DOC_CYCLIN_D_Helix_1

Cyclins are major regulators of cell cycle progression that act by regulating cyclin dependent kinase (Cdk) activity. Several cyclins are able to target specific substrates for phosphorylation by different cyclin/Cdk complexes, yet the molecular determinants underlying this specificity are still poorly understood in mammalian cyclins. Cyclins from budding yeast and animals harbour a highly conserved surface patch called the hydrophobic pocket (hp), which docks RxL sequences present in substrates and inhibitors (DOC_CYCLIN_RxL_1) (Lowe,2002; Cheng,2006). In budding yeast, the evolution of cyclin specificity led to the appearance of distinct sets of RxL-like motifs that target the hp and drive the phosphorylation of G2-specific targets by Clb3/4 (DOC_CYCLIN_yClb3_PxF_3), mitotic targets by Clb1/2 (DOC_CYCLIN_yClb1_LxF_4) (Ord,2019; Ord,2019) and S phase targets by Clb 5/6 (DOC_CYCLIN_yClb5_NLxxxL_5; Faustova,2021). The phosphorylation of G1 targets by Cln2 in yeast is mediated by a leucine- and proline-rich (LP) docking motif (DOC_CYCLIN_yCln2_LP_2) that resembles the core hydrophobic sequence of RxL-like motifs but whose binding site has not been conclusively determined (Koivomagi,2011; Bhaduri,2011). These motifs allow different yeast cyclins to specifically target cell cycle regulators at distinct cell cycle stages, and often the RxL-like motifs act cooperatively with additional docking sites (DOC_CKS1_1) to allow processive target phosphorylation (Koivomagi,2013). Independent evidence from mammalian Cyclin D regulation indicates that a novel helical motif present in metazoan Rb family proteins (DOC_CYCLIN_D_Helix_1) targets a separate and yet unidentified binding surface on Cyclin D proteins, cooperating with the RxL and LxCxE motifs to direct Rb protein family phosphorylation during the G1 phase of the cell cycle (Topacio,2019).

Cyclin D proteins bind specifically to cyclin-dependent kinases 4 and 6 (Cdk4/6) to phosphorylate substrates during the early G1 phase of the cell cycle. The retinoblastoma (Rb) family proteins Rb, p107 and p130, also called pocket proteins, are among the best-studied CyclinD-Cdk4/6 substrates (Giacinti,2006; Burkhart,2008). Pocket proteins bind to the E2F family of transcription factors, repressing their activity and acting as major controllers of the G1/S transition (van den Heuvel,2008). Pocket protein phosphorylation by CyclinD-Cdk4/6 and by CyclnE-Cdk2 in early and late G1 respectively leads to destabilization of the Rb-E2F repressive complex and to the transcription of E2F-regulated genes necessary for entry into S-phase (Dick,2013; Rubin,2013). The phosphorylation of Rb by CyclinD/Cdk4-6 is an important first step in driving the G1/S transition (Topacio,2019).

Deregulation of the CyclinD-Rb-E2F axis, and prominently Cyclin D1 and Rb deregulation are a hallmark of many types of human cancer (Musgrove,2011; Bosco,2007). Increased Cyclin D1 expression leads to a hyper activation of Cdk4/6 promoting Rb phosphorylation, entry into S-phase and the ensuing proliferation of tumor cells. Specific Cdk4/6 inhibitors such as Palbociclib are able to block deregulated proliferation, and combination therapies have emerged as a promising treatment for ER+ breast cancers (Johnson,2016). Because of this, SLiM-mediated interactions in the CyclinD-Rb-E2F axis also emerge as potential drug targets for cancer.