PROTEIN MOTIFS .3. THE PARALLEL BETA-HELIX AND OTHER COILED FOLDS

A new type of structural domain, composed of all parallel beta strands , has been observed within the last year. An analysis of the basic typ es suggests that there are two distinct classes: the parallel beta hel ices, which belong to a tri beta-strand category, and the beta roll, w hich belongs to a di beta-strand category. The novel structural featur es of each class are described and the proteins belonging to each cate gory are summarized. Proteins with the parallel beta helix fold includ e three pectate lyases and the tailspike protein from P22 phage. Prote ins with the beta roll fold include two alkaline proteases. Although t he parallel beta composition is emphasized, the same set of proteins s hare another common structural feature with several other proteins con taining alpha helices: the polypeptide backbone is folded into a coile d structure in which each coil has the same 3-dimensional arrangement of a group of secondary structural elements, In addition to parallel b eta domains, the other groups include the alpha/beta coiled fold, as r epresented by ribonuclease inhibitor, and the alpha/alpha coiled fold, as represented by lipovitellin and soluble lytic transglycoslyase. No vel features of the alpha/beta and alpha/alpha coiled folds are summar ized.