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...

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Concomitant CT offered improved disease control and survival in advanced head and neck cancer patients. Due to increased acute toxicity, more supportive care is demanded when CT is given simultaneously. Increased total treatment time does not exert a negative impact on outcome in this combined modality regimen.

The structural basis for the phosphoryla- tion-dependent regulation of smooth muscle myosin ATPase activity was investigated by forming two- dimensional (2-D) crystalline arrays of expressed unphosphorylated and thiophosphorylated smooth muscle heavy meromyosin (HMM) on positively charged lipid monolayers. A comparison of averaged 2-D projections of both forms at 2.3-nm resolution reveals distinct structural differences. In the active, thiophosphorylated form, the two heads of HMM interact intermolecularly with adjacent molecules. In the unphosphorylated or inhibited state, intramolecular interactions position the actin-binding interface of one head onto the converter domain of the second head, thus providing a mechanism whereby the activity of both heads could be inhibited.

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We used cryo-electron microscopy and image reconstruction to investigate the structure and microtubulebinding con®gurations of dimeric non-claret disjunctional (ncd) motor domains under various nucleotide conditions, and applied molecular docking using ncd's dimeric X-ray structure to generate a mechanistic model for force transduction. To visualize the a-helical coiled-coil neck better, we engineered an SH3 domain to the N-terminal end of our ncd construct (296±700). Ncd exhibits strikingly different nucleotide-dependent three-dimensional conformations and microtubule-binding patterns from those of conventional kinesin. In the absence of nucleotide, the neck adapts a con®guration close to that found in the X-ray structure with stable interactions between the neck and motor core domain. Minus-end-directed movement is based mainly on two key events: (i) the stable neck±core interactions in ncd generate a binding geometry between motor and microtubule which places the motor ahead of its cargo in the minus-end direction; and (ii) after the uptake of ATP, the two heads rearrange their position relative to each other in a way that promotes a swing of the neck in the minus-end direction.

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