THE ROLE OF TURNS IN THE STRUCTURE OF AN ALPHA-HELICAL PROTEIN

THE turns joining segments of secondary structure have been proposed t o be key elements in dictating the folded structures of native protein s1-9. An alternative view assumes that turns play a passive role and a re merely default structures that occur as a consequence of interactio ns between antiparallel segments of secondary structure, with chain re versal being dictated by the context surrounding the turn and not by t he sequence of the turn itself10,11. The solvent-exposure of turns and their tolerance to evolutionary variance suggests that they may have little or no effect on the formation of native structures. Previous in vestigations have focused on various types of beta-turns that connect antiparallel beta-strands1-3,12,13, with comparatively little reported on the structural role of interhelical turns. Here we probe the struc tural importance of such a turn in an antiparallel 4-helix bundle by r andomly substituting an interhelical tripeptide in cytochrome b-562 wi th many different amino-acid sequences. Thirty-one of the resulting su bstituted proteins were characterized and all of them were shown to fo ld into stable, native-like structures. These results suggest that thi s interhelical turn does not does not play a dominant role in determin ing the folded structure of this antiparallel 4-helix bundle.