Phenotype of the fission yeast cell cycle regulatory mutant pim1-46 is suppressed by a tobacco cDNA encoding a small, Ran-like GTP-binding protein.

Mutations in which the onset of mitosis is uncoupled from the completion of DNA replication has recently been described. Characterization of these mutants led to the identification of Pim1/Spi1 in fission yeast and RCC1/Ran proteins in mammalian cells. Their Saccharomyces cerevisae homologues, the MTR1/CNR1 proteins, appear to be involved in controlling RNA metabolism and transport. Here the isolation and partial characterization of plant cDNA clones which encode proteins homologous to the mammalian/fission yeast/budding yeast Ran/Spi/CNR proteins are reported. Higher plants appear to contain more than one gene per haploid genome which codes for Ran proteins. These genes are expressed in different plant tissues, including root tips and stems, known to contain mitotically active cells. The tobacco Ran-like proteins, like their mammalian and yeast homologues, are soluble proteins which are found in the cytoplasm and in the nucleus. In addition, it has been shown that overexpression of the tobacco Nt-Ran-A1 cDNA suppressed the phenotype of the temperature-sensitive fission yeast pim1-46 mutant. These results suggest that the plant Ran genes can be functionally equivalent to the mammalian/fission yeast/budding yeast Ran/Spi/CNR genes and that they may play a role: (i) in maintaining a coordinated cell cycle; (ii) in controlling RNA metabolism and transport in higher plants; and/or (iii) in protein import into the nucleus.