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J. Cell Biol.,
Volume 143, Number 3, November 2, 1998 751-765
Department of Molecular Biology, Princeton University, Princeton, New Jersey 08544-1014
Cdc31p is the yeast homologue of centrin, a
highly conserved calcium-binding protein of the calmodulin superfamily. Previously centrins have been implicated only in microtubule-based processes. To elucidate the functions of yeast centrin, we carried out a two-hybrid screen for Cdc31p-interacting proteins and
identified a novel essential protein kinase of 1,080 residues, Kic1p (kinase that interacts with Cdc31p). Kic1p
is closely related to S. cerevisiae Ste20p and the p-21-
activated kinases (PAKs) found in a wide variety of
eukaryotic organisms. Cdc31p physically interacts with Kic1p by two criteria; Cdc31p coprecipitated with
GST-Kic1p and it bound to GST-Kic1p in gel overlay
assays. Furthermore, GST-Kic1p exhibited in vitro kinase activity that was CDC31-dependent. Although kic1 mutants were not defective for spindle pole body
duplication, they exhibited a variety of mutant phenotypes demonstrating that Kic1p is required for cell integrity. We also found that cdc31 mutants, previously
identified as defective for spindle pole body duplication, exhibited lysis and morphological defects. The
cdc31 kic1 double mutants exhibited a drastic reduction
in the range of permissive temperature, resulting in a
severe lysis defect. We conclude that Kic1p function is
dependent upon Cdc31p both in vivo and in vitro. We
postulate that Cdc31p is required both for SPB duplication and for cell integrity/morphogenesis, and that the
integrity/morphogenesis function is mediated through the Kic1p protein kinase.
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