The Magic of Bicelles Lights Up Membrane Protein Structure

1.1. Why Study Membranes and Membrane Proteins? Biological membranes and membrane proteins, responsible for numerous exciting biological processes, present one of the paramount challenges in biophysics today. Membranes are present in great number and variety in all organisms. They form the boundary between the inside and outside for any bacterium or cell, and they delimit the host of organelles that make up their inner subunits. Each biological membrane is made up of dozens of different types of lipids and sterols, and any particular type of membrane has a characteristic content of these different constituents. As a very basic example, we mention that prokaryotic membranes contain a notable component of negatively charged lipids but almost no cholesterol, while eukaryotic membranes are mostly zwitterionic but have a significant amount of cholesterol. Since the driving biophysical principles of membrane formation are very simple—they lie in the amphipathic properties of any lipid molecule—a single lipid type is sufficient to form membrane-like bilayers in an aqueous environment. Such model membranes are used extensively to study biophysical properties that are representative for most membrane systems. A particularly interesting effect is observed when detergent molecules are added to lipid bilayer samples: the detergents solubilize the bilayers, and in certain regimes so-called bilayered mixed micelles or “bicelles” are formed. In the simplest case, they can be described as microscopic disks where a bilayer patch is encircled by a “rim” of detergent molecules. Bicelles represent a new instance of lipid morphology and are extensively applicable to structural studies of lipid membranes and protein structure.1 Membranes delimit any cell and all of its compartments. They form natural borders for metabolic substances and signaling molecules. Membrane proteins are the porters and gatekeepers that make sure that only proper molecules or signals make it across the membrane. Since membrane proteins perform numerous key functions in cell metabolism and signaling, they contribute over 30% of the genes in typical eukaryotic genomes,2 and they form the targets for over 50% of drugs in use today.3 The number of elucidated structures of membrane proteins has grown exponentially after the first structure was published in 1985, thus equaling the rate at which structure determination of soluble proteins emerged early on.4 Still, the number of available high-resolution structures of membrane proteins is limited. There are Internet sites that keep track of newly published structures of membrane proteins. The crystallography-oriented Web site of Dr. Stephen White [http://blanco.biomol. uci.edu/mpstruc] has recently been joined by another site maintained by Dr. Dror Warschawski that is dedicated to structures of membrane proteins elucidated by nuclear magnetic resonance (NMR) spectroscopy [www.drorlist. com/nmr/MPNMR.html]. Another equally important site of Dr. Hartmut Michel [www.mpibp-frankfurt.mpg. de/michel/public/memprotstruct.html] with an emphasis on crystallization conditions is no longer updated, but states that access is still enabled. In this review article, we aim to give a general overview of lipid bicelles as employed in the study of protein structure. Recent advances in the field of protein structural biology that have been made possible by exploiting the unique properties of lipid bicelles, in both solution and solid-state NMR spectroscopy, will be discussed. During the last five years, review contributions have presented bicelles either within the far more general context of reconstitution media for solution NMR studies (see section 1.4) or have focused on macroscopically aligned bicelles as used for solid-state NMR studies.5,6 One very recent contribution has tackled the formidable task of reviewing all membrane mimetics employed in both solution and solid-state NMR studies.7 As mentioned above, we will limit the contents of this review article to applications of lipid bicelles, but will cover both the isotropic and the aligned bicelles as used in NMR studies. Some parts of this article can be viewed as an update on the review articles of Opella and Marassi,8 Marcotte and Auger,9 and Prosser et al.10 In addition, some of our own recent research involving bicelles is presented in detail.