γ-Tubulin and the C-terminal motor domain kinesin-like protein, KLPA, function in the establishment of spindle bipolarity in Aspergillus nidulans

Abstract

Previous research has found that a γ-tubulin mutation inSchizosaccharomyces pombe is synthetically lethal with a deletion of the C-terminal motor domain kinesin-like protein genepkl1, but the lethality of the double mutant prevents a phenotypic analysis of the synthetic interaction. We have investigated interactions between klpA1, a deletion of an Aspergillus nidulans homolog of pkl1, and mutations in the mipA, γ-tubulin gene. We find that klpA1 dramatically increases the cold sensitivity and slightly reduces the growth rate at all temperatures, of threemipA alleles. In synchronized cells we find thatklpA1 causes a substantial but transient inhibition of the establishment of spindle bipolarity. At a restrictive temperature,mipAD123 causes a slight, transient inhibition of spindle bipolarity and a more significant inhibition of anaphase A. In the mipAD123/klpA1 strain, formation of bipolar spindles is more strongly inhibited than in theklpA1 single mutant and many spindles apparently never become bipolar. These results indicate, surprisingly, that γ-tubulin and the klpA kinesin have overlapping roles in the establishment of spindle bipolarity. We propose a model to account for these data.