| Phospho-PLK1 (Thr210) denotes the activated form of polo-like kinase 1 in which a threonine residue within the activation loop of the N‑terminal catalytic domain is phosphorylated, converting PLK1 from an autoinhibited state into a mitotic master kinase that coordinates multiple G2/M and mitotic processes including CDK1 activation, centrosome maturation, bipolar spindle assembly, chromosome segregation, and cytokinesis. PLK1 belongs to the CDC5/Polo subfamily of serine/threonine kinases and is organized into an N‑terminal kinase domain followed by a C‑terminal polo‑box domain that recognizes pre‑phosphorylated docking motifs on substrates and scaffolds, thereby conferring spatial precision and substrate selectivity once Thr210 phosphorylation has engaged the activation loop and relieved intramolecular inhibition. Aurora A kinase in complex with the cofactor Bora phosphorylates Thr210 at the G2/M transition through a CDK1-dependent switch, and this T‑loop phosphorylation is the principal event that generates catalytically active PLK1 at centrosomes and on mitotic structures, while dephosphorylation at this site returns PLK1 to a low-activity state. Active, Thr210‑phosphorylated PLK1 phosphorylates CDC25C and cyclin B1 to promote their nuclear accumulation and activation of CDK1, phosphorylates and inactivates inhibitory kinases such as PKMYT1/Myt1 and Wee1, and modifies components of the anaphase-promoting complex/cyclosome including Apc1, CDC16, and CDC27, thereby linking CDK1 activation, checkpoint satisfaction, and APC/C-dependent proteolysis into a coherent mitotic entry and exit program. PLK1 also phosphorylates a broad set of substrates at centrosomes, kinetochores, central spindle, and midbody, including NEDD1, KIZ, NINL, CEP55, PRC1, RACGAP1, BUB1B, SGOL1, STAG2, and CEP55, supporting centrosome maturation, attachment error correction, sister chromatid cohesion dynamics, spindle midzone organization, and completion of cytokinesis, and phospho‑Thr210 PLK1 shows characteristic enrichment at these structures during defined mitotic stages. Additional regulation of PLK1 activity superimposes on Thr210 phosphorylation through sites such as Ser99, which is phosphorylated downstream of PI3K–Akt to create a 14‑3‑3γ docking interface that further elevates PLK1 catalytic activity and is required for the timely metaphase–anaphase transition, indicating that Thr210 phosphorylation establishes a core active kinase state that can be modulated by parallel signaling inputs. Thr210 phospho‑status is sensitive to DNA damage signaling, with checkpoint activation suppressing T‑loop phosphorylation and PLK1 activity to prevent mitotic entry, whereas phospho‑mimetic Thr210 variants maintain kinase activity despite damage, highlighting phospho‑PLK1 (Thr210) as a direct indicator of checkpoint override or unscheduled mitosis in transformed cells. PLK1 expression and Thr210 phosphorylation levels are elevated in many solid tumors and hematologic malignancies and correlate with a high proliferative index and adverse prognosis. |
Lane 1: Hela, Lane 2: Hela (thymidine, 2mM, 16 h; nocodazole, 10nM, 24 h), Lane 3: Hela (thymidine, 2mM, 16 h; nocodazole, 10nM, 24 h; phosphatase treated)
