X-ray solution scattering of squid heavy meromyosin: strengthening the evidence for an ancient compact off state.

The overall conformations of regulated myosins or heavy meromyosins from chicken/turkey, scallop, tarantula, limulus, and scorpion sources have been studied by a number of techniques, including electron microscopy, sedimentation, and pulsed electron paramagnetic resonance. These studies have indicated that the binding of regulatory ions changes the conformation of the molecule from a compact shape found in the "off" state of the muscle to extended relationships between the tail and independently mobile heads that predominate in the "on" state. Here we strengthen the argument for the generality of this conformational change by using small angle X-ray scattering on heavy meromyosin from squid.

Purification, crystallization and preliminary X-ray crystallographic analysis of squid heavy meromyosin.

All muscle-based movement is dependent upon carefully choreographed interactions between the two major muscle components, myosin and actin. Regulation of vertebrate smooth and molluscan muscle contraction is myosin based (both are in the myosin II class), and requires the double-headed form of myosin. Removal of Ca2+ from these muscles promotes a relatively compact conformation of the myosin dimer, which inhibits its interaction with actin.

Crystal structure of a phosphorylated light chain domain of scallop smooth-muscle myosin.

We have determined the crystal structure of a phosphorylated smooth-muscle myosin light chain domain (LCD). This reconstituted LCD is of a sea scallop catch muscle myosin with its phosphorylatable regulatory light chain (RLC SmoA). In the crystal structure, Arg(16), an arginine residue that is present in this isoform but not in vertebrate smooth-muscle RLC, stabilizes the phosphorylation site.

Visualizing key hinges and a potential major source of compliance in the lever arm of myosin.

We have determined the 2.3-Å-resolution crystal structure of a myosin light chain domain, corresponding to one type found in sea scallop catch ("smooth") muscle. This structure reveals hinges that may function in the "on" and "off" states of myosin.

Myosin cleft closure determines the energetics of the actomyosin interaction.

Formation of the strong binding interaction between actin and myosin is essential for force generation in muscle and in cytoskeletal motor systems. To clarify the role of the closure of myosin's actin-binding cleft in the actomyosin interaction, we performed rapid kinetic, spectroscopic, and calorimetric experiments and atomic-level energetic calculations on a variety of myosin isoforms for which atomic structures are available. Surprisingly, we found that the endothermic actin-binding profile of vertebrate skeletal muscle myosin subfragment-1 is unique among studied myosins.

The on-off switch in regulated myosins: different triggers but related mechanisms.

In regulated myosin, motor and enzymatic activities are toggled between the on-state and off-state by a switch located on its lever arm domain, here called the regulatory domain (RD). This region consists of a long alpha-helical "heavy chain" stabilized by a "regulatory" light chain (RLC) and an "essential" light chain (ELC). The on-state is activated by phosphorylation of the RLC of vertebrate smooth muscle RD or by direct binding of Ca(2+) to the ELC of molluscan RD.

Rigor-like structures from muscle myosins reveal key mechanical elements in the transduction pathways of this allosteric motor.

Unlike processive cellular motors such as myosin V, whose structure has recently been determined in a "rigor-like" conformation, myosin II from contracting muscle filaments necessarily spends most of its time detached from actin. By using squid and sea scallop sources, however, we have now obtained similar rigor-like atomic structures for muscle myosin heads (S1). The significance of the hallmark closed actin-binding cleft in these crystal structures is supported here by actin/S1-binding studies.

Conformation and dynamics of the SH1-SH2 helix in scallop myosin.

Atomic structures of scallop myosin subfragment 1(S1) with the bound MgADP, MgAMPPNP, and MgADP.BeF(x) provide crystallographic evidence for a destabilization of the helix containing reactive thiols SH1 (Cys703) and SH2 (Cys693). A destabilization of this helix was not observed in previous structures of S1 (from chicken skeletal, Dictyostelium discoideum, and smooth muscle myosins), including complexes for which solution experiments indicated such a destabilization.