Fission yeast ptr1-1 is one of the mRNA transport mutants that accumulate poly(A)(+) RNA in the nuclei at the nonpermissive temperature. We found that the ptr1(+) gene encodes a homolog of Saccharomyces cerevisiae Tom1p, a hect type ubiquitin ligase. In ptr1-1, a conserved amino acid in the hect domain of Ptr1p is mutated. The ptr1(+) gene is essential for growth and its mutation did not affect nuclear protein export. A ptr1-1 rae1-167 double mutant showed a synthetic effect on a growth defect, indicating that Ptr1p functionally interacts with an essential mRNA export factor Rae1p. We also isolated a multi-copy suppressor for ptr1-1 and found that it is the mpd2(+) gene isolated as a multi-copy suppressor of cdc7-PDl (C) 2004 Elsevier Inc. All rights reserved.