Intrinsically Disordered Proteins and Their Mysterious (Meta)Physics

Recognition of the natural abundance and functional importance of intrinsically disordered proteins (IDPs), and protein hybrids that contain both intrinsically disordered protein regions (IDPRs) and ordered regions, is changing protein science. IDPs and IDPRs, i.e., functional proteins and protein regions without unique structures, can often be found in all organisms, and typically play vital roles in various biological processes. Disorder-based functionality complements the functions of ordered proteins and domains. However, by virtue of their existence, IDPs/IDPRs, which are characterized by remarkable conformational flexibility and structural plasticity, break multiple rules established over the years to explain structure, folding, and functionality of well-folded proteins with unique structures. Despite the general belief that unique biological functions of proteins require unique 3D-structures (which dominated protein science for more than a century), structure-less IDPs/IDPRs are functional, being able to engage in biological activities and perform impossible tricks that are highly unlikely for ordered proteins. With their exceptional spatio-temporal heterogeneity and high conformational flexibility, IDPs/IDPRs represent complex systems that act at the edge of chaos and are specifically tunable by various means. In this article, some of the wonders of intrinsic disorder are discussed as illustrations of their mysterious (meta)physics.