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The 2.5-[Angstrom] resolution crystal structure is reported for an actin dimer, composed of two protomers cross-linked along the longitudinal (or vertical) direction of the F-actin filament. The crystal structure provides an atomic resolution view of a molecular interface between actin protomers, which we argue represents a near-native interaction in the F-actin filament. The interaction involves subdomains 3 and 4 from distinct protomers. The atomic positions in the interface visualized differ by 5-10 A from those suggested by previous models of F-actin. Such differences fall within the range of uncertainties allowed by the fiber diffraction and electron microscopy methods on which previous models have been based. In the crystal, the translational arrangement of protomers lacks the slow twist found in native filaments. A plausible model of F-actin can be constructed by reintroducing the known filament twist, without disturbing significantly the interface observed in the actin dimer crystal. actin crystal structure | F-actin filament | motor proteins

A novel heterobifunctional photo-cross-linking reagent, N-(4-azido-2-nitrophenyl)-putrescine, was synthesized and covalently bound to Gln-41 of rabbit skeletal muscle actin. The G-actin was polymerized and the resulting F-actin was intermolecularly cross-linked by irradiation with ultraviolet light. Experimental results showed that the reagent is capable of cross-linking Gln-41 to Cys-374 on two adjacent monomers of the skeletal muscle actin.

The structural and functional properties of the intrastrand actin filaments of the heterobifunctional photo-cross-linking reagent, N-(4-azido-2-nitrophenyl)-putrescine, between Gln-41 and Cys-374 on adjacent monomers were investigated. Extensive cross-linking of F-actin results in its loss of ability to activate myosin ATPase at 60 degrees Celsius. Results also showed the uncoupling between force generation and other events in actomyosin interactions and demonstrated the important role of actin filament structure and function in the contractile process.

Actin filaments partially cross-linked with the heterobifunctional photo-cross-linking reagent, N-(4-azido-2-nitrophenyl)-putrescine, between Gln-41 and Cys-374 on adjacent monomers in the long-pitch helix were described. The filaments were depolymerized and fractionated into pools of longitudinal cross-linked dimers, trimers and higher-order oligomers. Experimental results showed the importance of lateral actin-actin interaction for the activation of myosin ATPase and the polymerization of actin by myosin subfragment.