Myo2p Function in Secretory Vesicle
Targeting & Cell Polarity

The budding yeast unconventional myosin encoded by the S. cerevisiae MYO2 gene was first identified in a genetic screen for conditional yeast mutants that become large unbudded spheres at the restrictive temperature. It is a typical unconventional myosin V. Two Myo2p heavy chains dimerize to form a myosin with two N-terminal motor domain heads followed by long necks bearing six binding sites each for calmodulin/light chains. An extended coiled-coil central region maintains the dimer, and C-terminal globular tails on each chain link the motor to membrane cargoes.

The phenotype of the original conditional mutation (myo2-66) after shifting to the restrictive temperature revealed that Myo2p is required to properly target cell growth to the bud. Myo2p does this by transporting post-Golgi secretory vesicles along polarized tropomyosin-containing actin cables. The mutation in myo2-66, which is located in the motor head domain, results in failure of the motor to transport either itself or post-Golgi secretory vesicles under restrictive conditions. Conversely, Myo2p containing tail mutations does concentrate at regions where actin cables focus, but without secretory cargo. In yeast, Myo2p also is involved in polarized segregation of the lysosome-like vacuole.

Genetic studies reveal that myo2-66 shows genetic interactions with several conditional mutations affecting the post-Golgi secretory pathway, suggesting that they may also play a role in Myo2p function. In particular, sec2 mutants also fail to polarize secretory vesicles. Current evidence suggests that association of secretory vesicles with Myo2p requires the nucleotide exchange activity of Sec2p to activate the GTPase Sec4p, the Rab associated with post-Golgi secretory vesicles. In addition, genetic and two-hybrid data suggest that Smy1p, a divergent kinesin, associates with Myo2p. Although microtubules are not needed for targeted secretion and loss of SMY1 has no phenotype in an otherwise wild-type strain, Smy1p probably binds to the tail of Myo2p and in some way participates in its function.

Yeast has a second unconventional myosin V, encoded by the non-essential gene MYO4, that is involved in delivery into the bud of mRNA for the Ash1p transcriptional regulator to suppress mating type switching.

Contributed by Tony Bretscher, Cornell University