Crystal structure of vitelline membrane outer layer protein I (VMO‐I): a folding motif with homologous Greek key structures related by an internal three‐fold symmetry.

The crystal structure of vitelline membrane outer layer protein I (VMO‐I), which is isolated from the vitelline membrane outer layer of hen's eggs, has been determined by the multiple isomorphous replacement method and refined to an R‐factor of 18.8% at 2.2 A resolution. The main chain folds into an unusual structure that consists of three beta‐sheets forming Greek key motifs, which are related by an internal pseudo three‐fold symmetry. The internal portion surrounded by these three beta‐sheets is filled with hydrophobic side chains. This conformational feature coincides with three internal repeats in the sequence. Although a similar fold exists in the second domain of delta‐endotoxin, there are significant structural differences between the two proteins, with the three‐fold symmetry being most regular in VMO‐I.