Poly(A) polymerase phosphorylation is dependent on novel interactions withcyclins

We have previously shown that poly(A) polymerase (PAP) is negatively regula ted by cyclin B-cdc2 kinase hyperphosphorylation in the M phase of the cell cycle. Here we show that cyclin B-1 binds PAP directly, and we demonstrate further that this interaction is mediated by a stretch of amino acids in P AP with homology to the cyclin recognition motif (CRM), a sequence previous ly shown in several cell cycle regulators to target specifically G(1)-phase -type cyclins. We find that PAP interacts with not only G(1)- but also G(2) -type cyclins via the CRM and is a substrate for phosphorylation by both ty pes of cyclin-cdk pairs. PAP's CRM shows novel, concentration-dependent eff ects when introduced as an 8-mer peptide into binding and kinase assays, Wh ile higher concentrations of PAP's CRM block PAP-cyclin binding and phospho rylation, lower concentrations induce dramatic stimulation of both activiti es. Our data not only support the notion that PAP is directly regulated by cyclin-dependent kinases throughout the cell cycle but also introduce a nov el type of CRM that functionally interacts with both G(1)- and G(2)-type cy clins in an unexpected way.