Effects of Styrene-Maleic Acid (SMA) Copolymer on the Photoactivation Mechanism of Rhodopsin

Abstract

Integral membrane proteins (IMPs) allow ions and molecules to cross cellular membranes, perform enzymatic reactions, and initiate signal transduction for biochemical processes. Obtaining detailed information about their dynamics, mechanisms of action and structure in the native membrane environment can be challenging. Styrene-maleic acid (SMA) copolymers have been shown to solubilize biological membranes to form nanometer-sized particles that contain active proteins surrounded by native lipids (termed SMALPs). SMALPs have enabled scientists to use a variety of experimental techniques for biophysical and structural studies of IMPs. However, the effect of SMA solubilization on the functional properties of the proteins remains unknown. In this study, we address this question by comparing the photoactivation mechanism of rhodopsin, a G protein-coupled receptor (GPCR) that comprises the majority of ROS disc membrane, in its native membrane and in SMALPs. Samples with different molar ratios between rhodopsin and SMA were studied with time-resolved absorption spectroscopy. Kinetic analysis of the data shows that rhodopsin in SMALPs follow the reaction mechanism found in native membranes, although the reaction steps are significantly slowed down. These results demonstrate how different concentrations of SMA, which have been used to produce crystals for structural studies, can affect protein dynamics and ultimately physiological function.