Structure‐function studies on Acanthamoeba myosins IA, IB, and II

Abstract

Myosins IA and IB are globular proteins with only a single, short (for myosins) heavy chain (140,000 and 125,000 daltons for IA and IB, respectively) and are unable to form bipolar filaments. The amino acid sequence of IB heavy chain shows 55% similarity to muscle myosins in the N-terminal 670 residues, which contain the active sites, and a unique 500-residue C-terminus highly enriched in proline, glycine, and alanine. The C-terminal region contains a second actin-binding site which allows myosins IA and IB to cross-link actin filaments and support contractile activity. Myosins IA and IB are regulated solely by phosphorylation of one serine on the heavy chain positioned between the catalytic site and the actin-binding site that activates ATPase. Myosin II is a more conventional myosin in composition (two heavy chains and two pairs of light chains), heavy chain sequence (globular head 45% identical to muscle myosins and a coiled-coil helical tail), and structure (bipolar filaments). The tail of myosin II is much shorter than that of other conventional myosins, and it contains a 25 amino acid sequence in which helical structure is predicted to be weak or absent. The position of this sequence corresponds to the position of a bend in the monomer. Myosin II heavy chains also have a 29-residue nonhelical tailpiece which contains three regulatory, phosphorylatable serines. Phosphorylation at the tip of the tail regulates ATPase activity in the globular head apparently through an effect on filament structure.

The heavy chain of Acanthamoeba myosin IB is a fusion of myosin-like and non-myosin-like sequences.

“…These results indicate conclusively that myosin IB has no coiled-coil rod-like tail, which is in agreement with its monomeric nature and inability to self-assemble into filaments. These results support the conclusion, drawn from previous study of the protein (1), that filamentous myosin is not required for force generation. This conclusion is consistent with a number of recent findings (reviewed in ref.…”

“…The myosin I molecule has been shown to contain two actin binding sites (1 (Fig. 4) proline-, and alanine-rich carboxyl-terminal domain (Fig.…”

“…1). The only well-developed model of actomyosin-dependent contractile and motile activities is the sliding filament model, which depends specifically on the ability of myosins to selfassemble into bipolar filaments (2).…”

The structure and function of unconventional myosins: a review

“…Acanthamoeba myosins I have also been localized using monoclonal antibodies to the leading edge, the contractile vacuole, sites of cell:cell contacL and filopodia (Yonemura & Pollard, 1992). The known and inferred biochemical properties of these myosins (Korn et al, 1988), together with their intracellular localizations, suggest that they should play important roles in the dynamics of the cortical actin meshwork and the movement of membranes (plasma membranes and intracellular membranes) relative to actin. Creation of cells that lack these myosins should effect, therefore, a wide range of cellular motile functions, from cell migration to intracellular vesicle traffic.…”

Regulatory light chain mutations associated with cardiomyopathy affect myosin mechanics and kinetics

“…Furthermore, this neck region has been proposed to serve as the compliant element in the myosin crossbridge with the RLC playing a structural role, modulating the stiffness of the lever arm [8]. The RLC also contains a highly conserved phosphorylatable serine [9–12] that plays an important role in the activation and modulation of myosins (Reviewed in [13, 14]).…”