Polarized distribution of intracellular components by class V myosins in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae has three classes of myosins corresponding to three actin structures: class I myosin for endocytic actin structure, actin patches; class II myosin for contraction of the actomyosin contractile ring around the bud neck; and class V myosin for transport along a cable-like actin structure (actin cables), extending toward the growing cortex. Myo2p and Myo4p constitute respective class V myosins as the heavy chain and, like class V myosins in other organisms, function as actin-based motors for polarized distribution of organelles and intracellular molecules. Proper distribution of organelles is essential for autonomously replicating organelles that cannot be reproduced de novo, and is also quite important for other organelles to ensure their efficient segregation and proper positioning, even though they can be newly synthesized, such as those derived from endoplasmic reticulum. In the budding yeast, microtubule-based motors play limited roles in the distribution. Instead, the actin-based motor myosins, especially Myo2p, play a major role. Studies on Myo2p have revealed a wide variety of Myo2p cargo and Myo2p-interacting proteins and have established that Myo2p interacts with cargo and transfers it along actin cables. Moreover, recent findings suggest that Myo2p has another way to distribute cargo in that Myo2p conveys the attaching cargo along the actin track. Thus, the myosin have "dual paths" for distribution of a cargo. This dual path mechanism is proposed in the last section of this review.