Reflectin Proteins Bind and Reorganize Synthetic Phospholipid Vesicles

Abstract

The reflectin proteins have been extensively studied for their role in reflectance in cephalopods. In the recently evolved Loliginid squids, these proteins and the structural color they regulate are dynamically tunable, enhancing their effectiveness for camouflage and communication. In these species, the reflectins are found in highest concentrations within the structurally tunable, membrane enclosed, periodically stacked lamellae of subcellular Bragg reflectors and in the intracellular vesicles of specialized skin cells known as iridocytes and leuocophores, respectively. To better understand the interactions between the reflectins and the membrane structures that encompass them, we analyzed the interactions of two purified reflectins with synthetic phospholipid membrane vesicles similar in composition to cellular membranes, using confocal fluorescence microscopy and dynamic light scattering. The purified recombinant reflectins were found to drive multivalent vesicle agglomeration in a ratio-dependent and saturable manner. Extensive proteolytic digestion terminated with PMSF of the reflectin A1-vesicle complexes triggered energetic membrane rearrangement, resulting in vesicle fusion, fission, and tubulation. This behavior contrasted markedly with that of vesicles complexed with reflectin C, from which PMSF-terminated proteolysis only released the original size vesicles. Clues to the basis for this difference, residing in significant differences between the structures of the two reflectins, led to the suggestion that specific reflectin-membrane interactions may play a role in the ontogenetic formation, long-term maintenance, and/or dynamic behavior of their biophotonically active host membrane nanostructures. Similar energetic remodeling has been associated with osmotic stress in other membrane systems, suggesting a path to reconstitution of the biophotonic system in vitro.