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 i n vitro reconstitution of an active CDK7-cyclin H complex requires sto ichiometric amounts of a novel 36 kDa assembly factor termed MAT1 (men age a trois 1). Sequencing of MAT1 reveals a putative zinc binding mot if (a C3HC4 RING finger) in the N-terminus; however, this domain is no t required for ternary complex formation with CDK7-cyclin H, MAT1 is a ssociated with nuclear CDK7-cyclin H at all stages of the cell cycle i n vivo, Ternary complexes of CDK7, cyclin H and MAT1 display kinase ac tivity towards substrates mimicking both the T-loop in CDKs and the C- terminal domain of RNA polymerase II, regardless of whether they are i mmunoprecipitated from HeLa cells or reconstituted in a reticulocyte l ysate. MAT1 constitutes the first example of an assembly factor that a ppears to be essential for the formation of an active CDK-cyclin compl ex.