Transmembrane coupling accelerates the growth of liquid-like protein condensates

Abstract

Timely and precise assembly of protein complexes on membrane surfaces is essential to the physiology of living cells. Recently, protein phase separation has been observed at cellular membranes, suggesting it may play a role in the assembly of protein complexes. Inspired by these findings, we observed that protein condensates on one side of a planar suspended membrane spontaneously colocalized with those on the opposite side. How might this phenomenon contribute to the assembly of stable transmembrane complexes? To address this question, we examined the diffusion and growth of protein condensates on both sides of membranes. Our results reveal that transmembrane coupling of protein condensates on opposite sides of the membrane slows down condensate diffusion while accelerating condensate growth. How can the rate of condensate growth increase simultaneously with a decrease in the rate of condensate diffusion? We provide insights into these seemingly contradictory observations by distinguishing between diffusion-limited and coupling-driven growth processes. While transmembrane coupling slows down diffusion, it also locally concentrates condensates within a confined area. This confinement increases the probability of condensate coalescence and thereby enhances the overall rate of growth for coupled condensates, substantially surpassing the growth rate for uncoupled condensates. These findings suggest that transmembrane coupling could play a role in the assembly of diverse membrane-bound structures by promoting the localization and growth of protein complexes on both membrane surfaces. This phenomenon could help to explain the efficient assembly of transmembrane structures in diverse cellular contexts. SignificanceProtein assemblies that span biological membranes are critical to cellular physiology. In the past decade, liquid-like protein condensates, which are flexible, multivalent protein assemblies, have been discovered on diverse membrane surfaces. Recently, we observed that protein condensates on opposite sides of a membrane spontaneously colocalize to form coupled, transmembrane complexes. Interestingly, while transmembrane coupling slows down the diffusion of membrane-bound condensates, it substantially accelerates their growth by strongly localizing interactions between them. These findings suggest that transmembrane coupling of protein condensates may play a role in promoting the robust assembly of membrane-bound protein complexes in crowded, complex cellular environments.