THE JNM1 GENE IN THE YEAST SACCHAROMYCES-CEREVISIAE IS REQUIRED FOR NUCLEAR MIGRATION AND SPINDLE ORIENTATION DURING THE MITOTIC CELL-CYCLE

JNM1, a novel gene on chromosome XIII in the yeast Saccharomyces cerev isiae, is required for proper nuclear migration. jnm1 null mutants hav e a temperature-dependent defect in nuclear migration and an accompany ing alteration in astral microtubules. At 30-degrees-C, a significant proportion of the mitotic spindles is not properly located at the neck between the mother cell and the bud. This defect is more severe at lo w temperature. At 11-degrees-C, 60% of the cells accumulate with large buds, most of which have two DAPI staining regions in the mother cell . Although mitosis is delayed and nuclear migration is defective in jn m1 mutants, we rarely observe more than two nuclei in a cell, nor do w e frequently observe anuclear cells. No loss of viability is observed at 11-degrees-C and cells continue to grow exponentially with increase d doubling time. At low temperature the large budded cells of jnm1 mut ants exhibit extremely long astral microtubules that often wind around the periphery of the cell. jnm1 mutants are not defective in chromoso me segregation during mitosis, as assayed by the rate of chromosome lo ss, or nuclear migration during conjugation, as assayed by the rate of mating and cytoduction. The phenotype of a jnm1 mutant is strikingly similar to that for mutants in the dynein heavy chain gene (Eshel, D., L. A. Urrestarazu, S. Vissers, J.-C. Jauniaux, J. C. van Vliet-Reedij k, R. J. Plants, and I. R. Gibbons. 1993. Proc. Natl. Acad. Sci. USA. 90:11172-11176; Li, Y. Y., E. Yeh, T. Hays, and K. Bloom. 1993. Proc. Natl. Acad. Sci. USA. 90:10096-10100). The JNM1 gene product is predic ted to encode a 44-kD protein containing three coiled coil domains. A JNM1:lacZ gene fusion is able to complement the cold sensitivity and m icrotubule phenotype of a jnm1 deletion strain. This hybrid protein lo calizes to a single spot in the cell, most often near the spindle pole body in unbudded cells and in the bud in large budded cells. Together these results point to a specific role for Jnm1p in spindle migration , possibly as a subunit or accessory protein for yeast dynein.