Expression and activity of a Kip-related protein, Zeama;KRP1, during maize germination

Plant KRP proteins are cyclin/cyclin-dependent kinase subunit (Cdk) inhibitors that share a limited homology with mammalian p271(Kip1) proteins, Several KRPs have been reported in maize (Zea mays L.), of which Zeama;KRP1 was studied during maize germination. Expression of the Zeama;KRP1 gene did not vary during the 24h germination period. A homologous antibody raised against the 13 kDa carboxy end of the Zeama;KRP1 polypeptide, a sequence containing the cyclin/Cdk inhibitory domain, indicated the existence of a 22 kDa protein in maize embryonic axes, the amount of which also remained unchanged during germination. Neither abscisic acid nor cytokinins modified the amount of protein. The purified Zeama;KRP1 polypeptide inhibited the kinase activity associated with Zeama;PCNA and Zeama;CycD2;1, and also the kinase activity in p13(Suc1)-pulled-down complexes. However, there were differences in the inhibition pattern during germination. Whereas kinase activity in proliferating cell nuclear antigen (PCNA) or CycD2;1 immunoprecipitates was strongly inhibited mainly during early germination, that in p13(suc1)-pulled-down complexes was mainly inhibited at late times, suggesting that each protein complex is composed of different cyclins and/or Cdks.