Actin Age Orchestrates Myosin-5 and Myosin-6 Run Lengths

Abstract

Unlike a static and immobile skeleton, the actin cytoskeleton is a highly dynamic network of filamentous actin (F-actin) polymers that continuously turn over. In addition to generating mechanical forces and sensing mechanical deformation, dynamic F-actin networks serve as cellular tracks for myosin motor traffic. However, much of our mechanistic understanding of processive myosins comes from invitro studies in which motility was studied on pre-assembled and artificially stabilized, static F-actin tracks. In this work, we examine the role of actin dynamics in single-molecule myosin motility using assembling F-actin and two highly processive motors, myosin-5 and myosin-6. These two myosins have distinct functions in the cell and travel in opposite directions along actin filaments [1-3]. Myosin-5 walks toward the barbed ends of F-actin, traveling to sites of actin polymerization at the cell periphery [4]. Myosin-6 walks toward the pointed end of F-actin [5], traveling toward the cell center along older segments of the actin filament. We find that myosin-5 takes 1.3- to 1.5-fold longer runs on ADP•Pi (young) F-actin, whereas myosin-6 takes 1.7- to 3.6-fold longer runs along ADP (old) F-actin. These results suggest that conformational differences between ADP•Pi and ADP F-actin tailor these myosins to walk farther toward their preferred actin filament end. Taken together, these experiments define a new mechanism by which myosin traffic may sort to different F-actin networks depending on filament age.