EFFECT OF MICROTUBULE-ASSOCIATED PROTEIN MHP1 ON MICROTUBULE ASSEMBLYAND CELL-CYCLE PROGRESSION IN SACCHAROMYCES-CEREVISIAE

Microtubule-associated proteins (MAPs) promote the assembly of microtu bules from purified tubulin in vitro. In order to establish a model sy stem for the investigation of the role of MAPs in microtubule assembly in vivo, we have generated Saccharomyces cerevisiae strains that perm it the modulation of the expression levels of MHP1 ((M) under bar AP-( H) under bar omologous (P) under bar rotein 1) and of the a and beta-t ubulin genes. Simultaneous overexpression of alpha and beta tubulin re sults in the accumulation of long aberrant microtubules in interphase, a similar phenotype as was observed in cells overexpressing MHP1. We demonstrate that overexpression of MHP1 in asynchronously growing yeas t cultures leads to cell cycle arrest in G2. In cells that overexpress MHP1 and the tubulin genes, a suppression of both the MHP1 and the tu bulin overexpression phenotypes can be observed. Progressive induction of alpha and beta tubulin overexpression and constitutive overexpress ion of MHP1 lead to the formation of long cytoplasmic microtubules mor e frequently than observed in cells overproducing tubulin or Mhp1p ind ividually and the increased microtubule polymerization could be correl ated with the increase of alpha and beta tubulin expression. However, the overexpression of MHP1 did not alter the phenotypes of individual overexpression of alpha or beta-tubulin. These data indicate that Mhp1 p not only stabilizes microtubules but promotes microtubule assembly i n vivo, and suggest that the role of other mammalian MAPs in the promo tion of microtubule assembly could be tested in this yeast system.