HELIX CAPPING

Helix-capping motifs are specific patterns of hydrogen bonding and hyd rophobic interactions found at or near the ends of helices in both pro teins and peptides. In an alpha-helix, the first four >N-H groups and last four >C=O groups necessarily lack intrahelical hydrogen bonds. In stead, such groups are often capped by alternative hydrogen bond partn ers. This review enlarges our earlier hypothesis (Presta LG, Rose GD. 1988. Helix signals in proteins. Science 240:1632-1641) to include hyd rophobic capping. A hydrophobic interaction that straddles the helix t erminus is always associated with hydrogen-bonded capping. From a glob al survey among proteins of known structure, seven distinct capping mo tifs are identified-three at the helix N-terminus and four at the C-te rminus. The consensus sequence patterns of these seven motifs, togethe r with results from simple molecular modeling, are used to formulate u seful rules of thumb for helix termination. Finally, we examine the ro le of helix capping as a bridge linking the conformation of secondary structure to supersecondary structure.