Fibers and Zippers

Historically, coiled coils were identified with long fibrous molecules, from which their structural properties had been determined. Fiber diffraction studies on proteins of the k-m-e-f class were highly successful, initially on dried specimens but later also on native samples (Cohen and Holmes, 1963). However, these proteins turned out to be very difficult to analyze by high-resolution X-ray crystallography for the same reasons that made them so amenable to fiber diffraction—their tendency to aggregate into fibers rather than crystals and the extreme dimensions of their asymmetric units. It took decades to obtain a working structure for tropomyosin [at 15 A resolution (Phillips et al., 1986) at 9 A (Whitby et al., 

Coiled-coil helices usually have one stripe of residues engaged in knobs-into-holes interactions. There are, however, instances where a helix may engage in such interactions along two stripes (Cohen and Parry, 1990 Walshaw and Woolfson, 2003 Walshaw et ah, 2001). For example, helices of four-stranded coiled coils make their primary knobs-into-holes contacts 

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