cyclin box found in retinoblastoma-associated protein (RB) family
The RB family includes retinoblastoma-associated protein (RB), and two retinoblastoma-like proteins, RBL1 and RBL2. RB, also called p105-Rb, pRb, or pp110, is a key regulator of entry into cell division, and also acts as a tumor suppressor. It promotes G0-G1 transition when phosphorylated by CDK3/cyclin-C. It also acts as a transcription repressor of E2F1 target genes. RB is directly involved in heterochromatin formation by maintaining overall chromatin structure. It recruits and targets histone methyltransferases SUV39H1, KMT5B and KMT5C, leading to epigenetic transcriptional repression. RBL1 and RBL2 are also key regulators of entry into cell division. RBL1 and RBL2 recruit and target histone methyltransferases KMT5B and KMT5C, leading to epigenetic transcriptional repression. They control histone H4 'Lys-20' trimethylation and probably act as transcription repressors by recruiting chromatin-modifying enzymes to promoters. They may also act as tumor suppressors. Members of this family contain one cyclin box. The cyclin box is a protein binding domain.