Regulation of the Chlamydomonas cell cycle by a stable, chromatin-associated retinoblastoma tumor suppressor complex

Bradley J.S.C. Olson, Michael Oberholzer, Yubing Li, James M. Zones, Harjivan S. Kohli, Katerina Bisova, Su Chiung Fang, Jill Meisenhelder, Tony Hunter, James G. Umena

Research output: Contribution to journalArticlepeer-review

60 Scopus citations


We examined the cell cycle dynamics of the retinoblastoma (RB) protein complex in the unicellular alga reinhardtii that has single homologs for each subunit-RB, E2F, and DP. We found that Chlamydomonas RB (encoded by MAT3) is a cell cycle-regulated phosphorprotein, that E2F1-DP1 can bind to a consensus E2F site, and that all three proteins interact in vivo to form complex that can be quantitatively immunopurified. Yeast two-hybrid assays revealed the formation of a ternary complex between MAT3, DP1, and E2F1 that requires a C-terminal motif in E2F1 analogous to the RB binding domain of plant and animal E2Fs. We examined the abundance of MAT3/RB and E2F1-DP1 in highly synchronous cultures and found that they are synthesized and remain stably associated throughout the cell cycle with no detectable fraction of free E2F1-DP1. Consistent with their stable association, MAT3/RB and DP1 are constitutively nuclear, and MAT3/ RB does not require DP1-E2F1 for nuclear localization. In the nucleus, MAT3/RB remains bound to chromatin throughout the cell cycle, and its chromatin binding is mediated through E2F1-DP1. Together, our data show that E2F-DP complexes can regulate the cell cycle without dissociation of their RB-related subunit and that other changes may be sufficient to convert RB-E2F-DP from a cell cycle repressor to an activator.

Original languageEnglish
Pages (from-to)3331-3347
Number of pages17
JournalPlant Cell
Issue number10
StatePublished - Oct 2010


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