A myosin motor dictates the localization and movement of an organelle that expels excess water from Dictyostelium, report Jung et al.
To counteract the osmotic stress of an aqueous environment (when it rains, for example), Dictyostelia pump protons into a membranous organelle called the contractile vacuole (CV), generating an osmotic gradient that sucks up extra water from the cytoplasm. The vacuole swells up and fuses with other CVs to form a large bladder that expels the water through brief contact with the plasma membrane. After relieving itself, the bladder rapidly contracts before sending out new CV tubules around the actin-rich cell cortex to begin the cycle again.
Jung et al. found that a myosin called MyoJ localizes to CV membranes, and cells lacking the motor no longer accumulated CVs at their edges. Instead, the membranes clumped together in the center of the cell; video microscopy revealed that CVs traveled out to the cell cortex along microtubules but couldn't stay there in the absence of MyoJ. Cells rescued with a version of MyoJ lacking motor activity could capture CV membranes at the cell periphery, but their bladders were unable to spread out into tubules after discharging water at the cell surface. A truncated version of MyoJ that takes shorter steps along actin filaments was able to drive this process, but the tubules spread out more slowly than they did in the presence of full-length MyoJ.
Doubts have been raised about the true function of myosins like MyoJ, says senior author John Hammer III, but the group's results demonstrate that MyoJ acts as a “point-to-point” organelle motor. The next question is how actin filaments are arranged at the cell cortex to facilitate the MyoJ-driven spreading of CV tubules. BS