Regulation of the actin-activated ATPase of smooth muscle myosin II is known to involve an interaction between the two heads that is controlled by phosphorylation of the regulatory light chain. However, the three-dimensional structure of this inactivated form has been unknown. We have used a lipid monolayer to obtain two-dimensional crystalline arrays of the unphosphorylated inactive form of smooth muscle heavy meromyosin suitable for structural studies by electron cryomicroscopy of unstained, frozenhydrated specimens. The three-dimensional structure reveals an asymmetric interaction between the two myosin heads. The ATPase activity of one head is sterically ''blocked'' because part of its actin-binding interface is positioned onto the converter domain of the second head. ATPase activity of the second head, which can bind actin, appears to be inhibited through stabilization of converter domain movements needed to release phosphate and achieve strong actin binding. When the subfragment 2 domain of heavy meromyosin is oriented as it would be in an actomyosin filament lattice, the position of the heads is very different from that needed to bind actin, suggesting an additional contribution to ATPase inhibition in situ.phosphorylation ͉ 2-D crystalline arrays ͉ myosin regulation ͉ myosin light chains O f the 15 types of myosin in the myosin superfamily, only isoforms of myosin II are capable of forming filaments (1). Myosin II consists of six polypeptide chains, two of which are heavy chains that contain actin-binding, ATP catalysis, and filament forming activities. Two pairs of light chains, an essential light chain (ELC) and a regulatory light chain (RLC), together with part of the heavy chain form a lever arm through which force is transmitted to produce filament sliding (2). Myosin II can be cleaved into several soluble subfragments. Myosin subfragment 1 (S1, the head portion of myosin) contains the ATPase and actin-binding regions of the heavy chain (also called the motor domain) and the light chain lever arm. Subfragment 2 (S2, the N-terminal portion of the myosin rod), which is predicted to have an ␣-helical coiled-coil structure (3), links S1 to the filament backbone and forms the myosin heavy chain dimerization interface. Another soluble subfragment, heavy meromyosin (HMM), consists of the two S1 heads and S2. Myosin IIs are found in all eukaryotic cells but are most prevalent in muscle cells, where they are assembled into an elaborate contractile apparatus.The actin-activated ATPase of vertebrate striated muscle myosin II is regulated primarily by proteins bound to the actin filament. In contrast, the ATPase activity of smooth and nonmuscle myosin II is regulated by phosphorylation of S19 in the N-terminal region of the RLC (reviewed in ref. 4). The dephosphorylated form has low ATPase activity, which is greatly increased on phosphorylation (5). The structural basis of phosphorylation-dependent regulation in smooth muscle myosin has been studied extensively by using soluble subfragments. Twoheaded fragments...

In vitro analysis confirms talin binding is sufficient for activation and extension of membrane-embedded integrin.

Cryo-EM of relaxed myosin filament reveals unique molecular motor conformation and unprecedented details of the filament backbone.

Unconventional myosin V (myoV) is an actin-based molecular motor that has a key function in organelle and mRNA transport, as well as in membrane trafficking. MyoV was the first member of the myosin superfamily shown to be processive, meaning that a single motor protein can 'walk' hand-over-hand along an actin filament for many steps before detaching. Full-length myoV has a low actin-activated MgATPase activity at low [Ca2+], whereas expressed constructs lacking the cargo-binding domain have a high activity regardless of [Ca2+] (refs 5-7). Hydrodynamic data and electron micrographs indicate that the active state is extended, whereas the inactive state is compact. Here we show the first three-dimensional structure of the myoV inactive state. Each myoV molecule consists of two heads that contain an amino-terminal motor domain followed by a lever arm that binds six calmodulins. The heads are followed by a coiled-coil dimerization domain (S2) and a carboxy-terminal globular cargo-binding domain. In the inactive structure, bending of myoV at the head-S2 junction places the cargo-binding domain near the motor domain's ATP-binding pocket, indicating that ATPase inhibition might occur through decreased rates of nucleotide exchange. The actin-binding interfaces are unobstructed, and the lever arm is oriented in a position typical of strong actin-binding states. This structure indicates that motor recycling after cargo delivery might occur through transport on actively treadmilling actin filaments rather than by diffusion.