ZPR1 was initially identified in the mouse (Galcheva-Gargova et al., 1996
). Human ZPR1 was found to be very similar to the mouse protein (Figure ). Examination of ZPR1 expression in other organisms identified homologues of ZPR1 in the budding yeast S. cerevisiae
and the fission yeast S. pombe
(Figure ). Gene disruption studies demonstrated that ZPR1 is an essential gene in fission yeast (Figure ) and budding yeast (our unpublished results). The lethal phenotype in fission yeast was complemented by expression of yeast or mammalian ZPR1 proteins (Figure ). The identification of ZPR1 as an essential gene that has been conserved during evolution indicates that ZPR1 serves a similar function in many cell types.
The ZPR1 protein is located in the cytoplasm of quiescent mammalian cells. However, in proliferating cells, ZPR1 translocates to the nucleus (GalchevaGargova et al., 1996
). The mechanism that controls the nuclear redistribution of ZPR1 in proliferating cells is not understood. However, it is possible that the interaction of ZPR1 with cell surface receptor tyrosine kinases may contribute to the cytoplasmic sequestration of ZPR1 in quiescent cells (Galcheva-Gargova et al., 1996
). Treatment of mammalian cells with mitogens causes the dissociation of ZPR1 from receptor tyrosine kinases and the translocation of ZPR1 to the nucleus. Whether the binding of ZPR1 to tyrosine kinase receptors in quiescent cells is mechanistically related to the cytoplasmic retention of ZPR1 is unclear. Interestingly, the nuclear redistribution of ZPR1 observed in mammalian cells is also observed in yeast. Thus, ZPR1 is a nuclear protein in proliferating yeast and is a cytoplasmic protein in growth-arrested yeast (our unpublished results). This observation indicates that the growth-associated redistribution of ZPR1 from the cytoplasm to the nucleus is conserved in mammals and yeast. This conservation of function suggests that the mechanism of nuclear redistribution of ZPR1 may also be conserved in yeast and mammals. It is likely that a conserved mechanism would not involve cytoplasmic retention by cell surface receptor tyrosine kinases because such molecules are not expressed in yeast. Clearly, further studies are required to define the mechanism of proliferation-associated nuclear redistribution of ZPR1 and to determine the significance of the interaction of ZPR1 with receptor tyrosine kinases in mammalian cells.
Immunofluorescence analysis demonstrated that ZPR1 is accumulated within a subnuclear compartment, the nucleolus (Figure ). Fractionation experiments demonstrated that ZPR1 was not an integral component of the fibrillar or granular compartments of the nucleolous (Figure ), suggesting that either ZPR1 is peripherally associated with one of these nucleolar compartments or is located in a distinct subregion of the nucleolus. However, the nucleolar location of ZPR1 required RNA, suggesting that nucleolar integrity was required for the accumulation of ZPR1 (Figure ). The movement of ZPR1 from the cytoplasm to the nucleolus in response to proliferative signals indicates that ZPR1 may have a signaling role. However, the nuclear function of ZPR1 has not been established by previous studies (Galcheva-Gargova et al., 1996
The major functions of the nucleolus are thought to be the transcription of rRNA genes, the processing of the pre-rRNA precursor to form mature rRNA, and the assembly of rRNA into preribosomal particles (Shaw and Jordan, 1995
). Because ZPR1 is a nucleolar protein that is essential for cell viability, a plausible role for ZPR1 is to contribute to one step in the process of rRNA expression. Consistent with this hypothesis, we found that the repression of zpr1+
gene expression in fission yeast caused a marked decrease in the level of pre-rRNA (Figure C). This decreased rRNA is likely to account for the growth arrest (Figure A) and decreased protein translation (Figure D) observed after repression of zpr1+
The results of the present study provide direct experimental evidence that the cytoplasmic protein ZPR1 redistributes to the nucleolus in proliferating cells and that ZPR1 is required for normal nucleolar function. Further studies are required to determine the molecular basis for the role of ZPR1 in these processes.