Cell cycle-dependent variations in the distribution of the nuclear protein cyclin proliferating cell nuclear antigen in cultured cells: subdivision of S phase.

Immunofluorescence analysis of synchronously growing transformed human amnion cells (AMA) using autoantibodies specific for cyclin has revealed dramatic changes in the nuclear distribution of this protein during the S phase of the cell cycle. Cells in G1, G2, and mitosis exhibit weak staining with the antibody, while S-phase cells show variable patterns of staining in terms of both intensity and distribution of the antigen. Early in S phase, cyclin is localized throughout the nucleoplasm with the exception of the nucleoli. A similar, but stronger, staining pattern is observed as the cells progress through the S phase. At a later stage, before maximum DNA synthesis, cyclin redistributes to reveal a punctuated pattern with foci of staining throughout the nucleus. This pattern precedes a major change in the distribution of this protein, which is then detected in the nucleolus. At this stage, DNA synthesis is at or near a maximum. Thereafter, there are further changes in the distribution of this protein, with the pattern becoming punctuated and of decreasing intensity. All these staining patterns have also been detected in asynchronously growing normal human amnion cells (AF type), suggesting that the distribution of this protein is not a consequence of transformation. Analysis of cultured cells from several vertebrate species also revealed similar staining patterns. These results are consistent with the idea that cyclin is a central component of the pathway(s) leading to DNA replication and cell division.