Structural details of an interaction between cardiolipin and an integral membrane protein

Anionic lipids play a variety of key roles in biomembrane function, includi ng providing the immediate environment for the integral membrane proteins t hat catalyze photosynthetic and respiratory energy transduction. Little is known about the molecular basis of these lipid-protein interactions. In thi s study, x-ray crystallography has been used to examine the structural deta ils of an interaction between cardiolipin and the photoreaction center, a k ey light-driven electron transfer protein complex found in the cytoplasmic membrane of photosynthetic bacteria. X-ray diffraction data collected over the resolution range 30.0-2.1 Angstrom show that binding of the lipid to th e protein involves a combination of ionic interactions between the protein and the lipid headgroup and van der Waals interactions between the lipid ta ils and the electroneutral intramembrane surface of the protein. In the hea dgroup region, ionic interactions involve polar groups of a number of resid ues, the protein backbone, and bound water molecules. The lipid tails sit a long largely hydrophobic grooves in the irregular surface of the protein. I n addition to providing new information on the immediate lipid environment of a key integral membrane protein, this study provides the first, to our k nowledge, high-resolution x-ray crystal structure for cardiolipin, The poss ible significance of this interaction between an integral membrane protein and cardiolipin is considered.