Directed movement is a characteristic of many living organisms and occurs as a result of the transformation of chemical energy into mechanical energy. Myosin is one of three families of molecular motors that are responsible for cellular motility. The three-dimensional structure of the head portion of myosin, or subfragment-1, which contains both the actin and nucleotide binding sites, is described. This structure of a molecular motor was determined by single crystal x-ray diffraction. The data provide a structural framework for understanding the molecular basis of motility.

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The crystal structures of three quinoxaline antibioticsechinomycin 2QN, triostin C and the C222~ form of triostin A -have been determined, and the structure of the P212121 form of triostin A has been re-refined against our previously reported data. The molecular conformations are compared with those deduced from NMR data and those reported for two complexes of triostin A with oligonucleotides. Although the depsipeptide ring conformations are basically similar, the effective twofold molecular symmetry is violated by the folding of one of the quinoxaline chromophores in echinomycin 2QN and by a rotation of one of the ester planes with the formation of an intramolecular hydrogen bond in triostin C. In the oligonucleotide complexes of triostin A the chirality of the disulfide bridge is inverted. The alanine NH groups are involved in intermolecular hydrogen bonds in all four structures, and (except in echinomycin 2QN) the stacking of the chromophores in the crystal emulates the intercalation involved in DNA complex formation. In echinomycin 2QN, the antibiotic molecules are hydrogen bonded to form a helix along the crystallographic 65 screw axes, with a channel of disordered solvent running through the middle of the helix. Crystal data: (1), echinomycin 2QN,

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The crystal structure of a glycopeptide antibiotic A40926 aglycone was investigated by X-ray analysis at -120O. A-40926 crystallises in the orthorhombic space group P2,2,2, with two monomers in the asymmetric unit, a = 21.774(4), b = 28.603(7), c = 29.757(4) A. 'Conventional' direct methods approach failed to solve the structure, but a novel iterative real/reciprocal space procedure was successful. Refinement against 1 1248 F2 data led to R 1 = 13.3 % for 6770 F > 4o(F). The two monomers of A-40926 have similar conformations and are bound by antiparallel H-bonds to form a 'chain' structure of connecting dimers. The antibiotic molecule possesses a 'binding pocket' for the C-terminal carboxy group of the cell-wall protein, which is consistent with suggestions based on NMR ddta and the recently reported crystal structure of ureido-balhimycin. In A-40926 the monomers are polymerically linked by H-bonds, quite unlike the tight dimer formation observed in ureido-balhimycin.