PCNA protein associates to Cdk-A type protein kinases in germinating maize

In higher eukaryotes, the proliferating cell nuclear antigen (PCNA) can be found associated to Cyclin D and Cdk4/6, the kinase complex responsible for cell cycle commitment in response to growth and mitogenic signals. During maize germination, PCNA can be found in protein complexes between 131 and 163 kDa. The sizes of PCNA protein complexes seem to change during germination, so that by the time the S phase starts, a complex of 100 kDa (likely the homotrimeric ring) is the predominant one. PCNA complexes during early germination contain (any of) two PSTAIRE-containing protein kinases of 32 and 36 kDa that readily phosphorylate both histone H1 and maize retinoblastoma-related (RBR) proteins. Kinase activity in PCNA complexes is markedly inhibited by roscovitine and olomoucine, two known Cdk inhibitors. The protein p13(Suc1) only pulls down the 36 kDa PSTAIRE protein. Kinase activity in PCNA immunoprecipitates is maximal during early germination, before the onset of the S-phase, whereas kinase activity associated to pl3(Suc1) reaches a peak later, after the onset of the S-phase. We discuss the physiological repercussions of these findings.