The parallel β helix and other coiled folds

A new type of structural domain, composed of all parallel β strands, has been observed within the last year. An analysis of the basic types suggests that there are two distinct classes: the parallel β helices, which belong to a tri β ‐strand category, and the β roll, which belongs to a di β ‐strand category. The novel structural features of each class are described and the proteins belonging to each category are summarized. Proteins with the parallel β helix fold include three pectate lyases and the tailspike protein from P22 phage. Proteins with the β roll fold include two alkaline proteases. Although the parallel β composition is emphasized, the same set of proteins share another common structural feature with several other proteins containing a helices: the polypeptide backbone is folded into a coiled structure in which each coil has the same 3‐dimensional arrangement of a group of secondary structural elements. In addition to parallel β domains, the other groups include the α/β coiled fold, as represented by ribonuclease inhibitor, and the a/a coiled fold, as represented by lipovitellin and soluble lytic transglycoslyase. Novel features of the α/β and a/a coiled folds are summarized.—Yoder, M. D., and Jurnak, F. The parallel β helix and other coiled folds. FASEB J. 9, 335–342 (1995)