A role for the microtubule +end protein Bik1 (CLIP170) and the Rho1 GTPase in Snc1 trafficking

The diversity of microtubule functions is dependent on the status of tubulin C-termini. To address the physiological role of the C-terminal aromatic residue of α-tubulin, a tub1-Glu yeast strain expressing an α-tubulin devoid of its C-terminal amino-acid was used to perform a genome-wide-lethality screen. The identified synthetic lethal genes suggested links with endocytosis and related processes. In the tub1-Glu strain, the routing of the v-SNARE Snc1 was strongly impaired, with a loss of its polarized distribution in the bud and Abp1, an actin patch/endocytic marker, developed comet-tails structures. Snc1 trafficking necessitated dynamic microtubules but not dynein and kinesin motors. Interestingly, deletion of the microtubule +end protein Bik1 (CLIP170), which is preferentially recruited to the C-terminal residue of α-tubulin, similarly resulted in Snc1 trafficking defects. Finally, constitutively active Rho1 rescued both Bik1 localization at microtubule +ends in tub1-Glu strain and a correct Snc1 trafficking in a Bik1-dependent manner. Our results provide the first evidence for a role of microtubule +ends in membrane-cargo trafficking in yeast, through Rho1- and Bik1-dependent mechanisms and highlight the importance of α-tubulin last amino-acid in this process.