Protein Networks as Synergistic Drivers of Membrane Remodeling Membrane curvature is required for many cellular processes, from assembly of highly curved trafficking vesicles to extension of needle-like filopodia. Consequently, defects in membrane curvature play a role in most human diseases, including altered recycling of receptors in cancer and diabetes, targeting of filopodia by pathogens, and hijacking of vesicle traffic during virus replication. Therefore, understanding the basic molecular mechanisms that drive membrane remodeling is essential to our knowledge of cellular physiology and human disease. Research on membrane curvature has primarily focused on individual protein domains with specialized structures, such as crescent-shaped scaffolds and wedge-like amphipathic insertions. While this work has provided invaluable insights, this ?structure-centric? perspective ignores two essential facts. First, most membrane remodeling proteins contain large intrinsically disordered domains in addition to structured domains. And second these disordered domains drive assembly of large, multi-valent protein networks. During the past 5 years, our group has made pioneering discoveries in support of the hypothesis that disordered protein networks are essential drivers of membrane remodeling in the cell. Specifically, using clathrin-mediated endocytosis as a model pathway, we showed that intrinsically disordered domains generate steric pressure at membrane surfaces. This pressure provides a surprisingly potent driving force for membrane bending, especially when coupled synergistically to the contributions of structured domains. This work was the first to reveal the membrane remodeling abilities of disordered proteins, examples of which have since been discovered in diverse areas of biology. Additionally, we have recently found that disordered domains within endocytic proteins drive assembly of liquid-like protein networks which efficiently initiate endocytosis. Importantly, this liquid-like behavior has the potential to resolve a long-standing paradox by explaining how curved membrane structures can be simultaneously highly interconnected, yet dynamic and flexible. These findings suggest urgent questions about the role of disordered protein networks in the key steps of membrane remodeling: (i) initiation, (ii) curvature induction, and (iii) cargo selection. First, how do protein networks initiate remodeling events, controlling their spatial and temporal dynamics? Second, once an event is initiated, how do protein networks bend membranes, stabilizing either a convex or a concave shape? Third, as the membrane bends, how does the protein network select cargo, such as transmembrane proteins, which are essential to the structure?s biological function? Building on our recent discoveries, this work will shift the paradigm for understanding membrane curvature beyond its present focus on in vitro structure-function relationships toward an understanding of disordered protein networks. By demonstrating novel synergistic mechanisms, this research will provide a blueprint for the study of protein networks at membrane surfaces throughout the cell.
The proposed research is relevant to public health because defects in membrane remodeling underlie debilitating diseases from cancer to neurological disorders, and because membrane trafficking vesicles serve as the cellular entry mechanism for both harmful pathogens and therapeutic drugs. Therefore, the proposed research on the basic physical mechanisms underlying membrane remodeling is relevant to the part of NIH?s mission that seeks to develop fundamental knowledge that will help to reduce the burdens of human disability and disease. Ultimately, it is envisioned that a fundamental understanding of membrane remodeling will enable therapeutic manipulation of membrane traffic and other points of intersection between membranes and disease.
