| RBL1, known as p107, belongs to the retinoblastoma (Rb) family of pocket proteins alongside Rb and RBL2/p130, where it serves as a central regulator of cell cycle progression through transcriptional repression. RBL1 contains a conserved pocket domain flanked by C-terminal and N-terminal regions that facilitate binding to E2F4/5 transcription factors and LXCXE motif-containing proteins, enabling recruitment of repressive complexes to promoter elements. Hypophosphorylated RBL1 anchors the DREAM complex, comprising E2F4/5, DP1, MuvB core (LIN proteins, RBBP4), and CHK2 kinase, to cell cycle gene promoters bearing CHR elements during G0/G1 quiescence, where it enforces silencing of S-phase entry genes like CCNA2, CDC2, and TK1 alongside G2/M regulators such as PLK1 and AURKB through HDAC-dependent chromatin compaction. Cyclin D-CDK4/6 and cyclin E-CDK2 sequentially phosphorylate RBL1 at multiple sites upon mitogenic stimulation, disrupting pocket interactions and releasing E2F transactivators to drive E2F-responsive transcription while permitting MuvB redeployment to B-MYB-FOXM1 for mitotic gene activation. RBL1 further integrates with p53 signaling via direct interaction that stabilizes p21-mediated CDK inhibition, amplifying G1 arrest during DNA damage, and cooperates with BMI1 to modulate Polycomb repressive complex 1 activity at lineage-specific loci during differentiation. RBL1 maintains quiescence in stem cells and enforces senescence-associated gene repression in post-mitotic tissues, with dynamic phosphorylation cycling calibrated by PP1/PP2A phosphatases to tissue-specific demands in liver regeneration and neuronal maturation. Loss of RBL1 elevates E2F activity and deregulates DREAM targets in osteosarcoma and breast carcinoma, promoting unrestrained proliferation through G1/S checkpoint collapse. |
Lane 1: MCF7, Lane 2: HT29, Lane 3: THP1, Lane 4: COS-7
