RESTORED HEPTAD PATTERN CONTINUITY DOES NOT ALTER THE FOLDING OF A 4-ALPHA-HELIX BUNDLE

The sequences of alpha-helical coiled-coils and bundles are characteri zed by a specific pattern of hydrophobic and hydrophilic residues whic h is repeated every seven residues. Highly conserved breaks in this pa ttern frecquently occur in segments of otherwise continuous heptad sub structures. The hairpin bend of the ROP protein coincides with such a break and provides a model system for the study of the structural effe cts induced by heptad discontinuities. The structure of a ROP mutant w hich re-establishes a continuous heptad pattern, shows insignificant c hanges relative to the wild-type protein, as is also reflected in its conformational stability, spectroscopic properties and unfolding behav iour Thus, formation of alpha-alpha-hairpin bends may occur both in th e presence and absence of heptad breaks.