A mixed-oligonucleotide probe was used to identify four ras-like coding sequences in a human teratocarcinoma cDNA library. Two of these sequences resembled the rho genes, one was closely related to H-, K-, and N-ras, and one shared only the four sequence domains that define the ras gene superfamily. Homologs of the four genes were found in genomic DNA from a variety of mammals and from chicken. The genes were transcriptionally active in a range of human cell types.Mammalian ras genes (8,11,36) encode a family of proteins that show low but significant homology to the Go, subunits of G proteins (18). A number of genes encoding proteins with Mrs of 20,000 to 25,000 that share significant homology with the ras proteins have been isolated. The homology is greatest in four domains that have been shown through both mutagenic (see reference 2 for a review) and X-ray crystallographic (9, 17, 28) studies to be involved in the binding and hydrolysis of guanine nucleotides. Many ras-related proteins also contain a fifth conserved domain at their carboxy termini that, in H-, K-, and N-ras, is required for membrane localization and biological activity (13, 40). These ras-related proteins are found in a variety of eucaryotic organisms and appear to be well conserved over evolutionary time.The ras gene superfamily can be divided into several major groups on the basis of amino acid sequence: (i) the H-ras, K-ras, and N-ras proto-oncogenes (H, K, and N genes); (ii) the ral genes, which share about 50% homology with H-, K-, and N-ras (4, 5); (iii) the rap genes (29, 30) and R-ras (21), which differ significantly from each other but all share about 50 to 55% homology with the ras proteins, including strict conservation of the ras effector domain (amino acids 32 to 40 of H-ras); (iv) the rho genes, a more distantly related group that exhibits only about 35% identity with the ras proteins (23, 24); and (v)

Abstract. Ran/TC4, first identified as a well-conserved gene distantly related to H-RAS, encodes a protein which has recently been shown in yeast and mammalian systems to interact with RCC1, a protein whose function is required for the normal coupling of the completion of DNA synthesis and the initiation of mitosis. Here, we present data indicating that the nuclear localization of Ran/TC4 requires the presence of RCC1. Transient expression of a Ran/TC4 protein with mutations expected to perturb GTP hydrolysis disrupts host cell DNA synthesis. These results suggest that Ran/TC4 and RCC1 are components of a GTPase switch that monitors the progress of DNA synthesis and couples the completion of DNA synthesis to the onset of mitosis. AN/TC4, was initially described as a RAS-related transcript of unknown function. It was identified in a human teratocarcinoma cell line but is abundant in a variety of cultured cell lines, and is of interest because it defines a new, evolutionarily well-conserved branch of the RAS gene superfamily (Drivas et al., 1990(Drivas et al., , 1991a. Recent genetic and biochemical analyses (Matsumoto and Beach, 1991;Bischoff and Ponstingl, 1991a) suggest that Ran/TC4 also plays a key role in the regulation of cell cycle progression in eukaryotes, and that this role depends on its nuclear localization and interaction with the product of a second gene, RCC1, defined by the tsBN2 mutation of BHK cells (Nishimoto et al., 1978;Uchida et al., 1990) and the piml mutation of the fission yeast Schizosaccharomyces pombe (Matsumoto and Beach, 1991). The compound name used here reflects this: a teratocarcinoma-derived eDNA clone that encodes a Ras-related nuclear (Ran) protein.S. pombe piml (premature initiation of mitosis) mutants enter mitosis without completing chromosomal DNA replication. Overexpression of the wild-type allele of a second gene, spil (suppressor ofpiml), suppresses the piml mutant phenotype. The predicted amino acid sequences of spil (yeast) and Ran/TC4 (human) are 80% identical. The fact that spil overexpression cannot rescue null mutants and the existence of a cold-sensitive mutation in spil suggest direct interaction between Piml and Spil proteins (Matsumoto and Beach, 1991).The mammalian homolog of piml is RCC1 (regulator of chromosomal condensation), a gene originally defined by the tsBN2 mutation of BHK cells (Uchida et al., 1990;Nishitani et al., 1991). The wild-type activity of RCC1 is required both to initiate DNA synthesis (Dasso et al., 1992) and to prevent chromosome condensation until the completion of S phase (Uchida et al., 1990;Nishitani et al., 1991;Enoch and Nurse, 1991;Dasso and Newport, 1990). piml is predicted to encode a larger protein (539 aa) than RCC1 (421 aa), but over the region shared by the two proteins, their sequences are 30 % identical and share a sequence motif repeated seven times in each protein (Matsumoto and Beach, 1991).RCC1 protein can bind DNA and is associated with chromatin (Ohtsubo et al., 1989). It is present in Xenopus egg extracts in amounts...

Ran is one of the most abundant and best conserved of the small GTP binding and hydrolyzing proteins of eukaryotes. It is located predominantly in cell nuclei. Ran is a member of the Ras family of GTPases, which includes the Ras and Ras-like proteins that regulate cell growth and division, the Rho and Rac proteins that regulate cytoskeletal organization and the Rab proteins that regulate vesicular sorting. Ran differs most obviously from other members of the Ras family in both its nuclear localization, and its lack of sites required for post-translational lipid modification. Ran is, however, similar to other Ras family members in requiring a specific guanine nucleotide exchange factor (GEF) and a specific GTPase activating protein (GAP) as stimulators of overall GTPase activity. In this review, the multiple cellular functions of Ran are evaluated with respect to its known biochemistry and molecular interactions.