Large deletions in the cytoplasmic kinase domain of the epidermal growth factor receptor do not affect its laternal mobility

Apoptosis-inducing anti-HER2 agents operate through oligomerization-induced receptor immobilization

Overexpression of the receptor tyrosine kinase HER2 plays a critical role in the development of various tumors. Biparatopic designed ankyrin repeat proteins (bipDARPins) potently induce apoptosis in HER2-addicted breast cancer cell lines. Here, we have investigated how the spatiotemporal receptor organization at the cell surface is modulated by these agents and is distinguished from other molecules, which do not elicit apoptosis. Binding of conventional antibodies is accompanied by moderate reduction of receptor mobility, in agreement with HER2 being dimerized by the bivalent IgG. In contrast, the most potent apoptosis-inducing bipDARPins lead to a dramatic arrest of HER2. Dual-color single-molecule tracking revealed that the HER2 "lockdown" by these bipDARPins is caused by the formation of HER2-DARPin oligomer chains, which are trapped in nanoscopic membrane domains. Our findings establish that efficient neutralization of receptor tyrosine kinase signaling can be achieved through intermolecular bipDARPin crosslinking alone, resulting in inactivated, locked-down bipDARPin-HER2 complexes.

Kinetics of epidermal growth factor/receptor binding on cells measured by total internal reflection/fluorescence recovery after photobleaching

Total internal reflection fluorescence (TIRF) microscopy is used to measure the dissociation kinetic rate of fluorescein-labeled epidermal growth factor from its specific receptors on the surface of intact but mildly fixed A431 human epidermoid cells in culture. Prior applications of TIRF microscopy have been limited to nonreceptor binding or to model membrane systems. The evanescent field excites fluorescence selectively at the surface of the cell proximal to the coverslip. "Prismless" epiillumination TIR is employed to avoid space limitations and is achieved by passing the excitation laser beam through a high (1.4)-aperture objective so that the light is incident at the glass/water interface beyond the critical angle. Long-term focus is maintained by a special feedback system. Of the possible effects that can influence the time course of the postbleach fluorescence recoveries-the EGF/receptor dissociation ratek 2, the bulk solution diffusion rate of EGF, and the cell surface motion of the receptors-we infer that the dissociation ratek 2 dominates. Several fitting schemes are compared and indicate the presence of a multiplicity of values fork 2, ranging from about 0.05 to 0.004 s(-1), with an average value of about 0.012 s(-1). These results compare well with values previously obtained by radiolabel/washing techniques. The significance of the results in terms of kinetic models and the advantages of the TIRF technique for these sorts of measurements are discussed.

A mechanistic role for polypeptide hormone receptor lateral mobility in signal transduction

Lateral diffusion of membrane-integral receptors within the plane of the membrane has been postulated to be mechanistically important for signal transduction. Direct measurement of polypeptide hormone receptor lateral mobility using fluorescence photobleaching recovery techniques indicates that tyrosine kinase receptors are largely immobile at physiological temperatures. This is presumably due to their signal transduction mechanism which requires intermolecular autophosphorylation through receptor dimerization and thus immobilization for activation. In contrast, G-protein coupled receptors must interact with other membrane components to effect signal transduction, and consistent with this, the phospholipase C-activating vasopressin V1- and adenylate cyclase activating V2-receptors are highly laterally mobile at 37°C. Modulation of the V2-receptor mobile fraction (f) has demonstrated a direct correlation between f and receptor-agonist-dependent maximal cAMP productionin vivo at 37°C. This indicates that f is a key parameter in hormone signal transduction especially at physiological hormone concentrations, consistent with mobile receptors being required to effect V2-agonist-dependent activation of G-proteins. Measurements using a V2-specific antagonist show that antagonist-occupied receptors are highly mobile at 37°C, indicating that receptor immobilization is not the basis of antagonism. In contrast to agonist-occupied receptor however, antagonistoccupied receptors are not immobilized prior to endocytosis and down-regulation. Receptors may thus be freely mobile in the absence of agonistic ligand; stimulation by hormone agonist results in receptor association with other proteins, probably including cytoskeletal components, and immobilization. Receptor immobilization may be one of the important steps of desensitization subsequent to agonistic stimulation, through terminating receptor lateral movement which is instrumental in generating and amplifying the initial stimulatory signal within the plane of the membrane.

Hydrophobic fluorescent probes introduce artifacts into single molecule tracking experiments due to non-specific binding

Single-molecule techniques are powerful tools to investigate the structure and dynamics of macromolecular complexes; however, data quality can suffer because of weak specific signal, background noise and dye bleaching and blinking. It is less well-known, but equally important, that non-specific binding of probe to substrates results in a large number of immobile fluorescent molecules, introducing significant artifacts in live cell experiments. Following from our previous work in which we investigated glass coating substrates and demonstrated that the main contribution to this non-specific probe adhesion comes from the dye, we carried out a systematic investigation of how different dye chemistries influence the behaviour of spectrally similar fluorescent probes. Single-molecule brightness, bleaching and probe mobility on the surface of live breast cancer cells cultured on a non-adhesive substrate were assessed for anti-EGFR affibody conjugates with 14 different dyes from 5 different manufacturers, belonging to 3 spectrally homogeneous bands (491 nm, 561 nm and 638 nm laser lines excitation). Our results indicate that, as well as influencing their photophysical properties, dye chemistry has a strong influence on the propensity of dye-protein conjugates to adhere non-specifically to the substrate. In particular, hydrophobicity has a strong influence on interactions with the substrate, with hydrophobic dyes showing much greater levels of binding. Crucially, high levels of non-specific substrate binding result in calculated diffusion coefficients significantly lower than the true values. We conclude that the physic-chemical properties of the dyes should be considered carefully when planning single-molecule experiments. Favourable dye characteristics such as photostability and brightness can be offset by the propensity of a conjugate for non-specific adhesion.

Characterization of the diffusion of epidermal growth factor receptor clusters by single particle tracking

A number of studies have shown that receptors of the epidermal growth factor receptor family (ErbBs) exist as higher-order oligomers (clusters) in cell membranes in addition to their monomeric and dimeric forms. Characterizing the lateral diffusion of such clusters may provide insights into their dynamics and help elucidate their functional relevance. To that end, we used single particle tracking to study the diffusion of clusters of the epidermal growth factor (EGF) receptor (EGFR; ErbB1) containing bound fluorescently-labeled ligand, EGF. EGFR clusters had a median diffusivity of 6.8×10(-11)cm(2)/s and were found to exhibit different modes of transport (immobile, simple, confined, and directed) similar to that previously reported for single EGFR molecules. Disruption of actin filaments increased the median diffusivity of EGFR clusters to 10.3×10(-11)cm(2)/s, while preserving the different modes of diffusion. Interestingly, disruption of microtubules rendered EGFR clusters nearly immobile. Our data suggests that microtubules may play an important role in the diffusion of EGFR clusters either directly or perhaps indirectly via other mechanisms. To our knowledge, this is the first report probing the effect of the cytoskeleton on the diffusion of EGFR clusters in the membranes of live cells.

Mapping molecular diffusion in the plasma membrane by Multiple-Target Tracing (MTT)

Our goal is to obtain a comprehensive description of molecular processes occurring at cellular membranes in different biological functions. We aim at characterizing the complex organization and dynamics of the plasma membrane at single-molecule level, by developing analytic tools dedicated to Single-Particle Tracking (SPT) at high density: Multiple-Target Tracing (MTT). Single-molecule videomicroscopy, offering millisecond and nanometric resolution, allows a detailed representation of membrane organization by accurately mapping descriptors such as cell receptors localization, mobility, confinement or interactions. We revisited SPT, both experimentally and algorithmically. Experimental aspects included optimizing setup and cell labeling, with a particular emphasis on reaching the highest possible labeling density, in order to provide a dynamic snapshot of molecular dynamics as it occurs within the membrane. Algorithmic issues concerned each step used for rebuilding trajectories: peaks detection, estimation and reconnection, addressed by specific tools from image analysis. Implementing deflation after detection allows rescuing peaks initially hidden by neighboring, stronger peaks. Of note, improving detection directly impacts reconnection, by reducing gaps within trajectories. Performances have been evaluated using Monte-Carlo simulations for various labeling density and noise values, which typically represent the two major limitations for parallel measurements at high spatiotemporal resolution. The nanometric accuracy obtained for single molecules, using either successive on/off photoswitching or non-linear optics, can deliver exhaustive observations. This is the basis of nanoscopy methods such as STORM, PALM, RESOLFT or STED, which may often require imaging fixed samples. The central task is the detection and estimation of diffraction-limited peaks emanating from single-molecules. Hence, providing adequate assumptions such as handling a constant positional accuracy instead of Brownian motion, MTT is straightforwardly suited for nanoscopic analyses. Furthermore, MTT can fundamentally be used at any scale: not only for molecules, but also for cells or animals, for instance. Hence, MTT is a powerful tracking algorithm that finds applications at molecular and cellular scales.

ErbB1 dimerization is promoted by domain co-confinement and stabilized by ligand binding

The extent to which ligand occupancy and dimerization contribute to erbB1 signaling is controversial. To examine this, we utilized two-color Quantum Dot tracking for visualization of erbB1 homodimerization and quantification of the dimer off rate (k_off) on living cells. Kinetic parameters were extracted using a 3-state Hidden Markov Model to identify transition rates between free, co-confined, and dimerized states. We report that dimers composed of 2 ligand-bound receptors are long-lived and their k_off is independent of kinase activity. By comparison, unliganded dimers have >4-fold faster k_off. Transient co-confinement of receptors promotes repeated encounters and enhances dimer formation. Mobility decreases >6-fold when ligand-bound receptors dimerize. Blockade of erbB1 kinase activity or disruption of actin networks results in faster diffusion of receptor dimers. These results implicate both signal propagation and the cortical cytoskeleton in reduced mobility of signaling-competent erbB1 dimers.

Studies of distribution, location and dynamic properties of EGFR on the cell surface measured by image correlation spectroscopy

In this work, we have studied the distribution and dynamic properties of Epidermal Growth Factor (EGF) receptors in the plasma membrane of fixed and live cells as well as the extent of co-localization of this transmembrane protein with proteins specific for three-membrane microdomains: membrane rafts, caveolae and clathrin-coated pits. This was achieved using a family of image-processing tools called image correlation spectroscopy (ICS), image cross-correlation spectroscopy (ICCS) and dynamic image correlation spectroscopy (DICS). Our results indicate that EGFR is diffusely distributed on the cell surface at 37 degrees C and aggregates as the temperature is lowered to 4 degrees C. This aggregation takes place within 15 min and is reversible. Changes in temperature also affect the diffusion of EGFR by two orders of magnitude. The dynamic properties of EGFR are similar to the dynamic properties of a GPI-anchored protein known to be present in membrane rafts, which motivated us to explore the extent of co-localization of EGFR with this membrane raft protein using ICCS. Our results indicate that more than half of the EGFR population is present in membrane rafts and smaller percentages are present in caveolae and clathrin-coated pits.

Plasma membrane domain organization regulates EGFR signaling in tumor cells

Macromolecular complexes exhibit reduced diffusion in biological membranes; however, the physiological consequences of this characteristic of plasma membrane domain organization remain elusive. We report that competition between the galectin lattice and oligomerized caveolin-1 microdomains for epidermal growth factor (EGF) receptor (EGFR) recruitment regulates EGFR signaling in tumor cells. In mammary tumor cells deficient for Golgi β1,6N-acetylglucosaminyltransferase V (Mgat5), a reduction in EGFR binding to the galectin lattice allows an increased association with stable caveolin-1 cell surface microdomains that suppresses EGFR signaling. Depletion of caveolin-1 enhances EGFR diffusion, responsiveness to EGF, and relieves Mgat5 deficiency–imposed restrictions on tumor cell growth. In Mgat5^+/+ tumor cells, EGFR association with the galectin lattice reduces first-order EGFR diffusion rates and promotes receptor interaction with the actin cytoskeleton. Importantly, EGFR association with the lattice opposes sequestration by caveolin-1, overriding its negative regulation of EGFR diffusion and signaling. Therefore, caveolin-1 is a conditional tumor suppressor whose loss is advantageous when β1,6GlcNAc-branched N-glycans are below a threshold for optimal galectin lattice formation.

Physical and Functional Interactions of the Cytomegalovirus US6 Glycoprotein with the Transporter Associated with Antigen Processing

The endoplasmic reticulum-resident human cytomegalovirus glycoprotein US6 (gpUS6) inhibits peptide translocation by the transporter associated with antigen processing (TAP) to prevent loading of major histocompatibility complex class I molecules and antigen presentation to CD8+ T cells. TAP is formed by two subunits, TAP1 and TAP2, each containing one multispanning transmembrane domain (TMD) and a cytosolic nucleotide binding domain. Here we reported that the blockade of TAP by gpUS6 is species-restricted, i.e. gpUS6 inhibits human TAP but not rat TAP. Co-expression of human and rat subunits of TAP demonstrates independent binding of gpUS6 to human TAP1 and TAP2, whereas gpUS6 does not bind to rat TAP subunits. gpUS6 associates with preformed TAP1/2 heterodimers but not with unassembled TAP subunits. To locate domains of TAP required for gpUS6 binding and function, we took advantage of reciprocal human/rat intrachain TAP chimeras. Each TAP subunit forms two contact sites within its TMD interacting with gpUS6. The dominant gpUS6-binding site on TAP2 maps to an N-terminal loop, whereas inhibition of peptide transport is mediated by a C-terminal loop of the TMD. For TAP1, two gpUS6 binding domains are formed by loops of the C-terminal TMD. The domain required for TAP inactivation is built by a distal loop of the C-terminal TMD, indicating a topology of TAP1 comprising 10 endoplasmic reticulum transmembrane segments. By forming multimeric complexes, gpUS6 reaches the distant target domains to arrest peptide transport. The data revealed a nonanalogous multipolar bridging of the TAP TMDs by gpUS6.

Factors controlling fibroblast growth factor receptor-1's cytoplasmic trafficking and its regulation as revealed by FRAP analysis

Biochemical and microscopic studies have indicated that FGFR1 is a transmembrane and soluble protein present in the cytosol and nucleus. How FGFR1 enters the cytosol and subsequently the nucleus to control cell development and associated gene activities has become a compelling question. Analyses of protein synthesis, cytoplasmic subcompartmental distribution and movement of FGFR1-EGFP and FGFR1 mutants showed that FGFR1 exists as three separate populations (a) a newly synthesized, highly mobile, nonglycosylated, cytosolic receptor that is depleted by brefeldin A and resides outside the ER-Golgi lumen, (b) a slowly diffusing membrane receptor population, and (c) an immobile membrane pool increased by brefeldin A. RSK1 increases the highly mobile cytosolic FGFR1 population and its overall diffusion rate leading to increased FGFR1 nuclear accumulation, which coaccumulates with RSK1. A model is proposed in which newly synthesized FGFR1 can enter the (a) "nuclear pathway," where the nonglycosylated receptor is extruded from the pre-Golgi producing highly mobile cytosolic receptor molecules that rapidly accumulate in the nucleus or (b) "membrane pathway," in which FGFR1 is processed through the Golgi, where its movement is spatially restricted to trans-Golgi membranes with limited lateral mobility. Entrance into the nuclear pathway is favored by FGFR1's interaction with kinase active RSK1.

Cholesterol dictates the freedom of EGF receptors and HER2 in the plane of the membrane

The flow of information through the epidermal growth factor receptor (EGFR) is shaped by molecular interactions in the plasma membrane. The EGFR is associated with lipid rafts, but their role in modulating receptor mobility and subsequent interactions is unclear. To investigate the role of nanoscale rafts in EGFR dynamics, we used single-molecule fluorescence imaging to track individual receptors and their dimerization partner, human epidermal growth factor receptor 2 (HER2), in the membrane of human mammary epithelial cells. We found that the motion of both receptors was interrupted by dwellings within nanodomains. EGFR was significantly less mobile than HER2. This difference was likely due to F-actin because its depolymerization led to similar diffusion patterns between the EGFR and HER2. Manipulations of membrane cholesterol content dramatically altered the diffusion pattern of both receptors. Cholesterol depletion led to almost complete confinement of the receptors, whereas cholesterol enrichment extended the boundaries of the restricted areas. Interestingly, F-actin depolymerization partially restored receptor mobility in cholesterol-depleted membranes. Our observations suggest that membrane cholesterol provides a dynamic environment that facilitates the free motion of EGFR and HER2, possibly by modulating the dynamic state of F-actin. The association of the receptors with lipid rafts could therefore promote their rapid interactions only upon ligand stimulation.

The lateral mobility of NHE3 on the apical membrane of renal epithelial OK cells is limited by the PDZ domain proteins NHERF1/2, but is dependent on an intact actin cytoskeleton as determined by FRAP

The epithelial brush border (BB) Na+/H+ exchanger, NHE3, plays a major role in transcellular Na+ absorption in the renal proximal tubule. NHE3 activity is rapidly regulated by neurohumoral substances and growth factors via changes in its amount on the BB by a process partially involving vesicle trafficking. The PDZ domain-containing proteins, NHERF1/2, are scaffold proteins that link NHE3 to the actin cytoskeleton via their binding to both ezrin and NHE3. NHERF1/2 interact with both an internal C-terminal domain of NHE3 and the N-terminus of ezrin. We used fluorescence recovery after photobleaching (FRAP) to study the effect of NHERF1/2 on NHE3 mobility in the brush border of opossum kidney (OK) proximal tubule cells. A confocal microscope was used to allow the selective study of apical membrane versus intracellular NHE3. A chimera of NHE3-EGFP was transiently expressed in OK cells and its lateral diffusion in the apical membrane was measured with FRAP and confocal microscopy at 37 degrees C. The contribution of intracellular NHE3-EGFP to recovery on the OK surface not directly over the juxtanuclear area (non-JN) was negligible as exposure to the water soluble crosslinker BS3 (10 mM) at 4 degrees C resulted in no recovery of this component of surface NHE3-EGFP after photobleaching. The mobile fraction (Mf) of apical NHE3-EGFP was 47.5+/-2.2%; the effective diffusion coefficient (Deff) was (2.2+/-0.3) x10(-10) cm2/second. Overexpression of NHERF2 in OK cells decreased the Mf to 29.1+/-3.1% without changing Deff. In the truncation mutant, NHE3585-EGFP (aa 1-585), which lacks the NHERF1/2 binding domain, Mf increased to 66.4+/-2.2%, with no change in Deff, whereas NHE3660-EGFP, which binds NHERF1/2, had Mf (48.3+/-3.0%) and Deff both similar to full-length NHE3. These results are consistent with the PDZ domain proteins NHERF1 and NHERF2 scaffolding NHE3 in macromolecular complexes in the apical membrane of OK cells under basal conditions, which limits the lateral mobility of NHE3. It is probable that this is one of the mechanisms by which NHERF1/2 affects rapid regulation of NHE3 by growth factors and neurohumoral mediators. By contrast, disrupting the actin cytoskeleton by latrunculin B treatment (0.05 microM, 30 minutes) reduced the NHE3 Mf (21.9+/-4.5%) without altering the Deff. Therefore the actin cytoskeleton, independently of NHERF1/2 binding, is necessary for apical membrane mobility of NHE3.

Membrane lateral mobility obstructed by polymer-tethered lipids studied at the single molecule level

Obstructed long-range lateral diffusion of phospholipids (TRITC-DHPE) and membrane proteins (bacteriorhodopsin) in a planar polymer-tethered 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayer is studied using wide-field single molecule fluorescence microscopy. The obstacles are well-controlled concentrations of hydrophobic lipid-mimicking dioctadecylamine moieties in the polymer-exposed monolayer of the model membrane. Diffusion of both types of tracer molecules is well described by a percolating system with different percolation thresholds for lipids and proteins. Data analysis using a free area model of obstructed lipid diffusion indicates that phospholipids and tethered lipids interact via hard-core repulsion. A comparison to Monte Carlo lattice calculations reveals that tethered lipids act as immobile obstacles, are randomly distributed, and do not self-assemble into large-scale aggregates for low to moderate tethering concentrations. A procedure is presented to identify anomalous subdiffusion from tracking data at a single time lag. From the analysis of the cumulative distribution function of the square displacements, it was found that TRITC-DHPE and W80i show normal diffusion at lower concentrations of tethered lipids and anomalous diffusion at higher ones. This study may help improve our understanding of how lipids and proteins in biomembranes may be obstructed by very small obstacles comprising only one or very few molecules.

Lateral propagation of EGF signaling after local stimulation is dependent on receptor density

We analyzed lateral propagation of epidermal growth factor (EGF) signaling in single live COS cells following local stimulation, achieved by the use of laminar flows containing rhodamine-labeled EGF. The spatiotemporal pattern of EGF signaling was visualized by fluorescent indicators for Ras activation and tyrosine phosphorylation. Contrary to the findings in previous reports, both signals were localized to the stimulated regions in control COS cells expressing EGF receptor at the basal level. However, the signals spread over the entire cell when EGF receptors were overexpressed or when receptor/ligand endocytosis was blocked. We thus present evidence that ligand-independent propagation of EGF signaling occurs only when the receptor density on the plasma membrane is high, such as in carcinoma cells.

From fixed to FRAP: measuring protein mobility and activity in living cells

Experiments with fluorescence recovery after photobleaching (FRAP) started 30 years ago to visualize the lateral mobility and dynamics of fluorescent proteins in living cells. Its popularity increased when non-invasive fluorescent tagging became possible with the green fluorescent protein (GFP). Many researchers use GFP to study the localization of fusion proteins in fixed or living cells, but the same fluorescent proteins can also be used to study protein mobility in living cells. Here we review the potential of FRAP to study protein dynamics and activity within a single living cell. These measurements can be made with most standard confocal laser-scanning microscopes equipped with photobleaching protocols.

The major substrates for TAP in vivo are derived from newly synthesized proteins

The transporter associated with antigen processing (TAP) is a member of the family of ABC transporters that translocate a large variety of substrates across membranes. TAP transports peptides from the cytosol into the endoplasmic reticulum for binding to MHC class I molecules and for subsequent presentation to the immune system. Here we follow the lateral mobility of TAP in living cells. TAP's mobility increases when it is inactive and decreases when it translocates peptides. Because TAP activity is dependent on substrate, the mobility of TAP is used to monitor the intracellular peptide content in vivo. Comparison of the diffusion rates in peptide-free and peptide-saturated cells indicates that normally about one-third of all TAP molecules actively translocate peptides. However, during an acute influenza infection TAP becomes fully employed owing to the production and degradation of viral proteins. Furthermore, TAP activity depends on continuing protein translation. This implies that MHC class I molecules mainly sample peptides that originate from newly synthesized proteins, to ensure rapid presentation to the immune system.

Head-head/tail-tail relative orientation of the pore-forming domains of the heterodimeric ABC transporter TAP

BACKGROUND

The transporter associated with antigen processing (TAP) is a heterodimeric member of the large family of ABC transporters. The study of interactions between the subunits TAP1 and TAP2 can reveal the relative orientation of the transmembrane segments, which form a translocation pore for peptides. This is essential for understanding the architecture of TAP and other ABC transporters.

RESULTS

The amino-terminal six transmembrane segments (TMs) of human TAP1, TAP1 (1-6), and the amino-terminal five TMs of TAP2, TAP2(1-5), are thought to constitute the pore of TAP. Two new approaches are used to define dimer interactions. We show that TM6 of TAP1 (1-6) is able to change topology post-translationally. This TM, along with a cytoplasmic tail, is translocated into the endoplasmic reticulum lumen, unless TAP2 is expressed. Coexpression of TM(4-5) of TAP2 stabilizes the topology of TAP1 (1-6), even when the TM1 of TAP1 is subsitituted with another sequence. This suggests that the carboxy-terminal TMs of the pore-forming domains TAP1 (1-6) and TAP2(1-5) interact. An alternative assay uses photobleaching in living cells using TAP1 (1-6) tagged with the green fluorescent protein (GFP). Coexpression with TAP2(1-5) results in reduced movement of the heterodimer within the endoplasmic reticulum membrane, as compared with the single TAP1 (1-6) molecule. In contrast, TAP2(1-4) has no effect on the mobility of TAP1 (1-6)-GFP, indicating the importance of TM5 of TAP2 for dimer formation. Also, TM1 of both TAP1 and TAP2 is essential for formation of a complex with low mobility.

CONCLUSIONS

Dimerization of the pore-forming transmembrane domains of TAP1 (TM1-6) with its TAP2 counterpart (TM1-5) prevents the post-translational translocation of TM6 of TAP1 and results in a complex with reduced mobility within the endoplasmic reticulum membrane compared with the free subunit. These techniques are used to show that the pore-forming domains of TAP are aligned in a head-head/tail-tail orientation. This positions the following peptide-binding segments of the two TAP subunits to one side of the pore.

Rapid characterization of green fluorescent protein fusion proteins on the molecular and cellular level by fluorescence correlation microscopy

Fluorescence correlation microscopy (FCM) was applied to characterize fusion proteins of the green fluorescent protein (GFP) on the cellular as well as molecular level within seconds in an integrated instrument. FCM combines the inherent sensitivity and high spatial resolution of fluorescence correlation spectroscopy with fluorescence imaging and micropositioning, thereby providing a spectrum of molecular information in the cellular context. Signatures of characteristic parameters derived from the autocorrelation functions served to distinguish a GFP fusion protein of the epidermal growth factor receptor from GFP fluorescence in the endoplasmic reticulum and cytoplasm. Diffusion constants measured for free transiently expressed GFP reproduced values reported previously with other techniques. The accessible concentration range extends from millions to only a few thousand molecules per cell, with single molecule detectability in the femtoliter detection volume. The detailed molecular characterization offered by FCM is fully compatible with automation in sample identification and detection, offering new possibilities for highly integrated high-throughput screening.

Interleukin 1 (IL-1) causes changes in lateral and rotational mobilities of IL-1 type I receptors

To investigate IL-1-dependent interactions of IL-1 type I (IL-1 RI) receptors on intact cells, lateral and rotational mobilities and detergent insolubility were investigated. Lateral mobility was measured by fluorescence photobleaching recovery, using a Cy3-modified, noncompetitive mAb specific for IL-1RI (M5) bound to wild-type IL-1 RI or mutant IL-1 RI with a truncated cytoplasmic tail. Addition of IL-1 causes significant reduction in the mobile fraction of wild-type IL-1 RI for two different transfected cell lines. For the mutant IL-1 RI, no significant decrease in response to IL-1 is observed, indicating that the missing cytoplasmic segment is involved in IL-1-dependent interactions of IL-1 RI that lead to reduced lateral mobility on the cell surface. The rotational mobility of IL-1 RI was assessed with phosphorescence anisotropy decay measurements using erythrosin-labeled M5. IL-1 decreases the rotational mobility of cell surface IL-1 RI on the microsecond time scale and also increases the initial anisotropy, indicating loss in segmental motion. Measurements of resistance to solubilization by Triton X-100 showed that IL-1 binding increases the fraction of IL-1 RI sedimenting with cytoskeletal residues. The IL-1 receptor antagonist protein (IL-1ra) causes partial effects in reducing rotational mobility and increasing detergent insolubility of M5-lableled IL-1 RI, indicating that this ligand causes structural changes in the presence of the dimerizing M5 mAb. These ligand-dependent physical interactions of IL-1 RI on the cell surface may be related to signal initiation by this receptor.

Mobility of cytochrome P450 in the endoplasmic reticulum membrane

Cytochrome P450 2C2 is a resident endoplasmic reticulum (ER) membrane protein that is excluded from the recycling pathway and contains redundant retention functions in its N-terminal transmembrane signal/anchor sequence and its large, cytoplasmic domain. Unlike some ER resident proteins, cytochrome P450 2C2 does not contain any known retention/retrieval signals. One hypothesis to explain exclusion of resident ER proteins from the transport pathway is the formation of networks by interaction with other proteins that immobilize the proteins and are incompatible with packaging into the transport vesicles. To determine the mobility of cytochrome P450 in the ER membrane, chimeric proteins of either cytochrome P450 2C2, its catalytic domain, or the cytochrome P450 2C1 N-terminal signal/anchor sequence fused to green fluorescent protein (GFP) were expressed in transiently transfected COS1 cells. The laurate hydroxylase activities of cytochrome P450 2C2 or the catalytic domain with GFP fused to the C terminus were similar to the native enzyme. The mobilities of the proteins in the membrane were determined by recovery of fluorescence after photobleaching. Diffusion coefficients for all P450 chimeras were similar, ranging from 2.6 to 6.2 x 10(-10) cm2/s. A coefficient only slightly larger (7.1 x 10(-10) cm2/s) was determined for a GFP chimera that contained a C-terminal dilysine ER retention signal and entered the recycling pathway. These data indicate that exclusion of cytochrome P450 from the recycling pathway is not mediated by immobilization in large protein complexes.

Mobility of TrkA is regulated by phosphorylation and interactions with the low-affinity NGF receptor

The nerve growth factor (NGF) receptor is a complex of two proteins, gp75 and the tyrosine kinase TrkA. Using fluorescence recovery after photobleaching, we have studied the diffusion properties of the TrkA receptor. For PC12 cells that express both gp75 and TrkA, TrkA was relatively immobile with only 28 +/- 1% of receptor molecules free to diffuse with D = (3.64 +/- 0.23) x 10(-9) cm2/s. Addition of NGF decreased the mobile fraction to 21 +/- 1% with D = (4.11 +/- 0.18) x 10(-9) cm2/s. Using the Sf9 baculovirus expression system, we were able to study TrkA in the absence and presence of gp75. On Sf9 cells, TrkA showed a mobile fraction of 46 +/- 2% with D = (2.64 +/- 0.21) x 10(-9) cm2/s in the absence of gp75 and 43 +/- 2% with D = (2.31 +/- 0.25) x 10(-9) cm2/s in its presence. Thus, gp75 did not alter TrkA mobility. Addition of NGF to the medium approximately halved the mobile fraction for TrkA in both the absence and presence of gp75. However, using a kinase-deficient mutant of TrkA, we found that ligand-induced immobilization requires an active kinase in the absence of gp75 but not in its presence. In addition, using point mutations at specific TrkA autophosphorylation sites, we determined that mobility is controlled by multiple phosphorylation sites, but the SHC binding site at Y490 may be particularly important for ligand-induced immobilization of TrkA. Therefore, two mechanisms lead to NGF-induced immobilization of TrkA--the first resulting from autophosphorylation of TrkA and the second occurring through TrkA's association with gp75.

Persistence of dimerization and enzymatic activity of epidermal growth factor receptor in the absence of epidermal growth factor

Dimerized and enzymatically active epidermal growth factor receptor, in the presence of saturating epidermal growth factor (EGF), was passed over a column containing an immunoadsorbent for EGF. The immunoadsorbent removed both EGF free in solution and EGF bound to EGF receptor, while EGF receptor passed through unimpeded. Both the dimerization of EGF receptor and the activation of its tyrosine kinase were arrested in samples containing EGF receptor that had been mixed with saturating EGF and then immediately passed over the immunoadsorbent for EGF. The EGF receptor in these samples, however, dimerized completely, and its tyrosine kinase became fully active upon readdition of EGF. Samples of EGF receptor that were fully dimerized and enzymatically active prior to being passed over the immunoadsorbent for EGF remained dimerized and enzymatically active even in the absence of bound EGF. The first-order rate constant for the inactivation of the tyrosine kinase of EGF receptor depleted of EGF was estimated by subtracting the rate constant of inactivation of the tyrosine kinase in samples replenished with EGF from the rate constant of inactivation of the tyrosine kinase in samples that had been depleted of EGF. The rate constant of inactivation was found to be 0.26 +/- 0.06 h-1.

Effect of hydrodynamic interactions on the diffusion of integral membrane proteins: diffusion in plasma membranes

Tracer diffusion coefficients of integral membrane proteins (IMPs) in intact plasma membranes are often much lower than those found in blebbed, organelle, and reconstituted membranes. We calculate the contribution of hydrodynamic interactions to the tracer, gradient, and rotational diffusion of IMPs in plasma membranes. Because of the presence of immobile IMPs, Brinkman's equation governs the hydrodynamics in plasma membranes. Solutions of Brinkman's equation enable the calculation of short-time diffusion coefficients of IMPs. There is a large reduction in particle mobilities when a fraction of them is immobile, and as the fraction increases, the mobilities of the mobile particles continue to decrease. Combination of the hydrodynamic mobilities with Monte Carlo simulation results, which incorporate excluded area effects, enable the calculation of long-time diffusion coefficients. We use our calculations to analyze results for tracer diffusivities in several different systems. In erythrocytes, we find that the hydrodynamic theory, when combined with excluded area effects, closes the gap between existing theory and experiment for the mobility of band 3, with the remaining discrepancy likely due to direct obstruction of band 3 lateral mobility by the spectrin network. In lymphocytes, the combined hydrodynamic-excluded area theory provides a plausible explanation for the reduced mobility of sIg molecules induced by binding concanavalin A-coated platelets. However, the theory does not explain all reported cases of "anchorage modulation" in all cell types in which receptor mobilities are reduced after binding by concanavalin A-coated platelets. The hydrodynamic theory provides an explanation of why protein lateral mobilities are restricted in plasma membranes and why, in many systems, deletion of the cytoplasmic tail of a receptor has little effect on diffusion rates. However, much more data are needed to test the theory definitively. We also predict that gradient and tracer diffusivities are the same to leading order. Finally, we have calculated rotational diffusion coefficients in plasma membranes. They decrease less rapidly than translational diffusion coefficients with increasing protein immobilization, and the results agree qualitatively with the limited experimental data available.

Roles for a cytoplasmic tyrosine and tyrosine kinase activity in the interactions of Neu receptors with coated pits

The neu proto-oncogene product, p185neu (HER2, c-ErbB-2), encodes a cell-surface tyrosine kinase receptor with high oncogenic potential, which correlates with increased tyrosine kinase activity and a rapid receptor internalization rate. To investigate the interactions and signal(s) leading to the endocytosis of Neu receptors, we employed lateral mobility and internalization studies. Fluorescence photobleaching recovery measurements revealed that activation of Neu receptors (induced by mutation or by agonistic antibodies) markedly reduced their mobile fractions. To elucidate the signals involved, other mutants, all carrying a constitutively dimerizing oncogenic mutation, were analyzed. A kinase-negative mutant and a mutant lacking all cytoplasmic tyrosine phosphorylation consensus sequences exhibited high mobile fractions, similar to nonactivated Neu. Retention of a single tyrosine autophosphorylation site (Tyr-1253) out of the five known such sites was sufficient to immobilize a large fraction of the receptor. For all mutants, internalization correlated with receptor immobilization and was blocked by treatments that interfere with coated pit structure, indicating that the immobilization is due to interactions with coated pits. This was supported by the coimmunoprecipitation of alpha-adaptin only with the constitutively activated Neu mutants. We conclude that activated Neu receptors become stably associated with coated pits via plasma membrane adaptor complexes (AP-2). Efficient Neu receptor endocytosis requires activation, a functional kinase domain, and at least one tyrosine autophosphorylation site.

Interaction with TrkA immobilizes gp75 in the high affinity nerve growth factor receptor complex

It has been proposed that the high affinity nerve growth factor (NGF) receptor required for NGF response is a complex of two receptor proteins, gp75 and the tyrosine kinase TrkA, but direct biochemical or biophysical evidence has been lacking. We have previously shown using fluorescence recovery after photobleaching that gp75 is highly mobile on NGF-nonresponsive cells, but relatively immobile on NGF-responsive cells. In this report, we show that a physical interaction with TrkA causes gp75 immobilization. We found that gp75 is relatively mobile on TrkA negative nnr5 cells, a PC12 variant which is nonresponsive to NGF. In contrast, on T14 nnr5 cells (which bear a TrkA expression vector) gp75 is relatively immobile. Similarly, using baculoviruses to express gp75 and TrkA on Sf9 insect cells, we found that TrkA immobilizes gp75 molecules. The related receptor, TrkB, caused a more modest immobilization of gp75. Immobilization was found to require intact TrkA kinase and gp75 cytoplasmic domains, paralleling the requirements of high affinity binding of NGF. Analysis of gp75 diffusion coefficients indicates that mutated gp75 and TrkA molecules may form a complex, even in the absence of the ability to bind NGF with high affinity.

Compartmentalized structure of the plasma membrane for receptor movements as revealed by a nanometer-level motion analysis

Movements of transferrin and alpha 2-macroglobulin receptor molecules in the plasma membrane of cultured normal rat kidney (NRK) fibroblastic cells were investigated by video-enhanced contrast optical microscopy with 1.8 nm spatial precision and 33 ms temporal resolution by labeling the receptors with the ligand-coated nanometer-sized colloidal gold particles. For both receptor species, most of the movement trajectories are of the confined diffusion type, within domains of approximately 0.25 microns2 (500-700 nm in diagonal length). Movement within the domains is random with a diffusion coefficient approximately 10(-9) cm2/s, which is consistent with that expected for free Brownian diffusion of proteins in the plasma membrane. The receptor molecules move from one domain to one of the adjacent domains at an average frequency of 0.034 s-1 (the residence time within a domain approximately 29 s), indicating that the plasma membrane is compartmentalized for diffusion of membrane receptors and that long-range diffusion is the result of successive intercompartmental jumps. The macroscopic diffusion coefficients for these two receptor molecules calculated on the basis of the compartment size and the intercompartmental jump rate are approximately 2.4 x 10(-11) cm2/s, which is consistent with those determined by averaging the long-term movements of many particles. Partial destruction of the cytoskeleton decreased the confined diffusion mode, increased the simple diffusion mode, and induced the directed diffusion (transport) mode. These results suggest that the boundaries between compartments are made of dynamically fluctuating membrane skeletons (membrane-skeleton fence model).

Confined lateral diffusion of membrane receptors as studied by single particle tracking (nanovid microscopy). Effects of calcium-induced differentiation in cultured epithelial cells

The movements of E-cadherin, epidermal growth factor receptor, and transferrin receptor in the plasma membrane of a cultured mouse keratinocyte cell line were studied using both single particle tracking (SPT; nanovid microscopy) and fluorescence photobleaching recovery (FPR). In the SPT technique, the receptor molecules are labeled with 40 nm-phi colloidal gold particles, and their movements are followed by video-enhanced differential interference contrast microscopy at a temporal resolution of 33 ms and at a nanometer-level spatial precision. The trajectories of the receptor molecules obtained by SPT were analyzed by developing a method that is based on the plot of the mean-square displacement against time. Four characteristic types of motion were observed: (a) stationary mode, in which the microscopic diffusion coefficient is less than 4.6 x 10(-12) cm2/s; (b) simple Brownian diffusion mode; (c) directed diffusion mode, in which unidirectional movements are superimposed on random motion; and (d) confined diffusion mode, in which particles undergoing Brownian diffusion (microscopic diffusion coefficient between 4.6 x 10(-12) and 1 x 10(-9) cm2/s) are confined within a limited area, probably by the membrane-associated cytoskeleton network. Comparison of these data obtained by SPT with those obtained by FPR suggests that the plasma membrane is compartmentalized into many small domains 300-600 nm in diameter (0.04-0.24 microns2 in area), in which receptor molecules are confined in the time scale of 3-30 s, and that the long-range diffusion observed by FPR can occur by successive movements of the receptors to adjacent compartments. Calcium-induced differentiation decreases the sum of the percentages of molecules in the directed diffusion and the stationary modes outside of the cell-cell contact regions on the cell surface (which is proposed to be the percentage of E-cadherin bound to the cytoskeleton/membrane-skeleton), from approximately 60% to 8% (low- and high-calcium mediums, respectively).

Role of the cytoplasmic domains of viral glycoproteins in antibody-induced cell surface mobility

We have investigated the role of the cytoplasmic domains of the influenza virus hemagglutinin (HA) and the parainfluenza virus type 3 (PI3) fusion (F) glycoproteins as a determinant of their ability to undergo antibody-induced redistribution on plasma membranes. The viral envelope genes were truncated in their cytoplasmic domains by using oligonucleotide-directed mutagenesis and expressed by using recombinant vaccinia viruses. In HeLa cells, the truncated HA (HAt), like the full-length HA, did not cap in response to specific antibody. In CV-1 cells, HAt showed patchy surface immunofluorescence with few caps, whereas full-length HA exhibited capping in many cells in response to bivalent antibody. Quantitation of cap formation indicated a sevenfold decrease in the frequency of capping of HAt in comparison with full-length HA. Similarly, truncated F also exhibited a significant decrease in cap formation in comparison with full-length F. These results indicate that the ability of influenza virus HA and PI3 F to undergo redistribution in response to bivalent antibody has been altered by truncation of the viral glycoproteins and suggest that capping may involve interactions between the cytoplasmic domain of the viral glycoproteins and host cell components.

Protein lateral mobility as a reflection of membrane microstructure

The lateral mobility of membrane lipids and proteins is presumed to play an important functional role in biomembranes. Photobleaching studies have shown that many proteins in the plasma membrane have diffusion coefficients at least an order of magnitude lower than those obtained when the same proteins are reconstituted in artificial bilayer membranes. Depending on the protein, it has been shown that either the cytoplasmic domain or the ectodomain is the key determinant of its lateral mobility. Single particle tracking microscopy, which allows the motions of single or small groups of membrane molecules to be followed, promises not only to reveal new features of membrane dynamics, but also to help explain longstanding puzzles presented by the photobleaching studies, particularly the so-called immobile fraction. The combination of the two complementary technologies should measurably enhance our understanding of membrane microstructure.

Deletions in the regulatory or kinase domains of protein kinase C-alpha cause association with the cell nucleus

We have constructed expression plasmids carrying protein kinase C (PKC) cDNAs with deletions in the coding region. Two truncated molecules, consisting only of the kinase domain of PKC-alpha, were generated by removing parts of the cDNA coding for the regulatory region. Another mutant molecule was created by deleting 95 amino acids from the C-terminal part of the molecule. The full-length cDNA coding for PKC-alpha and its deletion constructs was expressed in COS cells. Using cell fractionation experiments and immunofluorescence staining, we demonstrate here that in contrast to the cytosolic localization of full-length PKC-alpha, the truncated forms, coding only for the kinase domain, were found exclusively in the cell nucleus. Further subfractionation of nuclei isolated from these transfected cells indicated partial association with the nuclear envelopes. Expression of the cDNA lacking the C-terminal part of the molecule in COS cells encoded a truncated molecule that was found both in the cytosol and in the nucleus. We also show that translocation of full-length PKC-alpha molecules to the cell nucleus occurred in response to phorbol ester treatment. Thus, it appears that accumulation of PKC-alpha in the nucleus results either by phorbol ester activation or by deletions of specific regions of the molecule. A molecular mechanism for the nuclear translocation of phorbol ester-activated PKC-alpha or its truncated molecules is suggested.

The protein kinase C-related PKC-L(eta) gene product is localized in the cell nucleus

The tumor promoters phorbol esters are thought to induce changes in cell growth and gene expression by direct activation of protein kinase C (PKC). However, the molecular mechanisms by which PKC molecules transduce signals into the cell nucleus are unknown. In this study, we provide evidence for a direct target for phorbol esters in the nucleus. We demonstrate that the new PKC-related family member, PKC-L, recently isolated by us, is expressed specifically in the cell nucleus. Localization of PKC-L in the cell nucleus is shown both by immunofluorescence staining and by subcellular fractionation experiments of several human cell lines, including the human epidermoid carcinoma line A431. Treatment of these cells by phorbol esters does not induce the down-regulation of PKC-L, in contrast to their effect on classical PKC family members. This is the only PKC isoenzyme described so far that resides permanently and specifically in the cell nucleus. PKC-L may function as an important link in tumor promoting, e.g., as a nuclear regulator of gene expression that changes the phosphorylation state of transcriptional components such as the AP-1 complex.

Cell-specific constraints to the lateral diffusion of a membrane glycoprotein

We have previously shown that the lateral diffusion, D, of the class I Major Histocompatibility Complex (MHC) glycoprotein H-2Ld is constrained by its glycosylation, when expressed in mouse L-cells. Removal of one or more of the 3 N-linked oligosaccharides of H-2Ld glycoproteins results in an increase in D. In order to further examine the influence of glycosylation on D, we compared lateral diffusion of H-2Ld expressed in wild-type CHO cells with lateral diffusion of the same molecule expressed in mutant CHO cells with aberrant surface glycosylation. In addition, we compared lateral diffusion of wild-type and unglycosylated H-2Ld antigens in these cells. In contrast to the large effect of glycosylation state on lateral diffusion of H-2Ld in mouse L-cells, there was little effect of glycosylation on lateral diffusion of H-2Ld in any of the CHO cells. This, together with similar results on hamster class I antigens, indicates that the constraints to D of H-2Ld and other class I MHC molecules are different in CHO cells than in L-cells. Measurements of lateral diffusion after treatment of cells with cytochalasin D make it clear that interactions between MHC class I molecules and a cytoskeleton are important in reducing the mobile fraction of diffusing molecules, R, though they cannot be shown to directly affect the diffusion coefficient, D.

The protein kinase C-related PKC-L(eta) gene product is localized in the cell nucleus

The tumor promoters phorbol esters are thought to induce changes in cell growth and gene expression by direct activation of protein kinase C (PKC). However, the molecular mechanisms by which PKC molecules transduce signals into the cell nucleus are unknown. In this study, we provide evidence for a direct target for phorbol esters in the nucleus. We demonstrate that the new PKC-related family member, PKC-L, recently isolated by us, is expressed specifically in the cell nucleus. Localization of PKC-L in the cell nucleus is shown both by immunofluorescence staining and by subcellular fractionation experiments of several human cell lines, including the human epidermoid carcinoma line A431. Treatment of these cells by phorbol esters does not induce the down-regulation of PKC-L, in contrast to their effect on classical PKC family members. This is the only PKC isoenzyme described so far that resides permanently and specifically in the cell nucleus. PKC-L may function as an important link in tumor promoting, e.g., as a nuclear regulator of gene expression that changes the phosphorylation state of transcriptional components such as the AP-1 complex.

Lateral mobility of integral proteins in red blood cell tethers

The red blood cell membrane is a complex material that exhibits both solid- and liquidlike behavior. It is distinguished from a simple lipid bilayer capsule by its mechanical properties, particularly its shear viscoelastic behavior and by the long-range mobility of integral proteins on the membrane surface. Subject to sufficiently large extension, the membrane loses its shear rigidity and flows as a two-dimensional fluid. These experiments examine the change in integral protein mobility that accompanies the mechanical phenomenon of extensional failure and liquidlike flow. A flow channel apparatus is used to create red cell tethers, hollow cylinders of greatly deformed membrane, up to 36-microns long. The diffusion of proteins within the surface of the membrane is measured by the technique of fluorescence redistribution after photobleaching (FRAP). Integral membrane proteins are labeled directly with a fluorescein dye (DTAF). Mobility in normal membrane is measured by photobleaching half of the cell and measuring the rate of fluorescence recovery. Protein mobility in tether membrane is calculated from the fluorescence recovery rate after the entire tether has been bleached. Fluorescence recovery rates for normal membrane indicate that more than half the labeled proteins are mobile with a diffusion coefficient of approximately 4 x 10(-11) cm2/s, in agreement with results from other studies. The diffusion coefficient for proteins in tether membrane is greater than 1.5 x 10(-9) cm2/s. This dramatic increase in diffusion coefficient indicates that extensional failure involves the uncoupling of the lipid bilayer from the membrane skeleton.

Evidence from lateral mobility studies for dynamic interactions of a mutant influenza hemagglutinin with coated pits

Replacement of cysteine at position 543 by tyrosine in the influenza virus hemagglutinin (HA) protein enables the endocytosis of the mutant protein (Tyr 543) through coated pits (Lazarovits, J., and M. G. Roth. 1988. Cell. 53:743-752). To investigate the interactions between Tyr 543 and the clathrin coats in the plasma membrane of live cells, we performed fluorescence photobleaching recovery measurements comparing the lateral mobilities of Tyr 543 (which enters coated pits) and wild-type HA (HA wt, which is excluded from coated pits), following their expression in CV-1 cells by SV-40 vectors. While both proteins exhibited the same high mobile fractions, the lateral diffusion rate of Tyr 543 was significantly slower than that of HA wt. Incubation of the cells in a sucrose-containing hypertonic medium, a treatment that disperses the membrane-associated coated pits, resulted in similar lateral mobilities for Tyr 543 and HA wt. These findings indicate that the lateral motion of Tyr 543 (but not of HA wt) is inhibited by transient interactions with coated pits (which are essentially immobile on the time scale of the lateral mobility measurements). Acidification of the cytoplasm by prepulsing the cells with NH4Cl (a treatment that arrests the pinching-off of coated vesicles from the plasma membrane and alters the clathrin lattice morphology) led to immobilization of a significant part of the Tyr 543 molecules, presumably due to their entrapment in coated pits for the entire duration of the lateral mobility measurement. Furthermore, in both untreated and cytosol-acidified cells, the restrictions on Tyr 543 mobility were less pronounced in the cold, suggesting that the mobility-restricting interactions are temperature dependent and become weaker at low temperatures. From these studies we conclude the following. (a) Lateral mobility measurements are capable of detecting interactions of transmembrane proteins with coated pits in intact cells. (b) The interactions of Tyr 543 with coated pits are dynamic, involving multiple entries of Tyr 543 molecules into and out of coated pits. (c) Alterations in the clathrin lattice structure can modulate the above interactions.

Lateral diffusion of membrane-spanning and glycosylphosphatidylinositol-linked proteins: toward establishing rules governing the lateral mobility of membrane proteins

In the plasma membrane of animal cells, many membrane-spanning proteins exhibit lower lateral mobilities than glycosylphosphatidylinositol (GPI)-linked proteins. To determine if the GPI linkage was a major determinant of the high lateral mobility of these proteins, we measured the lateral diffusion of chimeric membrane proteins composed of normally transmembrane proteins that were converted to GPI-linked proteins, or GPI-linked proteins that were converted to membrane-spanning proteins. These studies indicate that GPI linkage contributes only marginally (approximately twofold) to the higher mobility of several GPI-linked proteins. The major determinant of the high mobility of these proteins resides instead in the extracellular domain. We propose that lack of interaction of the extracellular domain of this protein class with other cell surface components allows diffusion that is constrained only by the diffusion of the membrane anchor. In contrast, cell surface interactions of the ectodomain of membrane-spanning proteins exemplified by the vesicular stomatitis virus G glycoprotein reduces their lateral diffusion coefficients by nearly 10-fold with respect to many GPI-linked proteins.

Nerve growth factor receptors are preaggregated and immobile on responsive cells

It has been hypothesized that signal transduction occurs by ligand-induced receptor clustering and immobilization. For many peptide receptors, cross-linking by anti-receptor antibodies is sufficient for receptor activation. This is not, however, the case for nerve growth factor receptor (NGFR). Using fluorescence microscopy and fluorescence recovery after photobleaching (FRAP), we have analyzed the distribution and diffusibility of NGFR on a series of cell lines. We have found the following: (1) Cells expressing high-affinity responsive NGFR's display clustered NGFR's even in the absence of ligand. In contrast, NGFR's in nonresponsive cell lines are diffusely distributed. (2) Receptors on responsive cell lines are largely nondiffusing while most receptors on nonresponsive cell lines are relatively free to diffuse. (3) NGF does not greatly alter the distribution or diffusion properties of the NGFR on either nonresponsive or responsive cell lines. Thus, NGFR is preclustered and immobile on responsive cells, which suggests that immobilization of NGFR prior to ligand binding is required for signal transduction.

Isolation and characterization of PKC-L, a new member of the protein kinase C-related gene family specifically expressed in lung, skin, and heart

We have isolated and characterized a new human cDNA, coding for a protein kinase, related to the protein kinase C (PKC) gene family. Although this protein kinase shares some homologous sequences and structural features with the four members of the PKC family initially isolated (alpha, beta I, beta II, and gamma), it shows more homology with the recently described PKC-related subfamily, encoded by the cDNAs delta, epsilon, and zeta. The transcript for this gene product, termed PKC-L, is most abundant in lung tissue, less expressed in heart and skin tissue, and exhibited very low expression in brain tissue. Thus, its tissue distribution is different from that described for other mammalian members of the PKC gene family, their expression being enriched in brain tissues. PKC-L is also expressed in several human cell lines, including the human epidermoid carcinoma line A431. The ability of phorbol esters to bind to and stimulate the kinase activity of PKC-L was revealed by introducing the cDNA into COS cells.

Isolation and characterization of PKC-L, a new member of the protein kinase C-related gene family specifically expressed in lung, skin, and heart

We have isolated and characterized a new human cDNA, coding for a protein kinase, related to the protein kinase C (PKC) gene family. Although this protein kinase shares some homologous sequences and structural features with the four members of the PKC family initially isolated (alpha, beta I, beta II, and gamma), it shows more homology with the recently described PKC-related subfamily, encoded by the cDNAs delta, epsilon, and zeta. The transcript for this gene product, termed PKC-L, is most abundant in lung tissue, less expressed in heart and skin tissue, and exhibited very low expression in brain tissue. Thus, its tissue distribution is different from that described for other mammalian members of the PKC gene family, their expression being enriched in brain tissues. PKC-L is also expressed in several human cell lines, including the human epidermoid carcinoma line A431. The ability of phorbol esters to bind to and stimulate the kinase activity of PKC-L was revealed by introducing the cDNA into COS cells.

Lateral diffusion of nerve growth factor receptor: modulation by ligand-binding and cell-associated factors

We compared the properties in human melanoma cell line A875 and rat pheochromocytoma cell line PC12 of nerve growth factor receptor (NGFr). We also analyzed NGFr and a truncated NGFR lacking the cytoplasmic domain, which were transiently expressed in COS cells. The full-length NGFR expressed in COS cells bound nerve growth factor (NGF) with positive cooperativity, but A875 NGFr and truncated NGFr in COS cells did not display positive cooperativity. The anti-human NGFr monoclonal antibody NGFR5 was characterized and found not to compete with NGF for binding to NGFr. Fabs were prepared from NGFR5 and 192, an anti-rat NGFR monoclonal antibody that was previously shown not to compete with NGF for binding. Fluorescein-labeled Fabs were used to measure the distribution and lateral diffusion of the NGFr. NGFr expressed on COS and A875 cells are diffusely distributed, but NGFr on the surface of PC12 cells appeared, for some cells, to be patched. In A875 cells, 51% of the NGFr was free to diffuse with diffusion coefficient (D) approximately 7 X 10(-10) cm2/s. In COS cells, 43% diffused with D approximately 5 X 10(-10) cm2/s. There was no significant difference in diffusibility between the full-length NGFr and the truncated NGFr. We compared NGFr diffusion on PC12 cells in suspension or adherent to collagen-coated coverslips. For suspension cells, we obtained 32% recovery with D approximately 2.5 X 10(-9) cm2/s. On adherent cells, we obtained 17% recovery with 6 X 10(-9) cm2/s. Binding of NGF enhanced lateral diffusion of NGFr in A875 cells and in PC12 cells in suspension but did not alter lateral diffusion of NGFr in COS cells or in adherent PC12 cells. NGF had no effect on the diffusing fraction or the distribution of NGFR for any cell line.

Binding and internalization of ricin labelled with fluorescein isothiocyanate

The toxic lectin ricin has been covalently labelled with fluorescein isothiocyanate on the enzymatically active A chain. The fluorescein reacted toxin maintains its biological activity. The lateral diffusion coefficient of cell surface bound ricin, studied in two cell lines by fluorescence photobleaching recovery, is D = 1 - 2 x 10(-10) cm2/s. Fluorescence microscopy provides preliminary evidence for secondary endosomes in the cytoplasm.

Small intestinal differentiation in human colon carcinoma HT29 cells has distinct effects on the lateral diffusion of lipids (ganglioside GM1) and proteins (HLA class 1, HLA class 2, and neoplastic epithelial antigens) in the apical cell membrane

We have studied the effect of maturation to small intestinal-like epithelial cells of the human colonic carcinoma cell line HT29 on the lateral mobility of different representative membrane components (lipid, proteins), as assessed with fluorescence recovery after photobleaching (FRAP). Maturation was induced in vitro in the HT29 cells by replacing glucose (Glu) with galactose (Gal) in the growth medium (DMEM) during a 21-day period. Scanning electron microscopy revealed an increased number of microvilli in the apical cell membrane, and enzyme analyses (alkaline phosphatase, aminopeptidase) in combination with aqueous countercurrent distribution, indicated that maturation was induced with DMEM-Gal. In comparison to control cells grown in DMEM-Glu medium, the more small intestinal-like cells grown in DMEM-Gal displayed no alteration of the lateral mobility of either cholera toxin (B subunit)-labelled ganglioside GM1 (diffusion coefficient, D [x 10(8)] = 0.8-0.9 cm2s-1; mobile fraction, R = 50-60%) or antibody-stained Class 2 histocompatibility (HLA-DR) antigen (D [x 10(9)] = 2 cm2s-1; R = 60-70%). However, antibody-labelled beta 2-microglobulin of HLA Class 1 antigen displayed increased mobility in HT29-Gal cells; D was x 1.4 and R x 1.8 larger in the HT29-Gal cells. By contrast, the mobility of a neoplastic antigen was reduced; D and R were x0.60 and x0.69 of the values seen in HT29-Glu cells. It is thus concluded that DMEM-Gal-induced differentiation in confluent HT29 cells is accompanied by specific rather than general effects on the lateral mobility of different membrane components.

Chemistry and biology of N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-labeled lipids: fluorescent probes of biological and model membranes

Lipids that are covalently labeled with the 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) group are widely used as fluorescent analogues of native lipids in model and biological membranes to study a variety of processes. The fluorescent NBD group may be attached either to the polar or the apolar regions of a wide variety of lipid molecules. Synthetic routes for preparing the lipids, and spectroscopic and ionization properties of these probes are reviewed in this report. The orientation of various NBD-labeled lipids in membranes, as indicated by the location of the NBD group, is also discussed. The NBD group is uncharged at neutral pH in membranes, but loops up to the surface if attached to acyl chains of phospholipids. These lipids find applications in a variety of membrane-related studies which include membrane fusion, lipid motion and dynamics, organization of lipids and proteins in membranes, intracellular lipid transfer, and bilayer to hexagonal phase transition in liposomes. Use of NBD-labeled lipids as analogues of natural lipids is critically evaluated.