| Phospho-Cyclin B1 (Ser133) represents the active phosphorylated form of Cyclin B1 at serine residue 133, a critical regulatory modification within the cytoplasmic retention signal domain that controls nuclear entry of the CDK1/Cyclin B1 complex during mitosis. Cyclin B1 belongs to the B-type cyclin family and binds CDK1 to form the mitosis-promoting factor, the master regulator driving the G2-to-M phase transition in eukaryotic cells. The protein contains an N-terminal cytoplasmic retention signal domain housing five phosphorylation sites and a C-terminal cyclin box domain that directly engages CDK1. Polo-like kinase 1 phosphorylates Cyclin B1 preferentially at Ser133 during prophase, generating a nuclear import signal that enables rapid translocation of the CDK1/Cyclin B1 complex from the cytoplasm into the nucleus. This nuclear entry represents the critical switch that commits cells to mitosis. The nuclear CDK1/Cyclin B1 complex phosphorylates histone H3, nuclear lamin, and microtubule-associated proteins to trigger chromosome condensation, nuclear envelope breakdown, and spindle assembly. Cyclin B1 accumulates progressively during the G2 phase and undergoes rapid ubiquitin-mediated degradation by the anaphase-promoting complex/Cdc20 at anaphase onset. Mitogen-activated protein kinase and CDK1 itself also phosphorylate Ser116, Ser126, and Ser128 within the cytoplasmic retention signal, working synergistically with Ser133 phosphorylation to ensure complete nuclear accumulation. Polo-like kinase 1 shows preferential specificity for Ser133 over other cytoplasmic retention signal sites. Phosphorylation at Ser133 directly controls nuclear import rate without affecting CDK1 binding affinity or kinase activity. Premature nuclear accumulation of Cyclin B1 correlates with chromosomal instability in malignant cells. Cyclin B1 overexpression occurs frequently in breast carcinoma, prostate adenocarcinoma, and non-small cell lung cancer tissues. |
