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.