The first motor for AP-1B–dependent polarized sorting in epithelial cells is identified by Au et al. on page 103. An isoform of myosin VI, the group shows, helps set basolateral proteins apart from the rest.
Myosin VI works in a complex containing optineurin and Rab8, according to the new results. Rab8 is important for the sorting of basolateral proteins from the Golgi, which the authors now find also relies on a myosin VI isoform. Deletion of the shortest of four myosin VI isoforms sent basolateral proteins that are sorted by AP-1B to the apical membrane instead.
Polarized transport is generally associated with microtubules and their motors, whereas myosins run along actin tracks. The authors suspect that myosin VI, which was concentrated with AP-1B around endosomes, is important during short range movements near these and other organelles, where microtubules are scarce and actin dominates.
Within the myosin family, only myosin VI moves toward actin's minus ends. Minus ends point away from the plasma membrane and from phagosomes, but whether they also point away from the Golgi or endosomes is unknown. If actin is in the right orientation, myosin VI might help create basolateral transport vesicles by pulling membrane away from these organelles.