Catalysis of guanine nucleotide exchange on Ran by the mitotic regulator RCC1

Abstract

THE product of the gene RCC1 (regulator of chromosome condensation) in a BHK cell line is involved in the control of mitotic events¹. Homologous genes have been found in Xenopus², Drosophila³ and yeast^4,5. A human genomic DNA fragment and complementary DNA that complement a temperature-sensitive mutation ofRCC1 in BHK21 cells^6,7 encode a protein of relative molecular mass 45,000 (M_r45K) which is located in the nucleus and binds to chromatin⁸. We have recently isolated a protein from HeLa cells that strongly binds an anti-RCCl antibody and has the same molecular mass, DNA-binding properties, and amino-acid sequence as the 205 residues already identified⁹. HeLa cell RCCl is complexed to a protein of M_r 25K (ref. 9). We have shown¹⁰ that this 25K protein has a sequence homologous to the translated reading frame ofTC4, a cDNA found by screening a human teratocarcinoma cDNA library with oligonucleotides coding for a ras consensus sequence¹¹, and that the protein binds GDP and GTP. We have referred to this protein as the Ran protein (ras-related nuclear protein). In addition to the fraction of Ran protein complexed to RCCl, a 25-fold molar excess of the protein over RCCl was found in the nucleoplasm of HeLa cells. Here we show that RCCl specifically catalyses the exchange of guanine nucleo-tides on the Ran protein but not on the protein c-Ha-ras p21 (p21^ras).