Characterization of a fission yeast SUMO-1 homologue, Pmt3p, required for multiple nuclear events, including the control of telomere length and chromosome segregation

Unlike ubiquitin, the ubiquitin-like protein modifier SUMO-1 and its buddin g yeast homologue Smt3p have been shown to be more important for posttransl ational protein modification than for protein degradation, sere we describe the identification of the SUMO-1 homologue of fission yeast, which we show to be required for a number of nuclear events including the control of tel omere length and chromosome segregation. A disruption of the pmt3(+) gene, the Schizosaccharomyces pombe homologue of SMT3, was not lethal, but mutant cells carrying the disrupted gene grew more slowly. The pmt3 Delta cells s howed various phenotypes such as aberrant mitosis, sensitivity to various r eagents, and high-frequency loss of minichromosomes. Interestingly, we foun d that pmt3(+) is required for telomere length maintenance. Loss of Pmt3p f unction caused a striking Increase in telomere length. When Pmt3p synthesis was restored, the telomeres became gradually shorter. This is the first de monstration of involvement of one of the Smt3p/SUMO-1 family proteins in te lomere length maintenance. Fusion of Pmt3p to green fluorescent protein (GF P) showed that Pmt3p was predominantly localized as intense spots in the nu cleus. One of the spots was shown to correspond to the spindle pole body (S PB). During prometaphase- and metaphase, the bright GFP signals at the SPB disappeared. These observations suggest that Pmt3p is required for kinetoch ore and/or SPB functions involved in chromosome segregation. The multiple f unctions of Pmt3p described here suggest that several nuclear proteins are regulated by Pmt3p conjugation.