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Research Article |
1 Department of Biophysics and Biochemistry, Graduate School of Science,
University of Tokyo, Hongo, Tokyo 113-0033, Japan
2 PRESTO, Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012,
Japan
3 Laboratory for Developmental Genomics, RIKEN Center for Developmental Biology,
Kobe, Hyogo 650-0047, Japan
* Author for correspondence (e-mail: myamamot{at}ims.u-tokyo.ac.jp )
Accepted 6 January 2002
The centrosome consists of two centrioles surrounded by the pericentriolar
material (PCM). In late G2 phase, centrosomes enlarge by recruiting extra PCM,
and concomitantly its microtubule nucleation activity increases dramatically.
The regulatory mechanisms of this dynamic change of centrosomes are not well
understood. Protein phosphatase 4 (PP4) is known to localize to mitotic
centrosomes in mammals and Drosophila. An involvement of PP4 in the
mitotic spindle assembly has been implicated in Drosophila, but in
vivo functions of PP4 in other organisms are largely unknown. Here we
characterize two Caenorhabditis elegans PP4 genes, named
pph-4.1 and pph-4.2. Inhibition of the function of each gene
by RNA-mediated interference (RNAi) revealed that PPH-4.1 was essential for
embryogenesis but PPH-4.2 was not. More specifically, PPH-4.1 was required for
the formation of spindles in mitosis and sperm meiosis. However, this
phosphatase was apparently dispensable for female meiotic divisions, which do
not depend on centrosomes. In the cell depleted of pph-4.1 activity,
localization of 
-tubulin and a Polo-like kinase homologue to the
centrosome was severely disturbed. Immunofluorescence staining revealed that
PPH-4.1 was present at centrosomes from prophase to telophase, but not during
interphase. These results indicate that PPH-4.1 is a centrosomal protein
involved in the recruitment of PCM components to the centrosome, and is
essential for the activation of microtubule nucleation potential of the
centrosome. Furthermore, chiasmata between homologous chromosomes were often
absent in oocytes that lacked pph-4.1 activity. Thus, besides
promoting spindle formation, PPH-4.1 appears to play a role in either the
establishment or the maintenance of chiasmata during meiotic prophase I.
Key words: C. elegans, PP4, Centrosome, -Tubulin, Spindle assembly, Chiasma, Meiosis
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