Identification and properties of an atypical catalytic subunit (p34PSK-J3/cdk4) for mammalian D type G1 cyclins.

Murine D type cyclins associate with a catalytic subunit (p34PSK-J3) with properties distinct from known cyclin-dependent kinases (cdks). Mouse p34PSK-J3 shows less than 50% amino acid identity to p34cdc2, p33cdk2, and p36cdk3, lacks a PSTAIRE motif, and does not bind to p13suc1. Cyclin D1-p34PSK-J3 complexes accumulate in macrophages during G1 and decline in S phase, whereas complexes involving cyclins D2 and D3 form in proliferating T cells. Although histone H1 kinase activity is not detected in cyclin D or PSK-J3 immunoprecipitates, cyclin D-p34PSK-J3 complexes assembled in vitro stably bind and phosphorylate the retinoblastoma gene product (pRb) and an Rb-like protein (p107) but do not interact with pRb mutants that are functionally inactive. Thus, p34PSK-J3 is a cyclin D-regulated catalytic subunit that acts as an Rb (but not H1) kinase.