How partners define Myosin VI function in cells: towards a mechanistic understanding of the cellular roles of Myosin VI – MyoActions

Force generation by molecular motors powers numerous cellular processes. Activated by partners, motors can use this force for distinct mechanistic roles, such as transport, anchoring and track organization. However, the actual action the motor performs upon generating force has not been demonstrated in situ for most of their cellular roles. The design of the domains that follow the motor (lever arm, dimerization region if the motor is dimeric) greatly influences the coordination of heads and the reach of the motor during stepping. However, little is known about how motors control their action with a given design and the tuning of their motility properties. Even less is known about how the attachment of the motor to particular cargoes can contribute to defining the action it performs in cells. MyoActions will focus on Myosin VI (Myo6), a unique and puzzling motor since its mechanical properties allows it to transport, anchor or organize actin filaments. Myo6 produces force in the reverse direction than all other myosins and can be recruited in cells by distinct tail partners (cargoes). It is thus well-suited for delineating the roles cell partners play in specifying the action of molecular motors. Understanding the control of motor activation in cells is essential to gain information about its cellular function. It is critical to design experiments (1) to probe the exact role the motor plays in its own cellular environment, (2) to study how recruitment is regulated in space and time and (3) how partners dictate the role of the motor, in particular when it experiences reverse force (load).
We propose a multi-scale analysis (from structural biology, development of specific inhibitors/modulators of force generation, loaded motility assays and cell biology) to provide critical insights into how motors anchor and organize their tracks and how their partners specify and control these actions. A chemical/genetic approach will be developed with Partner 2 to identify small molecules able to influence the motor properties to generate selective anchors, or Myo6 motors unable to anchor. Taking advantage of the preliminary data recently obtained by Partner 3, we have designed a line of study to gain mechanistic insights into the role. Myo6 plays in the fission of tubular intermediates from melanosomes, a process that is required for their maturation and function. Myosins have been implicated in membrane fission in many different contexts, thus, the proposed studies on Myo6 will provide fundamental new insights into how motors act on membranes. The MyoActions proposal will result in a mechanistic understanding of the Myo6 cellular roles by innovative approaches not previously explored for studying molecular motors. The results will have direct implications for understanding pathologies associated with this motor. Myo6 dysfunction has been associated with deafness due to the degeneration of stereocilia in the inner ear. Similarly, mutation of its adaptors, such as OPTN and GIPC also result in disease pathologies. Finally, Myo6 has also been shown to contribute to cancer progression and may serve as a potential therapeutic target.