A gene, ryh1, of the fission yeast Schizosaccharomyces pombe encoding a GTP-binding protein of 201 amino acids and belonging to the ras superfamily was isolated using the protein-coding region of the cloned Saccharomyces cerevisiae YPT1 gene as hybridization probe. The ryh1 gene is interrupted by three introns. ryh1 null mutants are viable but unable to grow at temperatures greater than 35.5 degrees C. Invertase of ryh1- cells is properly secreted but has a faster electrophoretic mobility compared to that of wild-type cells. The temperature-sensitive phenotype of ryh1 null mutants is complemented by the human rab6 cDNA expressed either under transcriptional control of the S.pombe adh or the SV40 early promoter.