The directed cooperative assembly of proteorhodopsin into 2D and 3D polarized arrays

Proteorhoclopsin is the membrane protein used by marine bacterioplankton as a light-driven proton pump. Here, we describe a rapid cooperative assembly process directed by universal electrostatic interactions that spontaneously organizes proteorhoclopsin molecules into ordered arrays with well defined orientation and packing density. We demonstrate the charge density-matching mechanism that selectively controls the assembly process. The interactions among different components in the system are tuned by varying their charge densities to yield different organized transmembrane protein arrays: (i) a bacteriorhoclopsin purple membrane-like structure where proteorhoclopsin molecules are cooperatively arranged with charged lipids into a 2D hexagonal lattice; (h) selected liquid-crystalline states in which crystalline lamellae made up of the coassembled proteorhoclopsin and charged lipid molecules are coupled three-dimensionally with polarized proteorhoclopsin orientation persisting through the macroscopic scale. Understanding this rapid electrostatically driven assembly process sheds light on organizing membrane proteins in general, which is a prerequisite for membrane protein structural and mechanistic studies as well as in vitro applications.