Structural and functional diversity in 4-alpha-helical proteins.

Protein crystallographic studies show that many structural arrangements appear as common features among proteins which are otherwise unrelated in sequence or function. One of the more recently recognized recurring protein structural motifs is a nearly parallel arrangement of four alpha-helices to form a sequentially connected left-twisted bundle. We describe here the geometrical properties of these structures and suggest how they relate to the functional and aggregate properties of these molecules.