HELIX STOP SIGNALS IN PROTEINS AND PEPTIDES - THE CAPPING BOX

The alpha-helix [Pauling, L., Corey, R.B., & Branson, H.R. (1951) Proc . Natl. Acad. Sci. U.S.A. 37, 205-211] is a common motif in both prote ins and peptides. Despite intense investigation, predictive understand ing of helices is still lacking. A recent hypothesis [Presta, L. G., & Rose, G. D. (1988) Science 240, 1632-1641] proposed that the structur al specificity of helices resides, in part, in those residues that fla nk helix termini. If so, then signals that arrest helix propagation-i. e., helix stop signals-should be found among these flanking residues. Evidence is presented for the existence of one such signal, a reciproc al backbone-side-chain hydrogen-bonding interaction, dubbed the cappin g box. In proteins, the capping box is found uniquely at helix N-termi ni. In peptides, the capping box can function as a helix stop signal, as shown in the work of Kallenbach and co-workers.