Schizosaccharomyces pombe represents a genetic model system for studying cell polarity and division in eukaryotes. We report here the identification of Mac1, a novel fission yeast protein that localized predominantly to the cell tips and septum. Sequences corresponding to roughly the first 180 amino acids of Mac1, which exhibited weak homology to the transmembrane domains of the Aspergillus Pall protein [Mol. Microbiol. 30 (1998) 259], were found to specify localization to the cell periphery. The other 574 amino acids of Mac1 localized to the cytoplasm when expressed alone, thus suggesting that the N-terminal part of Mac1 functions as a plasma membrane anchor for the rest of the protein. In pom1 null mutant cells, which never switch from unipolar to bipolar growth but, instead, grow exclusively at the randomly chosen end [Genes Dev. 12 (1998) 1356], Mac1 was, nevertheless, found at both poles, thus suggesting that Mac1 does not specifically localize to the sites of growth. mac1 null mutant cells had no overt phenotype at 22-32 degrees C, but, nevertheless, displayed a marked decrease in viability at 34-36 degrees C, accompanied by severe separation defects. Overexpression of mac1 resulted in similar defects. Our data suggest that a correct dosage of Mac1 is needed for correct cell separation at elevated temperatures of growth.