In vitro assembly of a functional human CDK7‐cyclin H complex requires MAT1, a novel 36 kDa RING finger protein

It is proposed that the CDK7‐cyclin H complex functions in cell cycle progression, basal transcription and DNA repair. Here we report that in vitro reconstitution of an active CDK7‐cyclin H complex requires stoichiometric amounts of a novel 36 kDa assembly factor termed MAT1 (ménage à trois 1). Sequencing of MAT1 reveals a putative zinc binding motif (a C3HC4 RING finger) in the N‐terminus; however, this domain is not required for ternary complex formation with CDK7‐cyclin H. MAT1 is associated with nuclear CDK7‐cyclin H at all stages of the cell cycle in vivo. Ternary complexes of CDK7, cyclin H and MAT1 display kinase activity towards substrates mimicking both the T‐loop in CDKs and the C‐terminal domain of RNA polymerase II, regardless of whether they are immunoprecipitated from HeLa cells or reconstituted in a reticulocyte lysate. MAT1 constitutes the first example of an assembly factor that appears to be essential for the formation of an active CDK‐cyclin complex.