A small nuclear GTP-binding protein from tomato suppresses a Schizosaccharomyces pombe cell-cycle mutant.

Ran is a 25-kDa Ras-related nuclear GTP-binding protein which is very highly conserved in humans, Saccharomyces cerevisiae, and Schizosaccharomyces pombe. Ran has been found to form a stable, noncovalent complex with the chromatin-associated protein RCC1, a negative regulator of mitosis. In Sch. pombe, a temperature-sensitive mutation in the RCC1 homolog encoded by the pim1 gene causes premature induction of mitosis, and this mutation can be suppressed by overexpression of the Ran homolog encoded by spi1. We report here the cloning of three Ran cDNAs from tomato. The Ran protein is very highly conserved among plants, animals, and fungi. In tomato, Ran mRNA is expressed in all tissues examined, even those with little or no cell division, indicating that Ran in plants may have functions other than just control of mitosis. We have found that the tomato Ran protein can direct a beta-glucuronidase reporter protein to the plant cell nucleus, confirming that Ran is a nuclear protein in plants. We show that the tomato Ran protein can suppress the Sch. pombe pim1 mutation, indicating that the tomato Ran protein and the Sch. pombe spi1 protein are functionally homologous.