Nuclear binding of cell cycle-related proteins: Cyclin A versus proliferating cell nuclear antigen (PCNA).

We have investigated the cell cycle-dependent nuclear binding of cyclin A and of the proliferating cell nuclear antigen (PCNA) in asynchronously growing human fibroblasts. To this purpose, we have applied flow cytometry immunofluorescence, a powerful technique for elucidating the cell cycle phase during which the nuclear binding occurs. We have observed that, in striking contrast with the distribution of nuclear-bound PCNA which is restricted to S phase, the immunofluorescence signal of the nuclear-bound form of cyclin A is high in the G1 and G2 phases of the cell cycle. These results suggest the involvement of nuclear-bound cyclin A in the G1/S and G2/M phase transitions.