In eukaryotic cells, nucleus-cytoplasm exchanges play an important role in genomic regulation. We have analyzed the localization of four nuclear antigens in different growth conditions: two replicative proteins, DNA polymerase a and proliferating cell nuclear antigen (PCNA), and two oncogenic regulatory proteins, c-Myc and c-Fos. A kinetic study of subcellular localization of these proteins has been done. In cultures in which cells were sparse, these proteins were detected in the nucleus. When proliferation was stopped by the high density of culture cells or by serum starvation, these proteins left the nucleus for the cytoplasm with different kinetics. DNA polymerase a is the first protein to leave the nucleus, with the PCNA protein, c-Fos, and c-Myc leaving the nucleus later. In contrast, during serum stimulation c-Fos and c-Myc relocalize into the nucleus before the replicative proteins. We also noticed that in sparse cell cultures, 10%/ of the cells exhibit a perinuclear staining for the DNA polymerase a, PCNA, and c-Myc proteins but not for c-Fos. This peculiar staining was also observed as an initial step to nuclear localization after serum stimulation and in vivo in Xenopus embryos when the G1 phase is reintroduced in the embryonic cell cycle at the mid-blastula stage. We suggest that such staining could reflect specific structures involved in the initiation of the S phase.In eukaryotic cells there is continuous exchange of macromolecules between the nucleoplasm and the cytoplasm, and one possible way to regulate the activity of a nuclear protein is to store it in the cytoplasm and control its access to the nucleus. This provides a means for relaying signals originating from the plasma membrane to the nucleus while also offering the advantage of a rapid response time (7,22,24).Nuclear localization is regulated for several proteins involved in gene expression and development, as well as for proteins involved in the cell cycle (see reference 12 for a review). For example, the activity of the dorsal morphogen is directly regulated by nuclear transport in selected regions of the Drosophila embryo (26,28,34). For the c-Fos protein, nuclear localization is dependent on continuous stimulation of cells by serum factors during cell growth, and in the absence of serum the protein is kept in the cytoplasm (27).In this study, the localization of four nuclear proteins which respond to the stimulation of cell growth was reanalyzed. c-myc and c-fos are two proto-oncogenes which are involved in regulatory events leading to the Gl-to-S transition and which are synthesized shortly after serum stimulation (see references 15, 17, and 25 for recent reviews). DNA polymerase a and proliferating cell nuclear antigen (PCNA), a subunit of DNA polymerase 8, are two proteins involved in the mechanism of eukaryotic DNA replication (see reference 39 for review). They are coordinately expressed with cell proliferation as a secondary event of serum stimulation (18,38). All four proteins have been found in the nuclei of growing cells in c...