Tubulin heterodimers remain functional for one cell cycle after the inactivation of tubulin-folding cofactor D in fission yeast cells.

ABSTRACT

Tubulin-folding cofactor D plays a major role in the formation of functional tubulin heterodimers, the subunits of microtubules (MTs) that are essential for cell division. Previous work has suggested that, in Schizosaccharomyces pombe, cofactor D function is required during G(1) or S phases of the cell cycle, and when it fails to function due to the temperature-sensitive mutation alp1-t1, cells are unable to segregate their chromosomes in the subsequent mitosis. Here we report that another mutation in the cofactor D gene, alp1-1315, causes failures in either the first or second mitosis in cells synchronized in G(1) or G(2) phases, respectively. Other results, however, suggest that the kinetics of viability loss in these mutants does not depend on progression through the cell cycle. When cofactor D function is perturbed in cells blocked in G(2), cytoplasmic MTs appear normal for 2-3 h but thereafter they disintegrate quickly, so that only a few short MTs remain. These residual MTs are, however, stably maintained, suggesting that they do not require active cofactor D function. The abrupt disassembly of MT cytoskeleton at restrictive temperature in non-cycling cofactor D mutant cells strongly suggests that the life-span of folded tubulin dimers might be downregulated. Indeed, this period is significantly shorter than the previously determined dissociation time of bovine tubulins in vitro. The death of mutant cells occurs inevitably after 2-3 h at restrictive temperature in the following mitosis, and is explained by the idea that MT structures formed in the absence of cofactor D cannot support normal cell division.