A novel actin binding site of myosin required for effective muscle contraction

Abstract

F-actin serves as a track for myosin's motor functions and activates its ATPase activity by several orders of magnitude, enabling actomyosin to produce effective force against load. Although actin activation is a ubiquitous property of all myosin isoforms, the molecular mechanism and physiological role of this activation are unclear. Here we describe a conserved actin-binding region of myosin named the 'activation loop', which interacts with the N-terminal segment of actin. We demonstrate by biochemical, biophysical and in vivo approaches using transgenic Caenorhabditis elegans strains that the interaction between the activation loop and actin accelerates the movement of the relay, stimulating myosin's ATPase activity. This interaction results in efficient force generation, but it is not essential for the unloaded motility. We conclude that the binding of actin to myosin's activation loop specifically increases the ratio of mechanically productive to futile myosin heads, leading to efficient muscle contraction.

Conformationally Trapping the Actin-binding Cleft of Myosin with a Bifunctional Spin Label

“…There is evidence for coupling between the actin-binding cleft and nucleotide pocket (63)(64)(65), as well as between the nucleotide pocket and the force-generating domain (66,67). Recent work suggests that there is direct coupling between the actin-binding interface, particularly the myosin activation loop, and the relay helix in the myosin force-generating domain (68). We must next ask how conformationally trapping the actin-binding cleft affects the nucleotide-binding pocket structural dynamics, and the orientation of the myosin lever arm.…”

Functional Characterization of Human Myosin-18A and Its Interaction with F-actin and GOLPH3

“…8A) (37,38). In human myosin-18A, this loop is elongated by 13 extra amino acids that introduce additional positive charges (three arginine residues instead of one in D. discoideum myosin-2), which interferes with regular activation by this loop (cf.…”

Mammalian Myosin-18A, a Highly Divergent Myosin

“…Mutation of this residue dramatically reduces the actin-activated ATPase of D. discoideum myosin-2 while not affecting its motility or ADP release. It also appears to reduce the forcegenerating capacity of the myosin by reducing the duty ratio (59). This loop is longer in myosin-18A than in myosin-2 and lacks the conserved arginine (Fig.…”

“…Loop-2, an important surface loop that is involved in forming the weak binding interactions with actin, is longer in myosin-18A than in D. discoideum myosin-2. There are also alterations in the recently described activation loop (59), which contains a conserved arginine residue (Arg 520 in D. discoideum myosin-2) shown to directly interact with the negatively charged N-terminal region of actin. Mutation of this residue dramatically reduces the actin-activated ATPase of D. discoideum myosin-2 while not affecting its motility or ADP release.…”