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Keywords:

  • CycD;
  • plant cell cycle;
  • proteolysis;
  • protein stability;
  • CYCD3

Summary

In Arabidopsis, the D-type cyclin CYCD3 is rate-limiting for transition of the G1/S boundary, and is transcriptionally upregulated at this point in cells re-entering the cell cycle in response to plant hormones and sucrose. However, little is known about the regulation of plant cell-cycle regulators at the protein level. We show here that CYCD3;1 is a phosphoprotein highly regulated at the level of protein abundance, whereas another D-type cyclin CYCD2;1 is not. The level of CYCD3;1 protein falls rapidly on sucrose depletion, correlated with the arrest of cells in G1 phase, suggesting a rapid turnover of CYCD3;1. Treatment of exponentially growing cells with the protein synthesis inhibitor cycloheximide (CHX) confirms that CYCD3;1 is normally a highly unstable protein, with a half-life of approximately 7 min on CHX treatment. In both sucrose-starved and exponentially growing cells, CYCD3;1 protein abundance increases in response to treatment with MG132 (carbobenzoxyl-leucinyl-leucinyl-leucinal), a reversible proteasome inhibitor, but not in response to the cysteine protease inhibitor E-64 or the calpain inhibitor ALLN (N-acetyl-leucyl-leucyl-norleucinal). The increase on MG132 treatment is because of de novo protein synthesis coupled with the blocking of CYCD3;1 degradation. Longer MG132 treatment leads to C-terminal cleavage of CYCD3;1, accumulation of a hyperphosphorylated form and its subsequent disappearance. We conclude that CYCD3;1 is a highly unstable protein whose proteolysis is mediated by a proteasome-dependent pathway, and whose levels are highly dependent on the rate of CYCD3;1 protein synthesis.