Nek2B a novel maternal form of Nek2 kinase, is essential for the assembly or maintenance of centrosomes in early Xenopus embryos

Nek2, a NIMA-related kinase, has been postulated to play a role in both the meiotic and mitotic cell cycles in vertebrates. Xenopus has two Nek2 splic e variants, Nek2A and Nek2B, which are zygotic and maternal forms, respecti vely. Here we have examined the role of Nek2B in oocyte meiosis and early e mbryonic mitosis. Specific inhibition of Nek2B function does not interfere with the oscillation of Cdc2 activity in either the meiotic or mitotic cell cycles; however, it does cause abortive cleavage of early embryos, in whic h bipolar spindle formation is severely impaired due to fragmentation or di spersal of the centrosomes, to which endogenous Nek2B protein localizes. In contrast, inhibition of Nek2B function does not affect meiotic spindle for mation in oocytes, in which functional centrosomes are absent. Thus, striki ngly, Nek2B is specifically required for centrosome assembly and/or mainten ance (and hence for normal bipolar spindle formation and cleavage) in early Xenopus embryos. Finally, (ectopic) Nek2A but not NeK2B is very labile in cleaving embryos, suggesting that Nek2A cannot replace the centrosomal func tion of Nek2B in early embryos.