Collaborative Research: MODULUS: Protein droplets drive membrane bending and cytoskeletal organization

University Of Texas At Austin, Austin TX

Investigators

Abstract

Diverse classes of proteins within cells have recently been observed to self-assemble into liquid-like droplets. While they were first discovered in the cellular interior, or cytosol, it is increasingly clear that protein droplets frequently become coupled to other cellular structures, including biological membranes and the cell’s cytoskeleton. Inspired by these findings, in this project the investigators are seeking an understanding of the biophysical coupling between protein droplets, membranes, and cytoskeletal filaments. The project is executed in two parts. In the first part, the project, examines how protein droplets exert forces on biological membranes, resulting in membrane curvature. The second part of the project examines how protein droplets exert forces on cytoskeletal filaments, resulting in the organization of filaments into bundles and meshes. The project findings benefit society by improving knowledge on the mechanisms that pathogens use to invade cells, how endocytic vesicles internalize extracellular material and how cellular motility is established and regulated. The project also reveals fundamental mechanisms that organize soft matter, from surfactants and fuels to cosmetics and foods. The project couples scientific work to the implementation of a multi-level educational outreach program that benefits diverse students from K-12 classrooms to undergraduate researchers. This project sheds light on the role of protein condensates in two critical cellular functions: organization of cytoskeletal filaments and shaping of membrane surfaces. Traditional efforts to understand these functions have focused on specific interactions between structured proteins. In contrast, this work illustrates how protein condensates, composed largely of intrinsically disordered proteins, provide previously unknown mechanisms of force generation. This illustration suggests a new paradigm in which interfacial interactions among soft materials can be used to understand how cellular architectures arise. The project approaches are guided by the hypothesis that reducing the spatial dimensionality of 3D protein droplets, by interfacing either with 2D membranes or with 1D filaments, generates forces that will alter the morphology of the reduced-dimension composite. This hypothesis is tested by developing experimentally constrained, mathematical models of interfacial coupling between liquid protein droplets, cytoskeletal polymers, and membranes. Model predictions are tested with experimental systems that directly observe the coupling between droplets, filaments, and membranes, enabling measurement of the key geometrical and physical parameters required to validate and refine the models. A combination of theoretical, computational, and experimental tools is used to design studies that reveal the unique mechanical functions of protein condensates. The project enhances understanding of how proteins that are known to assemble into 3D liquid droplets, through liquid-liquid phase separation, can exert stresses on the membrane and on the cytoskeleton. This award is co-funded by the Systems and Synthetic Biology program in the Division of Molecular and Cellular Biosciences and the Division of Mathematical Biology in the Division of Mathematical Sciences. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.