Exploiting Bioorthogonal Chemistry to Elucidate Protein-Lipid Binding Interactions and Other Biological Roles of Phospholipids

Lipids play critical roles in a litany of physiological and pathophysiological events, often through the regulation of protein function. These activities are generally difficult to characterize, however, because the membrane environment in which lipids operate is very complex. Moreover, lipids have a diverse range of biological functions, including the recruitment of proteins to membrane surfaces, actions as small-molecule ligands, and covalent protein modification through lipidation. Advancements in the development of bioorthogonal reactions have facilitated the study of lipid activities by providing the ability to selectively label probes bearing bioorthogonal tags within complex biological samples. In this Account, we discuss recent efforts to harness the beneficial properties of bioorthogonal labeling strategies in elucidating lipid function. Initially, we summarize strategies for the design and synthesis of lipid probes bearing bioorthogonal tags. This discussion includes issues to be considered when deciding where to incorporate the tag, particularly the presentation within a membrane environment. We then present examples of the application of these probes to the study of lipid activities, with a particular emphasis on the elucidation of protein lipid binding interactions. One such application involves the development of lipid and membrane microarray analysis as a high-throughput platform for characterizing protein-binding interactions. Here we discuss separate strategies for binding analysis involving the immobilization of either whole liposomes or simplified isolated lipid structures. In addition, we present the different strategies that have been used to derivatize membrane surfaces via bioorthogonal reactions, either by using this chemistry to produce functionalized lipid scaffolds that can be incorporated into membranes or through direct modification of intact membrane surfaces. We then provide an overview of the development of lipid activity probes to label and identify proteins that bind to a particular lipid from complex biological samples. This process involves the strategy of activity-based proteomics, in which proteins are collectively labeled on the basis of function (in this case, ligand binding) rather than abundance. We summarize strategies for designing and applying lipid activity probes that allow for the selective labeling and characterization of protein targets. Additionally, we briefly comment on applications other than studying protein lipid binding. These include the generation of new lipid structures with beneficial properties, labeling of tagged lipids in live cells for studies involving fluorescence imaging, elucidation of covalent protein lipidation, and identification of biosynthetic lipid intermediates. These applications illustrate the early phase of the promising field of applying bioorthogonal chemistry to the study of lipid function.