CHANGES IN PROTEASOME LOCALIZATION DURING THE CELL-CYCLE

We have investigated proteasome localization in synchronized cells usi ng polyclonal anti-proteasome antibodies. Proteasomes were localized i n the nucleus and cytoplasm at all phases of the cycle, but changes in localization were observed which explain the different immunofluoresc ence patterns found in asynchronous cells. In the nucleus, the intensi ty of staining in early S phase was low and showed a punctate distribu tion which changed to a more diffuse and intense labeling during S to G(1). In the cytoplasm, proteasomes were concentrated in the perinucle ar region at G(1) and at the start of S phase and gradually moved towa rds the periphery of the cell as the cell cycle progressed to G(2). No cell cycle-dependent changes were detected in the rate of synthesis o r level of proteasomes. An apparent colocalization of proteasomes with elements of the cytoskeleton mainly observed in G(2) was investigated further in PtK2 cells. The overall distribution of proteasomes and ce ll cycle-dependent changes in PtK2 cells were similar to those in L-13 2 cells. Double-label immunofluorescence studies using anti-proteasome and anti cytokeratin (TROMA 1) antibodies showed that proteasomes do colocalize with intermediate filaments of the cytokeratin type, mainly during G(2) In mitosis, proteasomes were found by immunogold electron microscopy to be localized around the chromosomes in both PtK2 and L- 132 cells. Cell cycle-dependent changes in the localization of proteas omes suggest that they may have a regulatory function related to the c ell cycle, for example, in the degradation of proteins which control i ts progression.