αα-Hub domains and intrinsically disordered proteins: A decisive combo

Hub proteins are central nodes in protein-protein interaction networks with critical importance to all living organisms. Recently, a new group of folded hub domains, the alpha alpha-hubs, was defined based on a shared alpha alpha-hairpin supersecondary structural foundation. The members PAH, RST, TAFH, NCBD, and HHD are found in large proteins such as Sin3, RCD1, TAF4, CBP, and harmonin, which organize disordered transcriptional regulators and membrane scaffolds in interactomes of importance to human diseases and plant quality. In this review, studies of structures, functions, and complexes across the alpha alpha-hubs are described and compared to provide a unified description of the group. This analysis expands the associated molecular concepts of "one domain-one binding site", motif-based ligand binding, and coupled folding and binding of intrinsically disordered ligands to additional concepts of importance to signal fidelity. These include context, motif reversibility, multivalency, complex heterogeneity, synergistic alpha alpha-hub:ligand folding, accessory binding sites, and supramodules. We propose that these multifaceted protein-protein interaction properties are made possible by the characteristics of the alpha alpha-hub fold, including supersite properties, dynamics, variable topologies, accessory helices, and malleability and abetted by adaptability of the disordered ligands. Critically, these features provide additional filters for specificity. With the presentations of new concepts, this review opens for new research questions addressing properties across the group, which are driven from concepts discovered in studies of the individual members. Combined, the members of the alpha alpha-hubs are ideal models for deconvoluting signal fidelity maintained by folded hubs and their interactions with intrinsically disordered ligands.