|The PRK2 kinase is a potential effector target of both Rho and Rac GTPases and regulates actin cytoskeletal organization.|
Vincent, S and Settleman, J
Mol. Cell. Biol., 17: 2247-56 (1997)
The Ras-related Rho family GTPases mediate signal transduction pathways that regulate a variety of cellular processes. Like Ras, the Rho proteins (which include Rho, Rac, and CDC42) interact directly with protein kinases, which are likely to serve as downstream effector targets of the activated GTPase. Activated RhoA has recently been reported to interact directly with several protein kinases, p120 PKN, p150 ROK alpha and -beta, p160 ROCK, and p164 Rho kinase. Here, we describe the purification of a novel Rho-associated kinase, p140, which appears to be the major Rho-associated kinase activity in most tissues. Peptide microsequencing revealed that p140 is probably identical to the previously reported PRK2 kinase, a close relative of PKN. However, unlike the previously described Rho-binding kinases, which are Rho specific, p140 associates with Rac as well as Rho. Moreover, the interaction of p140 with Rho in vitro is nucleotide independent, whereas the interaction with Rac is completely GTP dependent. The association of p140 with either GTPase promotes kinase activity substantially, and expression of a kinase-deficient form of p140 in microinjected fibroblasts disrupts actin stress fibers. These results indicate that p140 may be a shared kinase target of both Rho and Rac GTPases that mediates their effects on rearrangements of the actin cytoskeleton.
|The p160 RhoA-binding kinase ROK alpha is a member of a kinase family and is involved in the reorganization of the cytoskeleton.|
Leung, T, et al.
Mol. Cell. Biol., 16: 5313-27 (1996)
The GTPase RhoA has been implicated in various cellular activities, including the formation of stress fibers, motility, and cytokinesis. We recently reported on a p150 serine/threonine kinase (termed ROK alpha) binding RhoA only in its active GTP-bound state and on its cDNA; introduction of RhoA into HeLa cells resulted in translocation of the cytoplasmic kinase to plasma membranes, consistent with ROK alpha being a target for RhoA (T. Leung, E. Manser, L. Tan, and L. Lim, J. Biol. Chem. 256:29051-29054, 1995). Reanalysis of the cDNA revealed that ROK alpha contains an additional N-terminal region. We also isolated another cDNA which encoded a protein (ROK beta) with 90% identity to ROK alpha in the kinase domain. Both ROK alpha and ROK beta, which had a molecular mass of 160 kDa, contained a highly conserved cysteine/histidine-rich domain located within a putative pleckstrin homology domain. The kinases bound RhoA, RhoB, and RhoC but not Rac1 and Cdc42. The Rho-binding domain comprises about 30 amino acids. Mutations within this domain caused partial or complete loss of Rho binding. The morphological effects of ROK alpha were investigated by microinjecting HeLa cells with DNA constructs encoding various forms of ROK alpha. Full-length ROK alpha promoted formation of stress fibers and focal adhesion complexes, consistent with its being an effector of RhoA. ROK alpha truncated at the C terminus promoted this formation and also extensive condensation of actin microfilaments and nuclear disruption. The proteins exhibited protein kinase activity which was required for stress fiber formation; the kinase-dead ROK alpha K112A and N-terminally truncated mutants showed no such promotion. The latter mutant instead induced disassembly of stress fibers and focal adhesion complexes, accompanied by cell spreading. These effects were mediated by the C-terminal region containing Rho-binding, cysteine/histidine-rich, and pleckstrin homology domains. Thus, the multidomained ROK alpha appears to be involved in reorganization of the cytoskeleton, with the N and C termini acting as positive and negative regulators, respectively, of the kinase domain whose activity is crucial for formation of stress fibers and focal adhesion complexes.