Our work suggests that protein native state structures occupy a novel phase of matter corresponding to the marginally compact conformations of a flexible tube. This phase arises from common attributes of proteins and is independent of amino acid sequences. Our approach provides a simple and unified framework to understand protein folding and amyloid formation. With a constraint on the local radius of curvature, the tube model is also shown to have ground state conformations similar to that of DNA toroids. © 2013 IFMBE.