A model for ninefold symmetry in α keratin and cilia

The dimensions of the α helix are such that certain simple repeats of a small number of amino acids within a helical protein can generate ninefold symmetry. The chemical data for α keratin suggest that within this molecule total or partial repeats might occur every 22 or 44 amino acids, accounting for the pattern of nine seen in electron microscope images of α keratin microfibrils, if the postulated repeating sequence contains a binding site such that spatial extension is possible. Centrioles and their derivatives may be generated via further extension by a fundamentally similar morphogenetic process.