A mutant form of the Ran/TC4 protein disrupts nuclear function in Xenopus laevis egg extracts by inhibiting the RCC1 protein, a regulator of chromosome condensation.
The Ran protein is a small GTPase that has been implicated in a large number of nuclear processes including transport. RNA processing and cell cycle checkpoint control. A similar spectrum of nuclear activities has been shown to require RCC1, the guanine nucleotide exchange factor (GEF) for Ran. We have used the Xenopus laevis egg extract system and in vitro assays of purified proteins to examine how Ran or RCC1 could be involved in these numerous processes. In these studies, we employed mutant Ran proteins to perturb nuclear assembly and function. The addition of a bacterially expressed mutant form of Ran (T24N-Ran), which was predicted to be primarily in the GDP-bound state, profoundly disrupted nuclear assembly and DNA replication in extracts. We further examined the molecular mechanism by which T24N-Ran disrupts normal nuclear activity and found that T24N-Ran binds tightly to the RCC1 protein within the extract, resulting in its inactivation as a GEF. The capacity of T24N-Ran-blocked interphase extracts to assemble nuclei from de-membranated sperm chromatin and to replicate their DNA could be restored by supplementing the extract with excess RCC1 and thereby providing excess GEF activity. Conversely, nuclear assembly and DNA replication were both rescued in extracts lacking RCC1 by the addition of high levels of wild-type GTP-bound Ran protein, indicating that RCC1 does not have an essential function beyond its role as a GEF in interphase Xenopus extracts.
Dasso
M
M
Laboratory of Molecular Embryology, NICHD, National Institutes of Health, Bethesda, MD 20892-5430.
Seki
T
T
Azuma
Y
Y
Ohba
T
T
Nishimoto
T
T
D000818:Animals; D018797:Cell Cycle Proteins; D002467:Cell Nucleus / Q000502:physiology*; D002875:Chromosomes / Q000502:physiology; D004261:DNA Replication / Q000235:genetics; D004268:DNA-Binding Proteins / Q000037:antagonists & inhibitors* / Q000502:physiology; D019204:GTP-Binding Proteins / Q000235:genetics*; D020662:Guanine Nucleotide Exchange Factors; D006801:Humans; D009154:Mutation; D009687:Nuclear Proteins / Q000037:antagonists & inhibitors* / Q000235:genetics* / Q000502:physiology; D010063:Ovum / Q000502:physiology / Q000648:ultrastructure; D011485:Protein Binding; D029867:Xenopus Proteins; D014982:Xenopus laevis; D020931:ran GTP-Binding Protein
D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D020662:Guanine Nucleotide Exchange Factors; D009687:Nuclear Proteins; C499304:RCC1 protein, Xenopus; C062521:RCC1 protein, human; D029867:Xenopus Proteins; D019204:GTP-Binding Proteins; D020931:ran GTP-Binding Protein