News and Publications

Cellular Functions of G Protein--Regulated Kinases

U.G. Knaus, S. Albrecht, M.-J. Kim, J.-P. Mira, T. Shalom-Barak, A. Reilly, A. Steele

Protein kinases and phosphatases play critical roles in regulating and coordinating aspects of gene expression, cellular growth and motility, differentiation, and division. As a result, normal cellular function depends not only on the catalytic activity of protein kinases and phosphatases but also on regulatory proteins or second messengers capable of switching the kinases and phosphatases on and off. A class of serine/threonine protein kinases known as p21-activated kinases (PAKs) are activated in response to the Rho family GTPases Rac and Cdc42. Rac and Cdc42 regulate signaling pathways involved in cytoskeletal reorganization, stress-induced activation of kinases, transformation of cells, and so forth. Our laboratory studies how GTPase effector protein kinases such as PAKs mediate the functions of Rac and Cdc42.

GTPase-dependent and GTPase-independent activation mechanisms of PAKs have been a focus of our recent work. In collaboration with J. Jackson, Department of Immunology, we have detected critical regions on the GTPase Rac and on PAK that determine the interaction of both proteins and are crucial for biological activity. In studies with G. Bokoch, Department of Immunology, we found a distinct, GTPase-independent activation mechanism of PAK activity: stimulation by lipid compounds.

We are interested in G protein--regulated processes in phagocytic leukocytes that can compromise the human immune response in inflammatory or infectious diseases. Rac and PAKs are implicated in a variety of leukocyte functions, ranging from production of reactive oxygen species to chemotaxis and phagocytosis. Stimulation of leukocytes by chemotactic factors or phagocytic stimuli leads to the generation of superoxide via a Rac-dependent NADPH oxidase.

We have identified several PAKs in leukocytes and have established the activation of these kinases by chemoattractants as well as by activated Rac. We are investigating the role of PAK in the NADPH oxidase activation leading to the generation of superoxide. Determination of (1) PAK-dependent phosphorylation of NADPH oxidase components and (2) potential PAK-dependent protein-protein interactions that precede the production of superoxide will enable us to define relevant PAK targets. Further studies are directed at the involvement of PAKs in other neutrophil-specific functions such as adhesion and phagocytosis.

PAKs are also integral elements of MAPK cascades leading to the activation of transcription factors. Preliminary data suggest a loss of regulatory control in these kinase cascades during tumor pathogenesis in breast cancer cells. Investigations on the contribution of PAK to transformation and invasiveness are under way. PAK is involved in cytoskeletal rearrangements and actin polymerization, which are prerequisites for shape changes and cell motility. Consequently, during malignant events, PAK might be involved not only in tumor progression but also in tumor metastasis.

PUBLICATIONS

Bokoch, G.M., Reilly, A.M., Daniels, R.H., King, C.C., Olivera, A., Spiegel, S., Knaus, U.G. A GTPase-independent mechanism of PAK activation: Regulation by sphingosine and other biologically active lipids. J. Biol. Chem. 273:8137, 1998.

Jones, S.L., Knaus, U.G., Bokoch, G.M., Springer, T.A., Brown, E.J. Two signaling mechanisms for activation of _Mß_a avidity in PMN. J. Biol. Chem. 273:10556, 1998.