Helical protein filaments formed by self-assembly. (a) General scheme for protein self-assembly into helices. For any globular protein of arbitrary shape, as shown at the top, considered as interacting with a second copy of itself in all possible orientations, there will be some pair of surface patches that result in optimal binding energy. It is highly unlikely that those two interface patches will happen to reflect any specific geometrical symmetry. When many copies of the same subunit self-associate by binding to one another through these surface interactions, a one-start helix with a single protofilament is the default structure formed, as shown in the middle. At bottom, if weaker interactions can also form laterally between subunits, multi-start helices may be stabilized (adapted with permission from the Royal Society of Chemistry ). (b) Electron micrograph showing a single filament of sickle-cell hemoglobin (HbS) (reprinted by permission from Macmillan Publishers Ltd: Nature 272:506–510, copyright 1978 ).