During transport from the cell center to the periphery, organelles are carried long distances along microtubules by kinesin and then locally along actin tracks by myosin Va (myoVa). What do these motors do when confronted with enmeshed cytoskeletal elements, and how do they pass their cargoes onward?
Ali et al. have addressed these questions by watching the movement of single molecules of myoVa (labeled with quantum dots) as they encountered intersecting filaments: either actin or microtubules. At actin-actin intersections, myoVa either stepped over the crossing filament, stopped moving altogether, or turned left or right, with the direction determined by the polarity of the second filament. The ratio of stepping versus turning events correlated with the ratio of binding sites within reach of a flexible myoVa head that samples actin monomers within a target zone defined by its 50- to 95-nm stepping range. Despite a tendency to switch tracks, myoVa has a high probability of reaching the cell periphery because of the strong bias for actin filaments to be oriented with their barbed ends aimed at the plasma membrane. At actin-microtubule intersections, myoVa could not step over the obstructing element (microtubules are significantly larger than actin filaments); however, in a few cases, myoVa turned onto the microtubule and diffused randomly along it, mimicking the search it would undertake for a cargo that was being delivered to the periphery by kinesin. -- VV
Proc. Natl. Acad. Sci. U.S.A. 104, 10.1073/pnas.0611471104 (2007).
