The ability of proteins to generate membrane curvature is essential to many cellular functions including endocytosis, cell motility, and the biogenesis of organelles. Previously characterized membrane bending mechanisms rely on protein domains with specific structural features like amphipathic helices and curved scaffolds. However, more recent work has shown that intrinsically disordered proteins (IDPs) can be potent drivers of membrane bending. Specifically, IDPs that repel on another can drive formation of protein-coated tubules through steric pressure, when crowded on the membrane. In contrast, IDPs with weak, multivalent interactions, which often participate in liquid-liquid phase separation (LLPS), generate protein-lined tubules by applying compressive stress on the membrane. How might IDPs that contain both repulsive and attractive domains impact membrane curvature? Here we examine a series of recombinant chimeras that link attractive and repulsive IDP domains within the same protein. We chose a model LLPS domain, the low complexity domain of Fused-In-Sarcoma (FUS LC) as the attractive domain, and the C-terminal domain of AP180 (AP180 CTD) as the repulsive domain. The two domains were linked in series, and a histidine tag was used to bind them to the membrane. When FUS LC was closer to the membrane, its attractive interactions helped to concentrate AP180 CTD, amplifying the crowding effect, resulting in protein-coated tubules. Interestingly, when the order of FUS LC and AP180 CTD switched, such that AP180CTD is closer to the membrane surface, attraction between FUS LC domains at a distance from the membrane overcame steric repulsion between AP180 CTD domains, resulting in protein-lined tubules. Further, a transition from protein-coated to protein-lined tubules was observed as salt concentration increased, likely arising from condensation of AP180 CTD in parallel with strengthening of the interactions between FUS LC domains. Collectively, these results, coupled with computational modeling indicate that the direction and curvature of membrane bending can be tuned by adjusting the balance of repulsive and cohesive forces among IDPs.

Balance of attractive and repulsive forces controls the direction of membrane bending by intrinsically disordered proteins

ACS Spring 2022

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