Expression of mitotic cyclins in higher plants: transcriptional and proteolytic regulation

Abstract

Cyclins control progression through the specific phases of the cell cycle by binding to and activating cyclin-dependent kinases (CDKs), which are key cell cycle regulators. Most cyclins with mitotic functions are known to be expressed in a cell-cycle-dependent manner, which in turn leads to the cell-cycle-stage-specific functions of CDKs. To achieve oscillating cyclin expression during the cell cycle, both transcriptional regulation and post-transcriptional regulation are required. In higher plants, mitotic cyclins of the CYCA1, CYCB1, and CYCB2 classes, which are expressed in the late G2 phase and during mitosis, have a cis-acting element called MSA in common that acts as a phase-specific enhancer element. A subclass of R1R2R3-Myb transcription factors bind to this element and activate transcription in a phase-dependent manner. At the post-transcriptional level, the anaphase-promoting complex/cyclosome (APC/C) E3 ubiquitin ligase plays a central role in the proteolytic regulation of mitotic cyclins. As well as the two conserved APC/C activators CDC20 and CCS52, two plant-specific inhibitors of APC/C, GIGAS CELL1 (GIG1)/OMISSION OF SECOND DIVISION1 (OSD1) and ULTRAVIOLET-B-INSENSITIVE4 (UVI4), have recently been identified. Genetic interactions between R1R2R3-Myb and APC/C inhibitors suggest that the balance between positive (transcription) regulation and negative (protein degradation) regulation may determine the cellular concentrations of mitotic cyclins, which is critical not only for cell division, but also for cell growth that is associated with increase of cellular ploidy.