The cofactor-dependent pathways for alpha- and beta-tubulins in microtubule biogenesis are functionally different in fission yeast

The biogenesis of microtubules in the cell comprises a series of complex st eps, including protein-folding reactions catalyzed by chaperonins. In addit ion a group of evolutionarily conserved proteins, called cofactors (A to E) , is required for the production of assembly-competent alpha-/beta-tubulin heterodimers. Using fission yeast, in which alp11(+), alp1(+), and alp21(+) , encoding the homologs for cofactors B, D, and E, respectively, are essent ial for cell viability, we have undertaken the genetic analysis of alp31(+) , the homolog of cofactor A. Gene disruption analysis shows that, unlike th e three genes mentioned above, alp31(+) is dispensable for cell growth and division. Nonetheless, detailed analysis of alp31-deleted cells demonstrate s that Alp31(A) is required for the maintenance of microtubule structures a nd, consequently, the proper control of growth polarity. alp31-deleted cell s show genetic interactions with mutations in beta-tubulin, but not in alph a-tubulin. Budding yeast cofactor A homolog RBL2 is capable of suppressing the polarity defects of alp31-deleted cells. We conclude that the cofactor- dependent biogenesis of microtubules comprises an essential and a nonessent ial pathway, both of which are required for microtubule integrity.