Membrane-Associated-Proteins: Self-Assembly, Interactions, and Biomolecular Materials

Abstract

Membrane-associated-proteins are active components of biological membranes facilitating functions such as energy conversion, molecular transport, and molecular recognition. Two-dimensional (2D) self-assemblies of membrane-proteins such as bacteriorhodopsin and bacterial self-assemblies of proteins are studied (1) from the biological interest in the elucidation of their structure-function correlations, and (2) for the development of biomolecular materials. In these lectures we describe a first series of recent high resolution synchrotron x-ray scattering studies of the purple membrane comprised of the 2D self-assembly of the membrane-protein bacteriorhodopsin (bR). At high humidity, the 2D lattice undergoes a reversible melting transition. The ordered phase of this multi- component and highly complex self-assembly is found to exhibit scattering characteristic of a 2D solid with power-law decay of positional correlations. Two significant consequences are (1) the first measurements of a protein-lattice elastic rigidity which is directly derived from the protein-protein interactions, and (2) the realization of the theoretical limit on the achievable resolution in protein crystallography of such two- dimensional crystals which exhibit power-law correlations and lack true long-range order. In the low humidity regime, a remarkable exceedingly high temperature phase of bR has been discovered. Higher order self-assemblies of the stacked 2D membranes, achieved by water removal, result in the complete suppression of the melting transition, and the absence of protein denaturation up to 140 °C.