Studies on the intracellular localization of hHR23B

Yeast Rad23, originally identified as a DNA repair protein, has been proposed to participate in other cellular functions, i.e., the proteasome-degradation pathway, the process of spindle pole body duplication and as a component of the anaphase checkpoint. Two human homologs of yeast Rad23, hHR23A and hHR23B, exhibit high sequence homology with yRad23 and also have been shown to be involved in DNA repair and proteasome-dependent degradation. Previous studies on the intracellular localization of hHR23A and hHR23B revealed their predominant localization in the nucleus during interphase and in the cytoplasm during mitosis. We have analyzed the localization of hHR23B during all the phases of the cell cycle using immunofluorescence. Unlike previous studies, our results suggest localization of hHR23B in the nucleus as well as in the cytoplasm during G1 phase. The nuclear levels of hHR23B decrease during S-phase of the cell cycle. When the cell enters mitosis, hHR23B relocalizes in the cytoplasm without association with chromatin. These results indicate that the intracellular distribution hHR23B is cell cycle dependent.