Mikhail Merzlyakov

Measuring the energetics of membrane protein dimerization in mammalian membranes.

Thus far, quantitative studies of lateral protein interactions in membranes have been restricted peptides or simplified protein constructs in lipid vesicles or bacterial membranes. Here we show how free energies of membrane protein dimerization can be measured in mammalian plasma membrane-derived vesicles. The measurements, performed in single vesicles, utilize the quantitative imaging FRET (QI-FRET) method. The experiments are described in a step-by-step protocol. The protein characterized is the transmembrane domain of glycophorin A, the most extensively studied membrane protein, known to form homodimers in hydrophobic environments. The results suggest that molecular crowding in cellular membranes has a dramatic effect on the strength of membrane protein interactions.

Energetics of ErbB1 transmembrane domain dimerization in lipid bilayers.

One of the most extensively studied receptor tyrosine kinases is EGFR/ErbB1. Although our knowledge of the role of the extracellular domains and ligands in ErbB1 activation has increased dramatically based on solved domain structures, the exact mechanism of signal transduction across the membrane remains unknown. The transmembrane domains are expected to play an important role in the dimerization process, but the contribution of ErbB1 TM domain to dimer stability is not known, with published results contradicting one another. We address this controversy by showing that ErbB1 TM domain dimerizes in lipid bilayers and by calculating its contribution to stability as -2.5 kcal/mol. The stability calculations use two different methods based on Förster resonance energy transfer, which give the same result. The ErbB1 TM domain contribution to stability exceeds the change in receptor tyrosine kinases dimerization propensities that can convert normal signaling processes into pathogenic processes, and is thus likely important for biological function.

Studies of Receptor Tyrosine Kinase Transmembrane Domain Interactions: The EmEx-FRET Method

The energetics of transmembrane (TM) helix dimerization in membranes and the thermodynamic principles behind receptor tyrosine kinase (RTK) TM domain interactions during signal transduction can be studied using Förster resonance energy transfer (FRET). For instance, FRET studies have yielded the stabilities of wild-type fibroblast growth factor receptor 3 (FGFR3) TM domains and two FGFR3 pathogenic mutants, Ala391Glu and Gly380Arg, in the native bilayer environment. To further our understanding of the molecular mechanisms of deregulated FGFR3 signaling underlying different pathologies, we determined the effect of the Gly382Asp FGFR3 mutation, identified in a multiple myeloma cell line, on the energetics of FGFR3 TM domain dimerization. We measured dimerization energetics using a novel FRET acquisition and processing method, termed “emission-excitation FRET (EmEx-FRET),” which improves the precision of thermodynamic measurements of TM helix association. The EmEx-FRET method, verified here by analyzing previously published data for wild-type FGFR3 TM domain, should have broad utility in studies of protein interactions, particularly in cases when the concentrations of fluorophore-tagged molecules cannot be controlled.

Forster Resonance Energy Transfer Measurements of Transmembrane Helix Dimerization Energetics

Lateral interactions between hydrophobic transmembrane (TM) helices in membranes underlie the folding of multispan membrane proteins and signal transduction by receptor tyrosine kinases (RTKs). Quantitative measurements of dimerization energetics in membranes are required to uncover the physical principles behind these processes. Here, we overview how FRET measurements can be used to determine the thermodynamics of TM helix homo- and heterodimerization in vesicles and in supported bilayers. Such measurements can shed light on the molecular mechanism behind pathologies arising due to single-amino acid mutations in membrane proteins.

Polar residues in transmembrane helices can decrease electrophoretic mobility in polyacrylamide gels without causing helix dimerization

There are only a few available methods to study lateral interactions and self assembly of transmembrane helices. One of the most frequently used methods is sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) which can report on strong interactions between peptides in SDS solution. Here we offer a cautionary tale about studying the folding and assembly of membrane proteins using peptides and SDS-PAGE experiments as a membrane mimetic system. At least for the specific peptide and detergent systems studied here, we show that a polar asparagine residue in the 12th position of an otherwise hydrophobic helical segment of 20 amino acids causes a peptide to migrate on SDS-PAGE gels with an apparent molecular weight that is twice its true molecular weight, suggesting dimerization. However when examined carefully in SDS solutions and in situ in the polyacrylamide gel itself using Forster resonance energy transfer no interaction can be detected. Instead we show evidence suggesting that differential interactions between peptide and detergent drive the differences in electrophoretic mobility without any interaction between peptides. These results emphasize the need to apply multiple independent techniques to the study of membrane protein folding, and they highlight the usefulness of studying folding and structure of membrane proteins in lipid membranes rather than in detergents.

Impedance spectroscopy of bilayer membranes on single crystal silicon

Bilayer membranes on solid supports are being developed as electrically addressable, robust, surface-supported membrane mimetics. These platforms are being explored for basic ion channel research as well as for detection and analyte sensing. The formation of bilayer membranes on semiconductor surfaces is an important step in device integration for transistor and sensor arrays. Here, the authors review the contributions to the impedance response of bilayer membranes on semiconductors, and highlight the important issues for experimental measurements. The authors also present experimental results for diphytanoyl phosphocholine bilayers formed on moderately doped and highly doped n-type silicon using Langmuir-Blodgett-based deposition techniques. The authors demonstrate that a detailed understanding of the contributions to the impedance response is important in developing silicon-based membrane platforms. The authors further report on the bias dependence of the impedance, and show that on highly doped n-type silicon, the membrane impedance can be measured over a 2 V range.

Hill Coefficient Analysis of Transmembrane Helix Dimerization

Here, we employed the Hill equation, used broadly to characterize cooperativity in protein–ligand binding, to describe the dimerization of transmembrane (TM) helices in hydrophobic environments. The Hill analysis of wild-type fibroblast growth factor receptor 3 (FGFR3) TM domain dimerization gives a Hill coefficient of ~1 for lipid bilayers but only ~0.2 for sodium dodecyl sulfate (SDS) micelles. We propose that this finding is indicative of heterogeneity in FGFR3 TM dimer structure and stability in SDS micelles. We further speculate that (1) the Hill equation can be used as a tool to assess the existence of multiple structural states of TM dimers in different hydrophobic environments and (2) the structural heterogeneity, detectable by Hill analysis, may be the underlying reason for the broad peaks and the low resolution NMR studies of peptides in detergents.

Utility of surface-supported bilayers in studies of transmembrane helix dimerization

This review focuses on the methods that are available to study transmembrane (TM) helix dimerization in membrane-like environments (either bacterial membranes or lipid bilayers, as mimics of the eukaryotic cellular membrane), with an emphasis on the utility of surface-supported bilayers in such studies.

Surface supported bilayer platform for studies of lateral association of proteins in membranes (Mini Review)

Here, the authors review how surface supported bilayers can be engineered and how Förster resonance energy transfer (FRET) can be used to quantify interactions between transmembrane peptides in these bilayers. The requirements for the surface supported platform are (1) lateral mobility of the peptides, (2) transmembrane orientation of the peptides, and (3) capabilities for FRET measurements. To satisfy these requirements, a new assembly method, termed “directed assembly” was developed. This assembly method could have broad utility in basic studies of proteins in membranes and in biotechnological applications.