The structures and mechanisms of assembly of biological macromolecules and macromolecular complexes are studied, primarily by high resolution electron microscopy and computer image processing, but also incorporating other information such as amino- acid sequence analyses. Over the past year, we have made headway primarily in (1) analysis of frozen-hydrated transverse thin sections of relaxed and rigor skeletal muscle; (2) formulating a molecular model for the fibrous receptor-recognition protein of reovirus; and (3) characterizing the regular surface lattice found in the outer membranes of two strains of Bordetella pertussis. (1) After restoring the cryo-section images for non-linear contrast transfer correcting for spatial disorders, and averaging to improve the signal-to-noise ratio, we have observed the distribution of mass within the projected unit cell of the myofilament lattice in both states. On entering rigor, mass is transferred from the periphery of the myosin filaments to surround the actin filaments. We also observe changes to affect myosin filaments in rigor even in the absence of actin. (2) Sigma-1 protein fibers released from reovirions upon heating are homotetrameric molecules, 48nm long x 5-9nm wide. The distal part of the rod is made up of two coiled-coil helical ropes placed side-by-side. This segment is followed by a wider portion of about the same length as the first (23nm) but which is wider, contains visible substructure, and is rich in beta-sheet conformation. Finally, the carboxy termini form globular domains where the receptor-combining sites reside. (3) Applying generalized image averaging techniques, we have found that the B. pertussis surface lattice is rectangular and observes p2 symmetry. Its unit cell contains two trimers of the bacterium's 40K anion-selective porin, in different orientations. That these porins are active in the crystalline state is implied by the normal growth rate of these strains that exhibit this structure.
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