Biochemical Properties of the Cdc42-associated Tyrosine Kinase ACK1

Abstract

ACK1 (activated Cdc42-associated kinase 1) is a non-receptor tyrosine kinase and the only tyrosine kinase known to interact with Cdc42. To characterize the enzy-matic properties of ACK, we have expressed and purified active ACK using the baculovirus/Sf9 cell system. This ACK1 construct contains (from N to C terminus) the kinase catalytic domain, SH3 domain, and Cdc42-binding Cdc42/Rac interactive binding (CRIB) domain. We characterized the substrate specificity of ACK1 using synthetic peptides, and we show that the specificity of the ACK1 catalytic domain most closely resembles that of Abl. Purified ACK1 undergoes autophosphorylation, and autophosphorylation enhances kinase activity. We identified Tyr 284 in the activation loop of ACK1 as the primary autophosphorylation site using mass spectrom-etry. When expressed in COS-7 cells, the Y284F mutant ACK1 showed dramatically reduced levels of tyrosine phosphorylation. Although the SH3 and CRIB domains of purified ACK1 are able to bind ligands (a polyproline peptide and Cdc42, respectively), the addition of ligands did not stimulate tyrosine kinase activity. To characterize potential interacting partners for ACK1, we screened several SH2 and SH3 domains for their ability to bind to full-length ACK1 or to the catalytic-SH3-CRIB construct. ACK1 interacts most strongly with the SH3 domains of Src family kinases (Src or Hck) via its C-terminal pro-line-rich domain. Co-expression of Hck with kinase-inactive ACK1(K158R) in mammalian cells resulted in ty-rosine phosphorylation of ACK1, suggesting that ACK1 is a substrate for Hck. Our data suggest that Hck is a novel binding partner for ACK1 that can regulate ACK1 activity by phosphorylation. Members of the Rho family of small GTP-binding proteins couple extracellular signals to the regulation of cell morphology , adhesion, differentiation, and proliferation (1-5). A number of proteins have been identified as targets for the Rho family proteins Rac and/or Cdc42, including p21-activated ki-nases (6-8), Wiscott-Aldrich syndrome proteins (9-11), mixed lineage kinases (12), and IQGAP (13-16). The ACK family nonreceptor tyrosine kinases (ACK1 and ACK2) associate specifically with Cdc42 and act as Cdc42 effectors in several sig-naling pathways (6, 17, 18). ACK1 has been reported to phos-phorylate Dbl, a guanine nucleotide exchange factor toward Rho family proteins, thereby promoting Dbl activity. In this way, ACK1 is thought to act as a mediator of EGF 1 signals to Rho family GTP-binding proteins (19). ACK2 mediates cell adhesion signals initiated by integrin 1 in a Cdc42-dependent manner (20). ACKs interact directly with the clathrin heavy chain and participate in the regulation of receptor-mediated endocyto-sis (21, 22). However, the physiological functions as well as specific target molecules of ACKs are still incompletely understood. The domain structure of ACK kinases consists of an N-terminal tyrosine kinase catalytic domain followed by an SH3 domain, a Cdc42/Rac interactive binding (CRIB) domain, and a proline-rich region (see Fig. 1A) (18). The position of the SH3 domain C-terminal to the catalytic domain is unique among families of cytoplasmic tyrosine kinases and in particular contrasts with the N-terminal SH3 domain observed in the Src, Csk, Abl, Frk, and Tec families. In Src family kinases, two intramolecular interactions tightly regulate enzymatic activity: (i) an interaction between the SH2 domain and the C-terminal tail and (ii) an interaction between the SH3 domain and a polyproline type II helix in the SH2-kinase linker region (23-25). Exogenous ligands for the SH2 and SH3 domains can disrupt the autoinhibitory interactions, promote autophospho-rylation at Tyr 416 , and stimulate Src kinase activity (25-27). The SH2 and SH3 domains of Src kinases also play an important role in substrate recognition (28, 29). The SH3 domains of Abl family kinases are likewise involved in autoinhibition and substrate binding. ACK2 has been shown to be activated by Cdc42 binding to the CRIB region (20, 30). However, the roles of the SH3 domain and polyproline region of ACK in enzyme regulation have not been elucidated. Apart from Cdc42 and clathrin, relatively few ACK-interacting proteins have been identified. The C-terminal proline-rich domain of ACK is involved in interaction with the Nck (22) and Grb2 (31) adaptor proteins and with sorting nexin protein 9 (SH3PX1) (32, 33). Co-immunoprecipitation studies indicate that ACK2, clathrin, and SH3PX1 form a complex; ACK2 cooperates with SH3PX1 to promote degradation of EGF receptor (32). HSH2, an adaptor protein found in hematopoietic cells, binds to the SH3 domain of ACK1 (34). Because of the importance of SH3 domains in Src kinase substrate targeting, it is likely that additional substrates/effectors of ACK kinases will be found that bind to the SH3 domain.