Both negative and positive G1 cell cycle regulators undergo proteasome-dependent degradation during sucrose starvation in Arabidopsis

Abstract

The proteasome pathway regulates many aspects of biological processes in plants, such as plant hormone signaling, light responses, the circadian clock and regulation of cell division. Key cell-cycle regulatory proteins including B-type cyclins, Cdc6, cyclin-dependent kinase inhibitors and E2Fc undergo proteasome-dependent degradation. We used the proteasome inhibitor MG132 to show that proteolysis of Arabidopsis RETINOBLASTOMA-RELATED 1 (AtRBR1) and three E2Fs is mediated by the proteasome pathway during sucrose starvation in Arabidopsis suspension MM2d cells. We found previously that estrogen-inducible RNAi-mediated downregulation of AtRBR1 leads to a higher frequency of arrest in G2 phase, instead of G1-phase arrest in the uninduced control, after sucrose starvation. Degradation of not only negative (AtRBR1 and E2Fc) but also positive (E2Fa and E2Fb) cell cycle regulators after sucrose starvation may be required for arrest in G1 phase, when cells integrate a variety of nutritional, hormonal and developmental signals to decide whether or not to commit to entry into the cell cycle.

Figure 1

Model of G₁-phase arrest after sucrose starvation. (A) Under normal sucrose conditions, RBR represses transcription by binding E2Fs during early G1 phase. However, CDKA/CYCD3 phosphorylates RBR during late G₁ phase, which results in releasing a functional E2F-DP to activate target genes involved in progression into S phase. (B) During sucrose starvation, CYCD3;1 undergoes proteasome-dependent degradation and disappears rapidly within several minutes. However, both AtRBR1 and E2Fs also undergo proteasome-dependent degradation, and this may be required for G₁-phase arrest after prolonged sucrose starvation.