Control of G(2)/M transition in Xenopus by a member of the p21-activated kinase (PAK) family: A link between protein kinase a and PAK signaling pathways?

X-PAKs are involved in negative control of the process of oocyte maturation in Xenopus (1), In the present study, we define more precisely the events targetted by the kinase in the inhibition of the G(2)/M transition. We show that microinjection of recombinant X-PAK1-Cter active kinase into progeste rone-treated oocytes prevents c-Mos accumulation and activation of both MAP K and maturation-promoting factor (MPF), In conditions permissive for MAPK activation, MPF activation still fails. We demonstrate that a constitutive truncated version of X-PAK1 (X-PAK1-Cter) does not prevent the association of cyclin B with p34(cdc2) but rather prevents the activation of the inacti ve complexes present in the oocyte. Proteins participating in the MPF ampli fication loop, including the Cdc25-activating Polo-like kinase are all bloc ked. Indeed, using active MPF, the amplification loop is not turned on in t he presence of X-PAK1. Our results indicate that X-PAK and protein kinase A targets in the control of oocyte maturation are similar and furthermore th at this negative regulation is not restricted to meiosis, because we demons trate that G(2)/M progression is also prevented in Xenopus cycling extracts in the presence of active X-PAK1.