Progression through the cell cycle requires activation of a series of enzymes designated cyclin dependent kinases (Cdks). The monomeric catalytic subunit, Cdk2, a critical enzyme for initiation of cell cycle progression, is completely inactive. Partial activation is achieved by the binding of regulatory cyclins such as cyclin D1, while full activation requires phosphorylation at Thr 160. The enzyme responsible for phosphorylation of Thr 160 in Cdk2 and also Thr 161 in Cdc2 p34, designated Cdk-activating kinase (CAK), has been partially purified and shown to be comprised of a catalytic subunit, a regulatory subunit and a subunit of unknown function. The regulatory subunit is a novel cyclin (cyclin H) and is required for activation of Cdk7. This previously undescribed protein, now termed Mat1, has been cloned as a protein that associates with the cyclin H-Cdk7 nuclear complex at all stages of the cell cycle. Cyclin H-Cdk7-Mat1 complexes display kinase activity towards Cdk activation domains, and the carboxy terminus of RNA polymerase II. Mat1 appears to constitute the first example of an assembly factor, essential for the formation of an active Cdk-cyclin complex.
CAP35; CDK-activating kinase assembly factor MAT1; CDK7/cyclin-H assembly factor; cyclin G1 interacting protein; Cyclin-G1-interacting protein; MAT1; Menage a trois; menage a trois 1 (CAK assembly factor); menage a trois homolog 1, cyclin H assembly factor; menage a trois-like protein 1 cyclin H assembly factor; MNAT CDK-activating kinase assembly factor 1; MNAT1; p35; p36; RING finger protein 66; RING finger protein MAT1; RNF66