Walking to work: roles for class V myosins as cargo transporters

Vertebrate myosin Va is structurally and kinetically designed to be able to move processively along actin filaments as a single molecule and is thus well suited to be a cargo transporter. The key features are the presence of two motor domains and kinetics with a high duty ratio. Not all class V myosins are structurally or kinetically similar to vertebrate myosin Va. Some do not have high duty ratios and are not two headed. These class V myosins must adopt other strategies to move cargo, such as having multiple motors bound to the cargo. Budding yeast and plants build actin cytoskeletons that are suitable for long range class V myosin-dependent cargo transport. Indeed, the yeast class V myosins Myo2 and Myo4 drive most, if not all, organelle transport in this organism. The organization of actin within the cortex of vertebrate cells is largely anisotropic, so the class V myosin-dependent transport of organelles in these cells, which in most cases will follow the long-range transport of the organelle on microtubules to the cell periphery, is likely to exhibit minimal directional persistence and be very local. Two recent and compelling examples of class V myosin-dependent organelle transport have been identified within the dendritic spines of neurons, one involving recycling endosomes, and the other involving the endoplasmic reticulum. The 'acid test' as to whether a class V myosin actually moves an organelle in vivo is to show that the organelle moves more slowly when the cell expresses a 'slower' version of the class V myosin.