HELIX STOP AND START SIGNALS IN PEPTIDES AND PROTEINS - THE CAPPING BOX DOES NOT NECESSARILY PREVENT HELIX ELONGATION

Recently several papers have addressed the existence of helix stop sig nals at the beginning of alpha-helices. It has been indicated that the existence of a reciprocal backbone-side-chain hydrogen-bond interacti on, designated the capping box, could be one of these signals. The fin gerprint sequence of this capping box is Ser/Thr-X-X-Glu/Gln. In the f ifth alpha-helix of the chemotactic alpha/beta parallel protein CheY t here is such a sequence in the middle of the helix. In a peptide corre sponding to this alpha-helix the capping box is bypassed, as deduced f rom NMR analysis. However, making the peptide shorter so that the capp ing box fingerprint is closer to the beginning of the peptide results in the formation of the capping box. These results indicate that, alth ough the capping box could play a role in stabilizing and nucleating h elical peptides in solution, it is not necessarily a stop signal and c an be bypassed when favourable interactions exist between the surround ing residues.