Complex Regulation of RETINOBLASTOMA-RELATED's Interactions With E2Fs via Phosphorylation

Area of Science:

• Plant molecular biology
• Cell cycle regulation
• Epigenetics

Background:

• Arabidopsis RETINOBLASTOMA-RELATED (RBR) protein controls cell proliferation via E2F and DREAM complexes.
• The exact phosphorylation events that inhibit RBR's cell cycle function are not fully understood.

Purpose of the Study:

• To investigate the precise phosphorylation events that regulate RBR's interaction with E2F/DREAM components and its cell cycle function.
• To elucidate the role of specific phosphorylation sites, particularly 911S, in RBR's regulatory mechanisms.

Main Methods:

• Phosphorylation analysis of RBR in Arabidopsis.
• Co-purification assays to study RBR interactions with E2F, DREAM, and RNA-binding proteins.
• Molecular modeling to assess site accessibility for phosphorylation.

Main Results:

• RBR is phosphorylated at 13 of 16 CDK sites in Arabidopsis; many forms retain E2F binding.
• Multi-phosphorylated RBR, especially at the 911S site, loses E2F/DREAM association and binds RNA-binding proteins.
• 911S phosphorylation is high in proliferating cells, decreases under DNA damage, and is modeled as inaccessible when RBR is bound to E2Fs.

Conclusions:

• Different phosphorylation patterns differentially regulate RBR's function.
• Multi-site phosphorylation at 911S inhibits RBR's interaction with E2F/DREAM, promoting a switch to post-transcriptional regulation.
• The 911S phosphorylation is crucial for transitioning cells from proliferation to quiescence under stress.