Before fertilization, vertebrate eggs are arrested in metaphase of meiosis
II by cytostatic factor (CSF), an activity that requires activation of the
mitogen-activated protein kinase (MAPK) pathway. To investigate whether CSF
arrest is mediated by the protein kinase p90(Rsk), which is phosphorylated
and activated by MAPK, a constitutively activated (CA) form of Rsk was exp
ressed in Xenopus embryos. Expression of CA Rsk resulted in cleavage arrest
, and cytological analysis showed that arrested blastomeres were in M phase
with prominent spindles characteristic of meiotic metaphase. Thus, Rsk app
ears to be the mediator of MAPK-dependent CSF arrest in vertebrate unfertil
ized eggs.