CLEAVAGE ARREST OF EARLY FROG EMBRYOS BY THE G-PROTEIN-ACTIVATED PROTEIN-KINASE PAK-I

PAK I is a member of the PAK (p21-activated protein kinase) family and is activated by Cdc42 (Jakobi, R., Chen, C.-J., Tuazon, P. T., and Tr augh, J. A. (1996) J. Biol. Chem. 271, 6206-6211). To examine the effe cts of PAK I on cleavage arrest, subfemtomole amounts of endogenously active (58 kDa) and inactive (60 kDa) PAK I and a tryptic peptide (37 kDa) containing the active catalytic domain were injected into one bla stomere of 2-cell frog embryos. Active PAK I resulted in cleavage arre st in the injected blastomere at mitotic metaphase, whereas the uninje cted blastomere progressed through mid- to late cleavage. Injection of other protein kinases at similar concentrations had no effect on clea vage. Endogenous PAK I was highly active in frog oocytes, and antibody to PAK I reacted specifically with protein of 58-60 kDa. PAK I protei n was decreased at 60 min post-fertilization, with little or no PAK I protein or activity detectable at 80 min post-fertilization or in 2-ce ll embryos. At the 4-cell stage PAK I protein increased, but the prote in kinase was present primarily as an inactive form. Rac2 and Cdc42, b ut not Rac 1, were identified in oocytes and throughout early embryo d evelopment. Thus, PAK I appears to be a potent cytostatic protein kina se involved in maintaining cells in a non-dividing state. PAK I activi ty is high in oocytes and appears to be regulated by degradation/synth esis and through autophosphorylation via binding of Cdc42. PAK I may a ct through regulation of the stress-activated protein kinase signaling pathway and/or by direct regulation of multiple metabolic pathways.