Synthesis and biochemical characterization of EGF receptor in a water-soluble membrane model system

Lipid and Protein Transfer between Nanolipoprotein Particles and Supported Lipid Bilayers

A nanolipoprotein particle (NLP) is a lipid bilayer disc stabilized by two amphipathic "scaffold" apolipoproteins. It has been most notably utilized as a tool for solubilizing a variety of membrane proteins while preserving structural and functional properties. Transfer of functional proteins from NLPs into model membrane systems such as supported lipid bilayers (SLBs) would enable new opportunities, for example, two-dimensional protein crystallization and studies on protein-protein interactions. This work used fluorescence microscopy and atomic force microscopy to investigate the interaction between NLPs and SLBs. When incubated with SLBs, NLPs were found to spontaneously deliver lipid and protein cargo. The impact of membrane composition on lipid exchange was explored, revealing a positive correlation between the magnitude of lipid transfer and concentration of defects in the target SLB. Incorporation of lipids capable of binding specifically to polyhistidine tags encoded into the apolipoproteins also boosted transfer of NLP cargo. Optimal conditions for lipid and protein delivery from NLPs to SLBs are proposed based on interaction mechanisms.

Cell-Free Co-Translational Approaches for Producing Mammalian Receptors: Expanding the Cell-Free Expression Toolbox Using Nanolipoproteins

Membranes proteins make up more than 60% of current drug targets and account for approximately 30% or more of the cellular proteome. Access to this important class of proteins has been difficult due to their inherent insolubility and tendency to aggregate in aqueous solutions. Understanding membrane protein structure and function demands novel means of membrane protein production that preserve both their native conformational state as well as function. Over the last decade, cell-free expression systems have emerged as an important complement to cell-based expression of membrane proteins due to their simple and customizable experimental parameters. One approach to overcome the solubility and stability limitations of purified membrane proteins is to support them in stable, native-like states within nanolipoprotein particles (NLPs), aka nanodiscs. This has become common practice to facilitate biochemical and biophysical characterization of proteins of interest. NLP technology can be easily coupled with cell-free systems to achieve functional membrane protein production for this purpose. Our approach involves utilizing cell-free expression systems in the presence of NLPs or using co-translation techniques to perform one-pot expression and self-assembly of membrane protein/NLP complexes. We describe how cell-free reactions can be modified to render control over nanoparticle size and monodispersity in support of membrane protein production. These modifications have been exploited to facilitate co-expression of full-length functional membrane proteins such as G-protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). In particular, we summarize the state of the art in NLP-assisted cell-free coexpression of these important classes of membrane proteins as well as evaluate the advances in and prospects for this technology that will drive drug discovery against these targets. We conclude with a prospective on the use of NLPs to produce as well as deliver functional mammalian membrane-bound proteins for a range of applications.

Single-Molecule Fluorescence Detection of the Epidermal Growth Factor Receptor in Membrane Discs

The epidermal growth factor receptor (EGFR) is critical to normal cellular signaling pathways. Moreover, it has been implicated in a range of pathologies, including cancer. As a result, it is the primary target of many anticancer drugs. One limitation to the design and development of these drugs has been the lack of molecular-level information about the interactions and conformational dynamics of EGFR. To overcome this limitation, this work reports the construction and characterization of functional, fluorescently labeled, and full-length EGFR in model membrane nanolipoprotein particles (NLPs) for in vitro fluorescence studies. To demonstrate the utility of the system, we investigate ATP-EGFR interactions. We observe that ATP binds at the catalytic site providing a means to measure a range of distances between the catalytic site and the C-terminus via Förster resonance energy transfer (FRET). These ATP-based experiments suggest a range of conformations of the C-terminus that may be a function of the phosphorylation state for EGFR. This work is a proof-of-principle demonstration of single-molecule studies as a noncrystallographic assay for EGFR interactions in real-time and under near-physiological conditions. The diverse nature of EGFR interactions means that new tools at the molecular level have the potential to significantly enhance our understanding of receptor pathology and are of utmost importance for cancer-related drug discovery.