Abstract
Recent studies in Xenopus egg extracts indicate that the small G protein Ran has a central role in spindle assembly and nuclear envelope reformation. We determined Ran localization and dynamics in cells during M phase. By immunofluorescence, Ran is accumulated on the chromosomes of meiosis-II-arrested Xenopus eggs. In living cells, fluorescently labeled Ran associated with the chromosomes in Xenopus and remained associated during anaphase when eggs were artificially activated. Fluorescent Ran associated with chromosomes in mouse eggs, during meiotic maturation and early embryonic divisions in starfish, and to a lesser degree during mitosis of a cultured mammalian cell line. Chromosomal Ran undergoes constant flux. From photobleach experiments in immature starfish oocytes, chromosomal Ran has a k(off) of approximately 0.06 second(-1), and binding analysis suggests that there is a single major site. The chromosomal interactions may serve to keep Ran-GTP in the vicinity of the chromosomes for spindle assembly and nuclear envelope reformation.
Highlights
The function of the small GTPase Ran was first studied in mutants, where its absence led to many defects such as in DNA replication, nuclear envelope structure, chromatin structure and cell cycle progression
Recent studies in Xenopus egg extracts indicate that the small G protein Ran has a central role in spindle assembly and nuclear envelope reformation
Ran is accumulated on the chromosomes of meiosis-II-arrested Xenopus eggs
Summary
The function of the small GTPase Ran was first studied in mutants, where its absence led to many defects such as in DNA replication, nuclear envelope structure, chromatin structure and cell cycle progression (reviewed in Sazer, 1996; Moore, 2001). Mattaj and Englmeier, 1998; Gorlich and Kutay, 1999) Work with another cellfree system has implicated Ran in M phase processes; this work has been extensively reviewed The M phase studies have made use of the cell-free extract system from Xenopus eggs, where addition of sperm chromatin or DNA coated beads induces the formation of a bipolar microtubule spindle centered on the added DNA. There remain unresolved issues about Ran’s role in M phase; for instance, verification in living cells of the roles for Ran identified in extracts is required, and has begun to be addressed (Bamba et al, 2002), and to what degree Ran is involved in all eucaryotic cell meioses and mitoses needs to be resolved
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