Stockpiling of Cdc25 During a DNA Replication Checkpoint Arrest inSchizosaccharomyces pombe

Abstract

The DNA replication checkpoint couples the onset of mitosis with the completion of S phase. It is clear that in the fission yeast Schizosaccharomyces pombe, operation of this checkpoint requires maintenance of the inhibitory tyrosyl phosphorylation of Cdc2. Cdc25 phosphatase induces mitosis by dephosphorylating tyrosine 15 of Cdc2. In this report, Cdc25 is shown to accumulate to a very high level in cells arrested in S. This shows that mechanisms which modulate the abundance of Cdc25 are unconnected to the DNA replication checkpoint. Using a Cdc2/cyclin B activation assay, we found that Cdc25 activity increased ϳ10-fold during transit through M phase. Cdc25 was activated by phosphorylations that were dependent on Cdc2 activity in vivo. Cdc25 activation was suppressed in cells arrested in G 1 and S. However, Cdc25 was more highly modified and appeared to be somewhat more active in S than in G 1 . This finding might be connected to the fact that progression from G 1 to S increases the likelihood that constitutive Cdc25 overproduction will cause inappropriate mitosis.

A role for the Cdc14-family phosphatase Flp1p at the end of the cell cycle in controlling the rapid degradation of the mitotic inducer Cdc25p in fission yeast

“…In the case of the Cdc25p mitotic inducer, CDKmediated phosphorylation is the signal to bring it to full activity (Hoffmann et al, 1993;Kovelman and Russell, 1996). Given that our in vivo biochemical analysis strongly suggests that once Flp1p (a Ser/Thr phosphatase) dephosphorylates Cdc25p (a Tyr phosphatase) the mitotic inducer becomes unstable, we speculate that the signal that targets Cdc25p for destruction will be given by Flp1p-mediated dephosphorylation.…”

“…The cdc25 + -encoded tyrosine phosphatase slowly accumulates during interphase to reach a critical level at the end of G2 (Moreno et al, 1990;Ducommun et al, 1990), to be first activated by polo, and then by Cdc2/Cdc13 kinases, to allow full activation of the latter kinase that induces entry into mitosis (Hoffmann et al, 1993;Kovelman and Russell, 1996;Glover et al, 1998;Nigg, 1998). During mitosis, Cdc25p is abruptly degraded (in a cell-cycle-dependent manner) (Moreno et al, 1990;Ducommun et al, 1990;Nefsky et al, 1996).…”

Alternative splicing of the human CDC25B tyrosine phosphatase. Possible implications for growth control?

“…In ®ssion yeast, the activity of CDC25 is regulated both at the mRNA and protein levels (Ducommun et al, 1990;Kovelman and Russell, 1996;Moreno et al, 1990). In HeLa cells, expression of CDC25B has been reported to be low through out the cell cycle with a sharp increase in G 2 , CDC25C is predominantly expressed in G 2 Sadhu et al, 1990) and CDC25A is abundant both at the mRNA and protein levels in late G 1 (Jinno et al, 1994).…”

Regulation of Schizosaccharomyces pombeWee1 Tyrosine Kinase

“…This second mechanism of inhibitory regulation of Wee1 is either directly or indirectly controlled by Cdc2-cyclin B kinase. Cdc25 phosphatase undergoes activating phosphorylation during M-phase, also by a mechanism that is either directly or indirectly controlled by Cdc2 (35)(36)(37)(38)(39)(40)(41)(42)(43). This type of Cdc25 regulation has been demonstrated in mammalian tissue culture cell, Xenopus oocyte extracts, and S. pombe.…”