How the Sum of Its Parts Gets Greater Than the Whole

Nature Methods. Feb, 2008  |  Pubmed ID: 18235433

Micropatterning for Quantitative Analysis of Protein-protein Interactions in Living Cells

Nature Methods. Dec, 2008  |  Pubmed ID: 18997782

We present a method to identify and characterize interactions between a fluorophore-labeled protein ('prey') and a membrane protein ('bait') in live mammalian cells. Cells are plated on micropatterned surfaces functionalized with antibodies to the bait extracellular domain. Bait-prey interactions are assayed through the redistribution of the fluorescent prey. We used the method to characterize the interaction between human CD4, the major co-receptor in T-cell activation, and human Lck, the protein tyrosine kinase essential for early T-cell signaling. We measured equilibrium associations by quantifying Lck redistribution to CD4 micropatterns and studied interaction dynamics by photobleaching experiments and single-molecule imaging. In addition to the known zinc clasp structure, the Lck membrane anchor in particular had a major impact on the Lck-CD4 interaction, mediating direct binding and further stabilizing the interaction of other Lck domains. In total, membrane anchorage increased the interaction lifetime by two orders of magnitude.

Plasma Membrane Fluidity Affects Transient Immobilization of Oxidized Phospholipids in Endocytotic Sites for Subsequent Uptake

The Journal of Biological Chemistry. Jan, 2009  |  Pubmed ID: 19043088

Oxidized phospholipids in serum initiate severe pathophysiological responses during the process of atherogenesis. On the cellular level it is known that these lipids induce apoptosis; however, the uptake mechanism remains enigmatic. We investigated here the behavior of the fluorescent oxidized phospholipid 1-palmitoyl-2-glutaroyl-sn-glycero-3-phospho-N-Alexa647-ethanolamine (PGPE-Alexa647) in the plasma membrane of various cell lines. The probe was taken up by the cells unspecifically via caveolae or clathrin-coated pits. Interestingly, we found the uptake to be facilitated by the overexpression of the scavenger receptor class B type I. Ultra-sensitive microscopy allowed us to follow the uptake process at the single molecule level; we observed rapid diffusion of PGPE-Alexa647 in the plasma membrane, interrupted by transient halts with duration of approximately 0.9 s at endocytotic sites. Scavenger receptor class B type I overexpression yielded a pronounced increase in the single molecule mobility, and in consequence an increased frequency of immobilization. Alternatively, the plasma membrane fluidity could also be increased by treating cells with high levels of the unlabeled oxidized phospholipid 1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphocholine; also in this case, the immobilization frequency of PGPE-Alexa647 was concomitantly increased. The data demonstrate the relevance of plasma membrane properties for uptake of oxidized phospholipids, and indicate a novel indirect mechanism for the control of endocytosis.

TCR-peptide-MHC Interactions in Situ Show Accelerated Kinetics and Increased Affinity

Nature. Feb, 2010  |  Pubmed ID: 20164930

The recognition of foreign antigens by T lymphocytes is essential to most adaptive immune responses. It is driven by specific T-cell antigen receptors (TCRs) binding to antigenic peptide-major histocompatibility complex (pMHC) molecules on other cells. If productive, these interactions promote the formation of an immunological synapse. Here we show that synaptic TCR-pMHC binding dynamics differ significantly from TCR-pMHC binding in solution. We used single-molecule microscopy and fluorescence resonance energy transfer (FRET) between fluorescently tagged TCRs and their cognate pMHC ligands to measure the kinetics of TCR-pMHC binding in situ. When compared with solution measurements, the dissociation of this complex was increased significantly (4-12-fold). Disruption of actin polymers reversed this effect, indicating that cytoskeletal dynamics destabilize this interaction directly or indirectly. Nevertheless, TCR affinity for pMHC was significantly elevated as the result of a large (about 100-fold) increase in the association rate, a likely consequence of complementary molecular orientation and clustering. In helper T cells, the CD4 molecule has been proposed to bind cooperatively with the TCR to the same pMHC complex. However, CD4 blockade had no effect on the synaptic TCR affinity, nor did it destabilize TCR-pMHC complexes, indicating that the TCR binds pMHC independently of CD4.

Temporal Resolution of Protein-protein Interactions in the Live-cell Plasma Membrane

Analytical and Bioanalytical Chemistry. Aug, 2010  |  Pubmed ID: 20574782

We have recently devised a method to quantify interactions between a membrane protein ("bait") and a fluorophore-labeled protein ("prey") directly in the live-cell plasma membrane (Schwarzenbacher et al. Nature Methods 5:1053-1060 2008). The idea is to seed cells on surfaces containing micro-patterned antibodies against the exoplasmic domain of the bait, and monitor the co-patterning of the fluorescent prey via fluorescence microscopy. Here, we characterized the time course of bait and prey micropattern formation upon seeding the cells onto the micro-biochip. Patterns were formed immediately after contact of the cells with the surface. Cells were able to migrate over the chip surface without affecting the micropattern contrast, which remained constant over hours. On single cells, bait contrast may be subject to fluctuations, indicating that the bait can be released from and recaptured on the micropatterns. We conclude that interaction studies can be performed at any time-point ranging from 5 min to several hours post seeding. Monitoring interactions with time opens up the possibility for new assays, which are briefly sketched in the discussion section.

Detection of Protein-protein Interactions in the Live Cell Plasma Membrane by Quantifying Prey Redistribution Upon Bait Micropatterning

Methods in Enzymology. 2010  |  Pubmed ID: 20580963

Our understanding of complex biological systems is based on high-quality proteomics tools for the parallelized detection and quantification of protein interactions. Current screening platforms, however, rely on measuring protein interactions in rather artificial systems, rendering the results difficult to confer on the in vivo situation. We describe here a detailed protocol for the design and the construction of a system to detect and quantify interactions between a fluorophore-labeled protein ("prey") and a membrane protein ("bait") in living cells. Cells are plated on micropatterned surfaces functionalized with antibodies to the bait exoplasmic domain. Bait-prey interactions are assayed via the redistribution of the fluorescent prey. The method is characterized by high sensitivity down to the level of single molecules, the capability to detect weak interactions, and high throughput, making it applicable as a screening tool. The proof-of-concept is demonstrated for the interaction between CD4, a major coreceptor in T-cell signaling, and Lck, a protein tyrosine kinase essential for early T-cell signaling.

Cholesterol Slows Down the Lateral Mobility of an Oxidized Phospholipid in a Supported Lipid Bilayer

Langmuir : the ACS Journal of Surfaces and Colloids. Nov, 2010  |  Pubmed ID: 20942393

We investigated the mobility and phase-partitioning of the fluorescent oxidized phospholipid analogue 1-palmitoyl-2-glutaroyl-sn-glycero-3-phospho-N-Alexa647-ethanolamine (PGPE-Alexa647) in supported lipid bilayers. Compared to the conventional phospholipid dihexadecanoylphosphoethanolamine (DHPE)-Bodipy we found consistently higher diffusion constants. The effect became dramatic when immobile obstacles were inserted into the bilayer, which essentially blocked the diffusion of DHPE-Bodipy but hardly influenced the movements of PGPE-Alexa647. In a supported lipid bilayer made of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), the differences in probe mobility leveled off with increasing cholesterol content. Using coarse-grained molecular dynamics simulations, we could ascribe this effect to increased interactions between the oxidized phospholipid and the membrane matrix, concomitant with a translation in the headgroup position of the oxidized phospholipid: at zero cholesterol content, its headgroup is shifted to the outside of the DOPC headgroup region, whereas increasing cholesterol concentrations pulls the headgroup into the bilayer plane.

Imaging of Mobile Long-lived Nanoplatforms in the Live Cell Plasma Membrane

The Journal of Biological Chemistry. Dec, 2010  |  Pubmed ID: 20966075

The plasma membrane has been hypothesized to contain nanoscopic lipid platforms, which are discussed in the context of "lipid rafts" or "membrane rafts." Based on biochemical and cell biological studies, rafts are believed to play a crucial role in many signaling processes. However, there is currently not much information on their size, shape, stability, surface density, composition, and heterogeneity. We present here a method that allows for the first time the direct imaging of nanoscopic long-lived platforms with raft-like properties diffusing in the live cell plasma membrane. Our method senses these platforms by their property to assemble a characteristic set of fluorescent marker proteins or lipids on a time scale of seconds. A special photobleaching protocol was used to reduce the surface density of labeled mobile platforms down to the level of well isolated diffraction-limited spots without altering the single spot brightness. The statistical distribution of probe molecules per platform was determined by single molecule brightness analysis. For demonstration, we used the consensus raft marker glycosylphosphatidylinositol-anchored monomeric GFP and the fluorescent lipid analog BODIPY-G(M1), which preferentially partitions into liquid-ordered phases. For both markers, we found cholesterol-dependent homo-association in the plasma membrane of living CHO and Jurkat T cells in the resting state, thereby demonstrating the existence of small, mobile, long-lived platforms containing these probes. We further applied the technology to address structural changes in the plasma membrane during fever-type heat shock: at elevated temperatures, the glycosylphosphatidylinositol-anchored monomeric GFP homo-association disappeared, accompanied by an increase in the expression of the small heat shock protein Hsp27.

Cationic Amphipathic Peptides Accumulate Sialylated Proteins and Lipids in the Plasma Membrane of Eukaryotic Host Cells

Biochimica Et Biophysica Acta. Oct, 2011  |  Pubmed ID: 21718688

Cationic antimicrobial peptides (CAMPs) selectively target bacterial membranes by electrostatic interactions with negatively charged lipids. It turned out that for inhibition of microbial growth a high CAMP membrane concentration is required, which can be realized by the incorporation of hydrophobic groups within the peptide. Increasing hydrophobicity, however, reduces the CAMP selectivity for bacterial over eukaryotic host membranes, thereby causing the risk of detrimental side-effects. In this study we addressed how cationic amphipathic peptides-in particular a CAMP with Lysine-Leucine-Lysine repeats (termed KLK)-affect the localization and dynamics of molecules in eukaryotic membranes. We found KLK to selectively inhibit the endocytosis of a subgroup of membrane proteins and lipids by electrostatically interacting with negatively charged sialic acid moieties. Ultrastructural characterization revealed the formation of membrane invaginations representing fission or fusion intermediates, in which the sialylated proteins and lipids were immobilized. Experiments on structurally different cationic amphipathic peptides (KLK, 6-MO-LF11-322 and NK14-2) indicated a cooperation of electrostatic and hydrophobic forces that selectively arrest sialylated membrane constituents.

Membrane-lipid Therapy in Operation: the HSP Co-inducer BGP-15 Activates Stress Signal Transduction Pathways by Remodeling Plasma Membrane Rafts

PloS One. 2011  |  Pubmed ID: 22174906

Aging and pathophysiological conditions are linked to membrane changes which modulate membrane-controlled molecular switches, causing dysregulated heat shock protein (HSP) expression. HSP co-inducer hydroxylamines such as BGP-15 provide advanced therapeutic candidates for many diseases since they preferentially affect stressed cells and are unlikely have major side effects. In the present study in vitro molecular dynamic simulation, experiments with lipid monolayers and in vivo ultrasensitive fluorescence microscopy showed that BGP-15 alters the organization of cholesterol-rich membrane domains. Imaging of nanoscopic long-lived platforms using the raft marker glycosylphosphatidylinositol-anchored monomeric green fluorescent protein diffusing in the live Chinese hamster ovary (CHO) cell plasma membrane demonstrated that BGP-15 prevents the transient structural disintegration of rafts induced by fever-type heat stress. Moreover, BGP-15 was able to remodel cholesterol-enriched lipid platforms reminiscent of those observed earlier following non-lethal heat priming or membrane stress, and were shown to be obligate for the generation and transmission of stress signals. BGP-15 activation of HSP expression in B16-F10 mouse melanoma cells involves the Rac1 signaling cascade in accordance with the previous observation that cholesterol affects the targeting of Rac1 to membranes. Finally, in a human embryonic kidney cell line we demonstrate that BGP-15 is able to inhibit the rapid heat shock factor 1 (HSF1) acetylation monitored during the early phase of heat stress, thereby promoting a prolonged duration of HSF1 binding to heat shock elements. Taken together, our results indicate that BGP-15 has the potential to become a new class of pharmaceuticals for use in 'membrane-lipid therapy' to combat many various protein-misfolding diseases associated with aging.

Detection and Quantification of Biomolecular Association in Living Cells Using Single-Molecule Microscopy

Methods in Enzymology. 2012  |  Pubmed ID: 22289453

During their random motion, biomolecules experience a manifold of interactions that transiently conjoin their paths. It is extremely difficult to measure such binding events directly in the context of a living cell: interactions may be short lived, they may affect only a minority fraction of molecules, or they may not lead to a macroscopically observable effect. We describe here a new single-molecule imaging method that allows for detecting and quantifying associations of mobile molecules. By "thinning out clusters while conserving the stoichiometry of labeling" (TOCCSL), we can virtually dilute the probe directly in the cell, without affecting the fluorescence labeling of single clusters. Essentially, an analysis region is created within the cell by photobleaching; this region is devoid of active probe. Brownian diffusion or other transport processes lead to reentry of active probe into the analysis region. At the onset of the recovery process, single spots can be resolved as well-separated, diffraction-limited signals. Standard single-molecule microscopy then allows for characterizing the spots in terms of their composition and mobility.