The Tight Packing in Parallel Helices

TOP shows an example of the tight interdigitating packing observed in helices of well-refined soluble protein structures. TOP-LEFT shows a side view of an interface; TOP-MIDDLE shows this rotated by 90 degrees; and TOP-RIGHT shows a slice through this, highlighting the interdigitation of the sidechains. Note how this precludes large motions at the interface without substantial repacking of the sidechains. BOTTOM-LEFT and BOTTOM-MIDDLE is a representation of the sliding motion in transmembrane helices, showing shifting relative to each other by 1 Angstrom, while still preserving the pattern of contacts at the interface. BOTTOM-RIGHT is a schematic of the overall chemotaxis receptor structure adapted from Kim et al., showing the location of the transmembrane helices. The ligand-binding site in the periplasmic domain is indicated by a 1. It is separated by >200 A from the "secondary" binding sites in the cytoplasmic domain -- the methylation region labelled by 2a and the signalling region labelled by 2b.