Spikes and fimbriae: alpha-helical proteins form surface projections on microorganisms.

Two basic alpha-helical plans appear to characterize portions of the proteins projecting from the surface of many microorganisms. Structure A is related to that of the antibody-binding protein A of Staphylococcus aureus, in which a single polypeptide chain doubles back to form an Anti-parallel domain; the other, structure M, is related to the M proteins of group A streptococci, in which Multiple parallel alpha-helical chains interlock in a coiled-coil conformation. A projection about 100 A long may be organized on either plan, but very long projections are likely to be of the M pattern. The alpha-helical motif may account for the length of these spikes, with other functions such as anchoring or antiphagocytic activity located in domains of different structure.