Cloning and characterization of Plx2 and Plx3, two additional polo-like kinases from Xenopus laevis

Members of the family of Polo-like kinases are implicated in the regulation of cell cycle progression in all eukaryotes. In Xenopus laevis, only one m ember of this family, Plx1, has previously been described. Here we report t he cloning and characterization of X. laevis Plx2 and Plx3, the likely homo logs of mammalian Plk2 (Snk) and Plk3 (Fnk/Prk), respectively. RNA expressi on studies indicate that all three Xenopus Plks are present in both oocytes and unfertilized eggs. Further analysis by in situ hybridization revealed that Plx1 RNA is ubiquitously expressed in early embryos, but shows more re stricted expression at later stages. In contrast, Plx2 and Plx3 expression is highly restricted in both early and late-stage embryos. Using Plx-specif ic antisera, Plx1 and Plx3 polypeptides could readily be detected on immuno blots of oocyte and egg extracts. Both Plx1 and Plx3 protein levels remaine d virtually constant during oocyte maturation. However, whereas Plx1 is mor e active in M phase than in I phase (P. Descombes and E. A. Nigg (1998) EMB O J. 17, 1328-1335), Plx3 protein and activity levels remained constant upo n release of meiotic metaphase II-arrested egg extracts into interphase. Fi nally, microinjection of in vitro-transcribed RNAs for Plx1, Plx2, and Plx3 increased the rate of progesterone-induced oocyte maturation, and concomit antly, all three kinases became activated. Conversely, overexpression of th e corresponding catalytically inactive kinases delayed maturation. This sug gests that, at least in oocytes, all three kinases may be regulated by simi lar mechanisms, and they may also share common substrates. However, the str ikingly restricted pattern of expression of Plx2 and Plx3 observed in embry os strongly suggests that individual Plk family members perform at least pa rtly distinct functions.