Functional sequences of the myosin head

Abstract

Muscle contraction originates from the sliding of myosin filaments on actin filaments, the energy for which is supplied by the hydrolysis of adenosine-5-triphosphate (ATP) by myosin. The nucleotide first binds to the acto-myosin complex in the myosin head (or subfragment-1), producing a conformational change which induces actin dissociation. The release of phosphate (Pi) then allows a return to the strong actin-myosin association, corresponding to the rigor state. We discuss here certain controversial points arising from current concepts of the actin and nucleotide binding regions at the amino acid sequence level within the subfragment-1 heavy chain. We consider the actin and nucleotide binding regions to be two distinct sites (for each of these regions) one of which is shared competitively between actin and the nucleotide. In our model the cyclical actin-S1 association-dissociation steps correspond to different ATP, actin and ADP affinities for the same amino acid sequence of the S1 heavy chain, contributing alternatively to a single hydrolytic nucleotide site or a strong actin site. We propose the existence of a flexible segment that forms or dismantles the nucleotide or actin sites. The large region (amino acids 540-707) overlapping the actin-myosin interface appears to be the main flexible region of the S1 molecule and we propose that this particular sequence plays a key role in the dissociation pathway of the actin-myosin complex and in the conversion of chemical energy into the mechanical energy of contraction.

Enzymatic Characterization and Functional Domain Mapping of Brain Myosin-V

“…Thus, in addition to myosin-II and myosin-I, which have been extensively studied as native proteins, there is now enough biochemical data on BM-V to allow for an initial description of the domain functions and mechanochemical properties of a class V myosin, as well as a comparison with specific class I and class II myosins. Sequence analysis has shown that, although the different classes of myosins have strikingly different tail domains, their head domains are conserved in general structure and organization (1,33,34). Purified myosins, however, exhibit quantitative differences in mechanochemical properties, such as ATPase activity and sliding filament velocity, as well as substantial differences in regulation.…”

“…Purified myosins, however, exhibit quantitative differences in mechanochemical properties, such as ATPase activity and sliding filament velocity, as well as substantial differences in regulation. It is now realized that all myosins contain a distinct neck domain responsible for light chain binding, which can have a regulatory role and perhaps also function as a lever for the generation of movement (33,(35)(36)(37)(38). Because the neck domain of the class V myosins is much larger than the neck domain of the conventional myosins, there is much interest in comparing the mechanochemical properties of these myosins.…”

Effects of sulphydryl modification on skinned rat skeletal muscle fibres using 5,5'‐dithiobis(2‐nitrobenzoic acid).

“…These thiol groups are probably not essential for normal ATPase activity of myosin 426V MYOFIBRILLAR SULPHYDR YL MODIFICATION42 (Okamoto & Sekine, 1987) but the SH1-SH2 region of the myosin head may participate in constructing the functional nucleotide hydrolytic site (Mornet, Bonet, Audemard & Bonicel, 1989). It is known that DTNB promotes cross-linking between SH1 and SH2 which modifies the enzymatic properties of myosin by trapping MgADP -at the active site and abolishing ATPase activity (Wells & Yount, 1982).…”

“…This implies that the sulphydryl groups on myosin in fibres in the rigor state undergo a different modification to that of myosin in the relaxed fibres. As the 8H1-8H2 region on myosin is thought to participate in the construction of the strong actin binding site (Mornet et al 1989) it is likely that cross-linking between SH, and 1SH2 cannot take place when myosin is strongly attached to actin as it is in rigor. The development of a Ca2+-independent MgATPase in the DTNB-rigor-treated fibres suggests that some myosin heads were specifically modified at the SH1 group since it has been recently reported that such specific modification of the SH1 group elevates the actin-activated myosin MgATPase activity in the absence of Ca21 (Titus et al 1989).…”

Myosin II Folding Is Mediated by a Molecular Chaperonin

“…The myosin motor domain has been studied extensively using proteolysis. Limited proteolysis of S1 by trypsin produces three distinct fragments of molecular mass 25, 50, and 20 kDa (38). The molecular basis of this cleavage pattern is clear from the structure of myosin S1 (3).…”