Differential stability of Arabidopsis D-type cyclins: CYCD3;1 is a highly unstable protein degraded by a proteasome-dependent mechanism
Article first published online: 4 MAY 2004
The Plant Journal
Volume 38, Issue 4, pages 616–625, May 2004
How to Cite
Planchais, S., Samland, A. K. and Murray, J. A. H. (2004), Differential stability of Arabidopsis D-type cyclins: CYCD3;1 is a highly unstable protein degraded by a proteasome-dependent mechanism. The Plant Journal, 38: 616–625. doi: 10.1111/j.0960-7412.2004.02071.x
- Issue published online: 4 MAY 2004
- Article first published online: 4 MAY 2004
- Received 24 October 2003; revised 28 January 2004; accepted 5 February 2004.
- plant cell cycle;
- protein stability;
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.