The MAK16 gene was first defined as a gene whose mutation resulted in loss of M1 double-stranded RNA virus-like particles. The mak16-1 mutation also produces temperature-sensitive cell growth. We report here that mak16-1 cells arrest at the nonpermissive temperature in G1 phase, such that they are mating competent. We sequenced the MAK16 gene and found an open reading frame of 306 amino acids encoding a predicted protein of Mr 35,694. Two typical nuclear localization signal sequences were found. MAK16-LacZ fusion proteins that include one of these putative signals entered the nucleus, while unfused beta-galactosidase did not, as judged by subcellular fractionation experiments. In the C-terminal third of the MAK16 open reading frame is an acidic region in which 25 of 41 residues are either glutamate or aspartate. This region contains potential phosphorylation sites for "casein kinases," protein kinases specific for serine or threonine residues in an acidic environment.