Cells diversify transmembrane signaling through the controlled chaos of protein disorder

Targeting Intramembrane Protein-Protein Interactions: Novel Therapeutic Strategy of Millions Years Old

Intramembrane protein-protein interactions (PPIs) are involved in transmembrane signal transduction mediated by cell surface receptors and play an important role in health and disease. Recently, receptor-specific modulatory peptides rationally designed using a general platform of transmembrane signaling, the signaling chain homooligomerization (SCHOOL) model, have been proposed to therapeutically target these interactions in a variety of serious diseases with unmet needs including cancer, sepsis, arthritis, retinopathy, and thrombosis. These peptide drug candidates use ligand-independent mechanisms of action (SCHOOL mechanisms) and demonstrate potent efficacy in vitro and in vivo. Recent studies surprisingly revealed that in order to modify and/or escape the host immune response, human viruses use similar mechanisms and modulate cell surface receptors by targeting intramembrane PPIs in a ligand-independent manner. Here, I review these intriguing mechanistic similarities and discuss how the viral strategies optimized over a billion years of the coevolution of viruses and their hosts can help to revolutionize drug discovery science and develop new, disruptive therapies. Examples are given.

Structural biology of intrinsically disordered proteins: Revisiting unsolved mysteries

The emergence of intrinsically disordered proteins (IDPs) has challenged the classical protein structure-function paradigm by introducing a new paradigm of "coupled binding and folding". This paradigm suggests that IDPs fold upon binding to their partners. Further studies, however, revealed a novel and previously unrecognized phenomenon of "uncoupled binding and folding" suggesting that IDPs do not necessarily fold upon interaction with their lipid and protein partners. The complex and often unusual biophysics of IDPs makes structural characterization of these proteins and their complexes not only challenging but often resulting in opposite conclusions. For this reason, some crucial questions in this field remain unsolved for well over a decade. Considering an important role of IDPs in cellular regulation, signaling and control in health and disease, more efforts are needed to solve these mysteries. Here, I focus on two long-standing contradictions in the literature concerning dimerization and membrane-binding activities of IDPs. Molecular explanation of these discrepancies is provided. I also demonstrate how resolution of these critical issues in the field of IDPs results in our expanded understanding of cell function and has multiple applications in biology and medicine.

Evolution of immunity: no development without risk

Signal transduction by cell surface receptors in the context of heterogeneous and variable cellular environments plays a pivotal role in regulating many biological processes, including development, activation, and homeostasis of the immune system. In some receptors, extracellular ligand-binding and intracellular signaling domains are located on the same protein chain (single-chain receptors), while in the so-called multichain immune recognition receptors (MIRRs), recognition and signaling functions are separated between different protein chains. Why did nature separate recognition and signaling functions for MIRRs, thereby increasing the risk of malfunction and potential attack by pathogens? The risk is real: in order to escape the immune response, viruses are able to disrupt functional coupling between recognition and signaling aspects of MIRR machinery. Intrinsic disorder of intracellular signal-generating regions of MIRRs adds further intrigue to the story. Why did nature select protein disorder for MIRRs to translate recognition of distinct antigens into appropriate activation signals that would induce specific functional outcomes? Here, I suggest that nature takes the risks associated with intrareceptor separation of functions as well as with the chaos and indeterminacy of protein disorder in exchange for providing diversity and variability of signal transduction. Not only does this phenomenon serve as the molecular basis for the development and evolution of the immune and other complex biological systems, but it fits closely to Darwinian evolutionary biology.