Existence of two populations of cyclin/proliferating cell nuclear antigen during the cell cycle: association with DNA replication sites

Pulse-chase experiments have revealed that cyclin, the auxiliary protein of DNA polymerase-delta, is stable during the transition from growth to quiescence in 3T3 cells. Immunoblotting together with immunofluorescence analysis has shown that the amount of cyclin after 24 h of quiescence is 30-40% of that of growing cells and that it presents a nucleoplasmic staining. Immunofluorescence studies show the existence of two populations of cyclin during the S phase, one that is nucleoplasmic as in quiescent cells and is easily extracted by detergent, and another that is associated to specific nuclear structures. By using antibromodeoxyuridine immunofluorescence to detect the sites of DNA synthesis, it was shown that the staining patterns of the replicon clusters and their order of appearance throughout the S phase are identical to those observed for cyclin. Two-dimensional gel analysis of Triton-extracted cells show that 20-30% of cyclin remains associated with the replicon clusters. This population of cyclin could not be released from the nucleus using high-salt extractions. This demonstrates that cyclin is tightly associated to the sites of DNA replication and that it must have a fundamental role in DNA synthesis in eukaryotic cells.