the autoinhibitory domain (AID) of its dimeric partner (Lei et al, 2000;Parrini et al, 2002;Rane and Minden, 2014). By contrast, type II PAKs are constitutively autophosphorylated on the activation loop and are generally not believed to be activated by GTPase binding (Ha et al, 2012(Ha et al, , 2015. Cdc42 binding has instead been proposed to determine cellular localization of type II PAKs Callow et al, 2002), although Cdc42 might also play a role in conformational activation of PAK4 (Baskaran et al, 2012).…”

“…PAKs are effector molecules of the small GTPases Cdc42 and Rac1, and thus participate in a variety of signaling pathways involved in cytoskeletal remodeling and cell adhesion dynamics. As such, all PAKs contain an N-terminal CRIB (Cdc42/Rac interactive binding) domain, capable of interacting with upstream GTPases, and a wellconserved C-terminal catalytic domain, shown to phosphorylate diverse downstream signaling molecules, such as β-catenin, LIM kinases, BAD, GEF-H1 (also known as ARHGEF2), pacsin and paxillin, with varying degrees of PAK isoform substrate specificity (Radu et al, 2014;Ha et al, 2015).…”

Fasudil inhibits tissue factor and plasminogen activator Inhibitor-1 secretion by peripheral blood mononuclear cells in CAPD patients

“…One of its main effectors, Rho-kinase (ROCK), has two known isoforms (ROCK1 and ROCK2) and regulates cytoskeletal reorganization by phosphorylating myosin phosphatase, which results in an increase in myosin light chain (MLC) phosphorylation. 17,18 The aims of this study were to investigate the effects of fasudil on TF and PAI-1 expression in peripheral blood mononuclear cells in CAPD patients.…”