Effects of vanadium-containing compounds on membrane lipids and on microdomains used in receptor-mediated signaling

There is increasing evidence for the involvement of plasma membrane microdomains in insulin receptor function. Moreover, disruption of these structures, which are typically enriched in sphingomyelin and cholesterol, results in insulin resistance. Treatment strategies for insulin resistance include the use of vanadium (V) compounds which have been shown in animal models to enhance insulin responsiveness. One possible mechanism for insulin-enhancing effects might involve direct effects of V compounds on membrane lipid organization. These changes in lipid organization promote the partitioning of insulin receptors and other receptors into membrane microdomains where receptors are optimally functional. To explore this possibility, we have used several strategies involving V complexes such as [VO(2)(dipic)](-) (pyridin-2,6-dicarboxylatodioxovanadium(V)), decavanadate (V(10)O(28)(6-), V(10)), BMOV (bis(maltolato)oxovanadium(IV)), and [VO(saltris)](2) (2-salicylideniminato-2-(hydroxymethyl)-1,3-dihydroxypropane-oxovanadium(V)). Our strategies include an evaluation of interactions between V-containing compounds and model lipid systems, an evaluation of the effects of V compounds on lipid fluidity in erythrocyte membranes, and studies of the effects of V-containing compounds on signaling events initiated by receptors known to use membrane microdomains as signaling platforms.

Fluorescence photobleaching recovery measurement of protein absolute diffusion constants

The technique of fluorescence photobleaching recovery [Axelrod et al., Biophys. J. 16 (1976) 1055] has been applied to the measurement of absolute diffusion constants of a number of fluoiescein isothiocyanate-labeled proteins. Measured diffusion constants agree to within +/- 7% of published values for the underivatized proteins. The method has sufficient sensitivity to reveal the concentration dependence at neutral pH of the diffusion constant of alpha-chymotrypsin. The rapidity with which the labelling and measurements can be performed and the small amount of material required suggest the technique may be useful in rapid characterization of small protein samples. Some developments in optical and electronic systems and in data processing for this technique are discussed.

Measuring rotational diffusion of MHC class I on live cells by polarized FPR

Clustering of membrane proteins is a dynamic process which can regulate cellular function and signaling. The size of receptor and other membrane protein clusters can in principle be measured in terms of their rotational diffusion. However, in practice, measuring rotation of membrane proteins of live cells has been difficult, largely because of the difficulty of rigidly attaching reporter groups to the molecules of interest. Here we show that polarized photobleaching recovery can detect rotation of membrane proteins genetically tagged with yellow fluorescent protein, YFP. MHC class I molecules were engineered with a rigid, in-sequence, YFP tag followed at the C-terminus by a pair of crosslinkable domains. When crosslinker was added we could detect changes in rotational anisotropy decay consistent with clustering of the MHC molecules. This result points the way to use of engineered fluorescent fusion proteins to measure rotational diffusion in native cell membranes.

Compartmentalization of the Type I Fcε receptor and MAFA on mast cell membranes

The Mast cell Function-associated Antigen (MAFA) is a membrane glycoprotein on rat mast cells (RBL-2H3) expressed at a ratio of approximately 1:30 with respect to the Type I Fc epsilon receptor (Fc epsilon RI). Despite this stoichiometry, clustering MAFA by its specific mAb G63 substantially inhibits secretion of both granular and de novo synthesized mediators induced upon Fc epsilon RI aggregation. Since the Fc epsilon RIs apparently signal from within raft micro-environments, we investigated possible co-localization of MAFA within these membrane compartments containing aggregated Fc epsilon RI. We used cholera toxin B subunit (CTB) to cluster the raft component ganglioside GM1 and studied the effects of this perturbation on rotation of Fc epsilon RI and MAFA by time-resolved phosphorescence anisotropy of erythrosin-conjugated probes. CTB treatment would be expected to substantially inhibit rotation of raft-associated molecules. Experimentally, CTB has no effect on rotational parameters such as the long-time anisotropy (r(infinity)) of unperturbed Fc epsilon RI or MAFA. However, on cells where Fc epsilon RI-IgE has previously been clustered by antigen (DNP(14)-BSA), CTB treatment increases the Fc epsilon RI-IgE's r(infinity) by 0.010 and MAFA's by 0.014. Similarly, CTB treatment of cells where MAFA had been clustered by mAb G63 increases MAFA's r(infinity) by 0.010 but leaves Fc epsilon RI's unaffected. Evaluation of raft localization of Fc epsilon RI and MAFA using sucrose gradient ultracentrifugation of Triton X-100 treated membrane fragments demonstrates that a significant fraction of MAFA molecules sediments with rafts when Fc epsilon RI is clustered by antigen or when MAFA itself is clustered by mAb G63. The large excess of Fc epsilon RI over MAFA explains why clustering MAFA does not substantively affect Fc epsilon RI dynamics. Moreover, in single-particle tracking studies of individual Fc epsilon RI-IgE or MAFA molecules, these proteins, upon clustering by antigen, move into small membrane compartments of reduced, but similar, dimensions. This provides additional indication of constitutive interactions between Fc epsilon RI and MAFA. Taken together, these results of distinct methodologies suggest that MAFA functions within raft microdomains of the RBL-2H3 cell membrane and thus in close proximity to the Fc epsilon RI which themselves signal from within the raft environment.

Fluorescence photobleaching recovery using total internal reflection interference fringes

Lateral diffusion measurements on cell membrane molecules, most commonly accomplished through fluorescence photobleaching recovery (FPR or FRAP), provide information on such molecules' size, environment, and participation in intermolecular interactions. However, difficulties arise in FPR measurements of lateral dynamics of materials, such as visible fluorescent protein (VFP) fusion proteins, where fluorescent intracellular species contribute to the fluorescence recovery signal and thus distort measurements intended to reflect surface molecules only. A new method helps eliminate these difficulties. In total internal reflection interference fringe FPR, interfering laser beams enter a 1.65-numercial aperture (NA) Olympus objective at the periphery of the back focal plane where the NA exceeds 1.38. This creates an extended interference pattern totally internally reflected at the coverslip-medium interface which excites fluorescence only from fluorescent molecules located where the cell contacts the coverslip. The large illuminated area interrogates many more membrane receptors than spot methods and hence obtains more diffusion information per measurement while rejecting virtually all interfering intracellular fluorescence. We report successful measurements of membrane dynamics of both VFP-containing and conventionally labeled molecules by this technique and compare them with results of other FPR methods.

High probe intensity photobleaching measurement of lateral diffusion in cell membranes

Lateral diffusion measurements, most commonly accomplished through Fluorescence Photobleaching Recovery (FPR or FRAP), provide important information on cell membrane molecules' size, environment and participation in intermolecular interactions. However, serious difficulties arise when these techniques are applied to weakly expressed proteins of either of two types: fusions of membrane receptors with visible fluorescent proteins or membrane molecules on autofluorescent cells. To achieve adequate sensitivity in these cases, techniques such as interference fringe FPR are needed. However, in such measurements, cytoplasmic species contribute to the fluorescence recovery signal and thus yield diffusion parameters not properly representing the small number of surface molecules. A new method helps eliminate these difficulties. High Probe Intensity (HPI)-FPR measurements retain the intrinsic confocality of spot measurements to eliminate interference from fluorescent cytoplasmic species. However, HPI-FPR methods lift the previous requirement that FPR procedures be performed at probe beam intensities low enough to not induce bleaching in samples during measurements. The high probe intensities now employed provide much larger fluorescence signals and thus more information on molecular diffusion from each measurement. We report successful measurement of membrane dynamics by this technique.

Luteinizing hormone receptors translocate to plasma membrane microdomains after binding of human chorionic gonadotropin

Receptor-mediated signal transduction by G protein-coupled receptors can involve redistribution of plasma membrane receptors into membrane structures that are characterized by insolubility in Triton X-100 and low buoyant density in sucrose gradients. Here we describe the translocation of wild-type (wt) rat LH receptors (LHR-wt) from the bulk membrane into membrane microdomains (rafts) after the binding of human chorionic gonadotropin (hCG). In sucrose gradient ultracentrifugation of plasma membranes from cells stably expressing FLAG-tagged LHR-wt, receptors were located in high-density membrane fractions before binding of hormone and in low-density fractions after hCG treatment. Receptor translocation to low-density sucrose fractions did not occur when cells were pretreated with 1% methyl-beta-cyclodextrin, which reduces membrane cholesterol and disrupts rafts. Single-particle tracking of individual FLAG-LHR-wt receptors showed that hCG-treated receptors become confined in small compartments with a diameter of 86 +/- 36 nm, significantly smaller than 230 +/- 79 nm diameter regions accessed by the untreated receptor. Receptors were no longer confined in these small compartments after disruption of rafts by methyl-beta-cyclodextrin, a treatment that also decreased levels of cAMP in response to hCG. Finally, translocation of LHR into rafts required a functional hormone-receptor complex but did not occur after extensive receptor cross-linking that elevated cAMP levels. Thus, retention of LHR in rafts or small membrane compartments is a characteristic of functional, hormone-occupied LHR-wt. Although raft translocation was not essential for cAMP production, it may be necessary for optimizing hormone-mediated signaling.

Chimeric GnRH-LH receptors and LH receptors lacking C-terminus palmitoylation sites do not localize to plasma membrane rafts

Luteinizing hormone and gonadotropin releasing hormone receptors (LHR and GnRHR, respectively) are G protein-coupled receptors with important functions in reproduction. We have developed chimeric GnRHR-LHR that contain the full GnRHR coupled to various forms of the LH receptor C-terminus to explore the role of the LH receptor C-terminus in raft localization of the receptor and signaling. Addition of the full-length LHR C-terminus to GnRHR resulted in localization of the resting chimeric receptor in the bulk membrane rather than plasma membrane rafts as has been reported for the wild-type GnRHR [A. Navratil, S. Bliss, K. Berghorn, J. Haughian, T. Farmerie, J. Graham, C. Clay, M. Roberson, Constitutive localization of the gonadotropin-releasing hormone (GnRH) receptor to low density membrane microdomains is necessary for GnRH signaling to ERK, J. Biol. Chem. 278 (2003) 31593-31602]. With truncation of the LHR C-terminus, approximately 3% of chimeric receptors appeared in low density membrane fractions. Palmitoylation of sites on the LHR C-terminus appears important for raft localization. Mutations to C-terminus palmitoylation sites eliminated translocation of LH receptors from the bulk membrane to rafts upon binding of hCG although these mutant receptors retained the ability to signal via cAMP.

The influence of actin microfilaments on signaling by the receptor with high-affinity for IgE

Aggregation of the receptors with high-affinity for IgE (FcRI) stimulates a variety of cellular responses, but excessive aggregation inhibits such responses. Actin filaments have been implicated in the inhibitory phenomenon because disrupting the filaments enhances the cellular reactions stimulated by the aggregated receptors. To clarify further the molecular mechanism and physiological importance of the actin-mediated inhibition, we assessed the effect of inhibitors of actin polymerization on the initial signaling events of mast cells alternatively stimulated by nitrophenyl ligands that dissociate slowly (high-affinity) or rapidly (low-affinity) from receptor-bound anti-dinitrophenyl IgE. The inhibitors amplified the phosphorylation of FcRI and of Syk induced by addition of either ligand but at physiological temperatures, the augmentation of the response to the low-affinity ligand was especially exaggerated. The effect of actin is on the earliest events, and although the molecular mechanism(s) by which the filaments regulate the intensity of proximal signaling remains unclear, several possibilities have been excluded. That the inhibitors only minimally augment the responses stimulated by preformed dimers of IgE, and in general show smaller effects with more limited aggregation, suggests that the actin-mediated "down-regulation" may be more prominent in laboratory experiments than under physiological circumstances.

Evidence that IgE molecules mediate a spectrum of effects on mast cell survival and activation via aggregation of the FcepsilonRI

We demonstrate that binding of different IgE molecules (IgEs) to their receptor, FcepsilonRI, induces a spectrum of activation events in the absence of a specific antigen and provide evidence that such activation reflects aggregation of FcepsilonRI. Highly cytokinergic IgEs can efficiently induce production of cytokines and render mast cells resistant to apoptosis in an autocrine fashion, whereas poorly cytokinergic IgEs induce these effects inefficiently. Highly cytokinergic IgEs seem to induce more extensive FcepsilonRI aggregation than do poorly cytokinergic IgEs, which leads to stronger mast cell activation and survival effects. These effects of both types of IgEs require Syk tyrosine kinase and can be inhibited by FcepsilonRI disaggregation with monovalent hapten. In hybridoma-transplanted mice, mucosal mast cell numbers correlate with serum IgE levels. Therefore, survival effects of IgE could contribute to the pathogenesis of allergic disease.

Imaging molecular interactions in cells by dynamic and static fluorescence anisotropy (rFLIM and emFRET)

We report the implementation and exploitation of fluorescence polarization measurements, in the form of anisotropy fluorescence lifetime imaging microscopy (rFLIM) and energy migration Förster resonance energy transfer (emFRET) modalities, for wide-field, confocal laser-scanning microscopy and flow cytometry of cells. These methods permit the assessment of rotational motion, association and proximity of cellular proteins in vivo. They are particularly applicable to probes generated by fusions of visible fluorescence proteins, as exemplified by studies of the erbB receptor tyrosine kinases involved in growth-factor-mediated signal transduction.

Interactions of the mast cell function-associated antigen with the type I Fcepsilon receptor

Clustering the mast cell function-associated antigen (MAFA), a membrane glycoprotein expressed on 2H3 cells, by its specific monoclonal antibody G63 substantially inhibits secretion normally triggered by aggregating these cells' Type I Fcepsilon receptor (FcepsilonRI). To explore possible MAFA-FcepsilonRI interactions giving rise to this inhibition, we have studied by time-resolved phosphorescence anisotropy the rotational behavior of both MAFA and FcepsilonRI as ligated by various reagents involved in FcepsilonRI-induced degranulation and MAFA-mediated inhibition thereof. From 4 to 37 degrees C the rotational correlation times (mean+/-S.D.) of FcepsilonRI-bound, erythrosin-conjugated IgE resemble those observed for MAFA-bound erythrosin-conjugated G63 Fab, 82+/-17 micros and 79+/-31 micros at 4 degrees C, respectively. Clustering the FcepsilonRI-IgE complex by antigen or by anti-IgE increases the phosphorescence anisotropy of G63 Fab and slows its rotational relaxation. Lateral diffusion of G63 Fab is also slowed by antigen clustering of the receptor. Taken together, these results suggest that unperturbed MAFA associates with clustered FcepsilonRI. They are also consistent with its interaction with the isolated receptor, a situation also suggested by FRET measurements on the system.

The mast cell function-associated antigen and its interactions with the type I Fcepsilon receptor

Rat mucosal-type mast cells of the RBL-2H3 line express a glycoprotein termed the MAst cell Function-associated Antigen (MAFA). When MAFA is clustered by its specific monoclonal antibody G63, secretion normally triggered by aggregating these cells' type I Fcepsilon receptor (FcepsilonRI) is substantially inhibited. The nature of MAFA-FcepsilonRI interactions giving rise to this inhibition remains unclear. Rotational diffusion of a membrane protein is a sensitive probe of its involvement in intermolecular interactions. We have therefore studied by time-resolved phosphorescence anisotropy the rotational behavior of both MAFA and FcepsilonRI as ligated by various reagents involved in FcepsilonRI-induced degranulation and MAFA-mediated inhibition thereof. From 4 to 37 degrees C, the rotational correlation times (mean +/- SD) of FcepsilonRI-bound, erythrosin-conjugated IgE resemble those observed for MAFA-bound, erythrosin-conjugated G63 Fab, 82 +/- 17 and 79 +/- 31 micros at 4 degrees C, respectively. Clustering the FcepsilonRI-IgE complex by antigen or by anti-IgE increases the phosphorescence anisotropy of G63 Fab and slows its rotational relaxation. Lateral diffusion of G63 Fab is also slowed by antigen clustering of the receptor. Taken together, these results indicate that unperturbed MAFA associates with clustered FcepsilonRI. They are also consistent with its interaction with the isolated receptor.

Molecular dynamics of point mutated I-A(k) molecules expressed on lymphocytes

We have recently reported the lateral and rotational diffusion parameters for I-A(k) molecules expressing various cytoplasmic truncations (Int. Immunol. 12 (2000) 1319). We now describe the membrane dynamics of I-A(k) with various mutations in the presumed contact region between alphabeta-heterodimers in an (alphabeta)2 dimer of dimers structure. Such mutations are known to strongly affect the antigen presentation ability of these molecules (Int. Immunol. 10 (1998) 1237-1249) but cause relatively small changes in the molecular dynamics of I-A(k). Lateral diffusion coefficients of I-A(k) wild-type molecules and mutants obtained via fringe fluorescence photobleaching recovery (FPR) ranged from 1.1 to 2.3x10(-10)cm2/s at room temperature while fractional mobilities averaged 75+/-6%. For all cell types examined, treatment with either hen egg lysozyme 46-61 peptide or db-cAMP reduced the I-A(k) mobile fraction by about 10% relative to untreated cells, suggesting that these treatments may increase lateral confinement of class II in lipid rafts or cytoskeletal interactions of the molecules. Wild-type I-A(k) and mutants capable of normal or partial antigen presentation exhibited, as a group, slightly longer rotational correlation times (RCT) at 4 degrees C than did mutants inactive in antigen presentation, 14+/-4 versus 10+/-1 micros, respectively. Moreover, peptide, cAMP and anti-CD40 mAb treatment all increased rotational correlation times for fully- and partially-functional I-A(k) but not for non-functional molecules. For example, 16 h peptide treatment yielded average RCTs of 28+/-12 and 10+/-1 micros for the groups of functional and non-functional molecules, respectively. Such modulation of the dynamics of functional class II molecules is consistent with these treatments' stabilization of class II or induction of new gene expression. Measurements of fluorescence resonant energy transfer between I-A(k), though complicated by cellular autofluorescence, averaged 6+/-7% over 15 cells or treatments, a result consistent with the presence of a small fraction of I-A(k) as a dimer of dimers species. In summary, our results suggest subtle changes in the molecular motions of class II molecules correlate with a significant impact on class II function. Molecules active in antigen presentation exhibit more restricted motion in the membrane, and thus presumably more extensive intermolecular interactions, than non-functional molecules. Further, treatments, such as db-cAMP and anti-CD40, which rescue antigen presentation by partially defective mutants, appear to increase such interactions, several types of which have already been reported for class II. A more detailed understanding of these phenomena will require both more sensitive biophysical tools and a more refined model of the role of class II intermolecular interactions in antigen presentation.

Binding of agonist but not antagonist leads to fluorescence resonance energy transfer between intrinsically fluorescent gonadotropin-releasing hormone receptors

We have used spot fluorescence photobleaching recovery methods to measure the lateral diffusion of GnRH receptor (GnRHR) fused at its C terminus to green fluorescent protein (GFP) after binding of either GnRH agonists or antagonist. Before ligand binding, GnRHR-GFP exhibited fast rates of lateral diffusion (D = 18 +/- 2.8 x 10(-10)cm2 x sec(-1)) and high values for fractional fluorescence recovery (%R) after photobleaching (73 +/- 1%). Increasing concentrations of agonists, GnRH or D-Ala6-GnRH, caused a dose-dependent slowing of receptor lateral diffusion as well as a decreased fraction of mobile receptors. Increasing concentrations of the GnRH antagonist Antide slowed the rate of receptor diffusion but had no effect on the fraction of mobile receptors, which remained high. To determine whether the decrease in %R caused by GnRH agonists was due, in part, to increased receptor self-association, we measured the fluorescence resonance energy transfer efficiency between GnRHR-GFP and yellow fluorescent protein-GNRHR: There was no energy transfer between GnRHR on untreated cells. Treatment of cells with GnRH agonists led to a concentration-dependent increase in the energy transfer between GnRH receptors to a maximum value of 16 +/- 1%. There was no significant energy transfer between GnRH receptors on cells treated with Antide, even at a concentration of 100 nM. These data provide direct evidence that, before binding of ligand, GnRHR exists as an isolated receptor and that binding of GnRH agonists, but not antagonist, leads to formation of large complexes that exhibit slow diffusion and contain receptors that are self-associated.

Luteinizing hormone receptors are self-associated in slowly diffusing complexes during receptor desensitization

We have previously shown that rat LH receptors (LHRs) occupied by human CG (hCG) exhibit slow receptor lateral diffusion and are self-associated. Here we have examined whether LHRs become self-associated and enter slowly diffusing structures in response to hormone binding and whether these receptors retain this organization while in the desensitized state. Before hormone exposure, wild-type rat LHRs coupled at the C terminus to enhanced green fluorescent protein (GFP-LHR-wt) exhibited fast lateral diffusion, as assessed by fluorescent photobleaching recovery (FPR) methods, and most receptors were laterally mobile. After 30 min exposure to hCG and subsequent removal of hormone by low pH wash, hormone challenge at any time within the next 4 h produced no increase in cellular cAMP levels. During this time, LHRs were either laterally immobile or exhibited slower lateral diffusion. When LHRs were again responsive to binding of hormone, the rate of receptor lateral diffusion had become significantly faster and the fraction of mobile receptors was again large. Desensitized LHRs were also self-associated and present in microscopically visible clusters on the plasma membrane. Fluorescence energy transfer (FET) methods were used to measure the extent of interaction between receptors coupled to either GFP or to yellow fluorescent protein (YFP). Before hormone treatment, there was essentially no energy transfer between LHRs. After desensitization of the receptors by 30 min exposure to hCG, energy transfer efficiency increased to 18%. Values for FET efficiency between desensitized receptors decreased over time, and receptors were responsive to hormone only after measurable energy transfer had completely disappeared. Together these results suggest that desensitized LHRs exist in large, slowly diffusing structures containing self-associated receptors and that these structures must dissipate before the receptor can again respond to hormone.

Steroidogenic acute regulatory protein and peripheral-type benzodiazepine receptor associate at the mitochondrial membrane

Steroidogenic acute regulatory protein (StAR) and peripheral-type benzodiazepine receptor (PBR) have both been implicated in the transport of cholesterol across mitochondrial membranes in steroidogenic cells. Therefore, we hypothesized that StAR and PBR were associated in this process. To test this hypothesis, we measured fluorescence energy transfer (FET) between these proteins by fusing enhanced green fluorescent protein (GFP, donor fluorophore) and yellow fluorescent protein (YFP, acceptor fluorophore) to the C-terminus of ovine StAR (37GFP) and ovine PBR (PBRYFP), respectively. These intrinsically fluorescent proteins were stably transfected into Cos-7 cells and determined to be biologically active. For FET to occur the appropriate fluorescent molecules need to be <100 A from each other. We observed 22.0 +/- 0.9% energy transfer efficiency for 37GFP and PBRYFP, a 4.9 fold increase above non-specific energy transfer between free GFP and PBRYFP (p <.0001). Thus, it appears that StAR and PBR are closely associated in mitochondrial membranes and that these molecules may interact in the transportation of cholesterol.

Luteinizing hormone receptors are self-associated in the plasma membrane

We have evaluated rat LH receptor self-association and lateral dynamics for functional and nonfunctional receptors after binding of hormone. We demonstrate, for the first time, that grouped receptors observed in electron or light microscopy represent actual receptor dimers or oligomers rather than simply the concentration of receptors within membrane microdomains. Fringe fluorescence photobleaching recovery methods showed that, after binding of either LH or human CG (hCG), functional wild-type LH receptors, expressed on 293 cells (LHR-wt cells), have mobilities that are 25% lower than those of nonfunctional LH receptors containing an arginine substitution for lysine at position 583 (LHR-K583R cells). Because lateral diffusion coefficients in two dimensions depend only on the logarithm of the molecular size of the diffusing species, this result implies that functional receptors exist in substantially larger membrane complexes than do nonfunctional receptors. In single-cell measurements of fluorescence energy transfer after hormone binding, functional LH receptors were also characterized by receptor self-aggregation. Values for fluorescence resonant energy transfer efficiency were 13 +/- 2% and 17 +/- 3% between fluorophore-conjugated LH or hCG, respectively, bound to receptors on LHR-wt cells. However, there was little or no energy transfer between receptors on LHR-K583R cells. These results suggest that receptor functionality involves receptor-receptor interactions and that the extent of such receptor self-association depends on whether LH or hCG binds the receptor.

Rotational and lateral dynamics of I-A(k) molecules expressing cytoplasmic truncations

Rotational and lateral diffusion of I-A(k) molecules with various alpha and beta chain cytoplasmic truncations known to affect class II function were measured to assess the role of cytoplasmic domains in regulating I-A(k) molecular motions. Deletion of all 12 alpha chain C-terminal residues and all 18 corresponding beta chain residues (alpha-12/beta-18) is known to abrogate translocation of protein kinase C to the nucleus upon class II cross-linking. Similarly, truncation of the entire cytoplasmic alpha chain domain and the 10 C-terminal residues of the beta chain impairs presentation of antigenic peptides to T cells. The rotational correlation time of the wild-type molecule, 11.9 +/- 2.6 micros as measured by time-resolved phosphorescence anisotropy, decreased to 7. 2 +/- 3.7 micros in the fully truncated alpha-12/beta-18 protein. Other truncated class II molecules exhibited only small changes in molecular rotation rates relative to the wild-type. The rate of lateral diffusion of the fully truncated molecule, measured with two independent methods, 2.3 x 10(-10) cm(2)/s, was comparable with that of the wild-type molecule. Thus, it appears that the alpha and beta chain cytoplasmic domains regulate the molecular motions of unperturbed I-A(k) molecules only modestly, despite the known involvement of these regions in class II signaling. Various explanations for this behavior are discussed, e.g. the possibility that class II membrane complexes are sufficiently large that association and dissociation of specific signaling proteins during antigen presentation do not significantly perturb the apparent molecular motions of the complex.

Mutations in specific I-A(k) alpha(2) and beta(2) domain residues affect surface expression

A previous investigation demonstrated that several mutations in class II dimer-of-dimers contact residues interfere with antigen presentation by transfectants but not with plasma membrane expression of the mutant class II. In the present study we examined other class II mutations in this region that did inhibit plasma membrane expression of mutant class II molecules. Molecules containing both mutations H alpha 181D in the alpha(2) domain and E beta 170K in the beta(2) domain exhibited low plasma membrane expression, but molecules with only one of these mutations were expressed normally. The mutant class II molecules were transported to organelles that were accessible to a fluid-phase protein, hen egg lysozyme (HEL). Culture of transfectants with lysozyme enhanced the amount of class II compact dimer (alpha beta plus peptide; CD), and this was especially marked for the class II mutant H alpha 181D/E beta 170K and for other molecules possessing both mutations. Formation of class II CD was not paralleled by an increase in class II surface expression. Thus the joint mutation of H alpha 181 and E beta 170 has two effects. In the absence o high concentrations of exogenous peptide, it prevents efficient CD formation, possibly by affecting invariant chain (Ii) proteolysis and/or the stability of the class II after Ii/CLIP is removed. At high peptide concentrations supplied by exogenous HEL, the mutations allow CD formation, but not expression of class II on the plasma membrane. Molecular modeling of the possible interaction of class II and Ii suggests that the mutant amino acids H alpha 181D and E beta 170K, besides affecting the overall stability of class II, might also interact with Ii via two loops in class II's alpha(2) and beta(2) domains respectively.

Biological function of the LH receptor is associated with slow receptor rotational diffusion

The biological activity of luteinizing hormone (LH) receptors can be affected by modifications to the receptor's amino acid sequence or by binding of hormone antagonists such as deglycosylated hCG. Here we have compared rotational diffusion of LH receptors capable of activating adenylate cyclase with that of non-functional hormone-occupied receptors at 4 degrees C and 37 degrees C using time-resolved phosphorescence anisotropy techniques. Binding of hCG to the rat wild-type receptor expressed on 293 cells (LHR-wt cells) or to the LH receptor on MA-10 cells produces functional receptors which exhibit rotational correlation times longer than 1000 micros. However, modification of the LH receptor by substitution of Lys583-->Arg (LHR-K583R) results in a receptor that is non-functional and which has a significantly shorter rotational correlation time of 130+/-12 micros following binding of hCG. When these receptors are treated with deglycosylated hCG, an inactive form of hCG, the rotational correlation times for the LH receptors on LHR-wt and MA-10 cells are also shorter, namely 64+/-8 and 76+/-14 micros, respectively. Finally, a biologically active truncated form of the rat LH receptor expressed in 293 cells (LHR-t631) has slow rotational diffusion, greater than 1000 micros, when occupied by hCG and a significantly shorter rotational correlation time of 103+/-12 micros when occupied by deglycosylated hCG. The effects of rat LH binding to LH receptors on these various cell lines were similar to those of hCG although the magnitude of the changes in receptor rotational diffusion were less pronounced. We suggest that functional LH receptors are present in membrane complexes that exhibit slow rotational diffusion or are rotationally immobile. Shorter rotational correlation times for non-functional hormone-receptor complexes may reflect the absence of essential interactions between these complexes and other membrane proteins.

Dynamics of molecules involved in antigen presentation: effects of fixation

Antigen presentation by MHC class II molecules can be enhanced by paraformaldehyde fixation of antigen-presenting cells prior to assay. This treatment might be expected to aggregate membrane proteins and thus stabilize and strengthen transient protein-protein interactions involved in intercellular cooperation. Lateral and rotational dynamics of the MHC class II antigen I-Ad on A20 cells fixed with various concentrations of paraformaldehyde were examined by fluorescence photobleaching recovery and time-resolved phosphorescence anisotropy, respectively. Probes were tetramethylrhodamine and erythrosin conjugates of MKD6 Fab fragments. Increasing concentrations of paraformaldehyde led to a progressive increase in the limiting anisotropy of I-Ad at 4 degrees C from the value of 0.042 for untreated cells, indicative of large aggregate formation, while leaving the rotational correlation time of 29 micros unchanged, a measure of the unperturbed molecule. On the other hand, the translational diffusion constants decreased from approximately 2x10(-10) cm2 s(-1), while the fractional recovery remained unchanged at about 40-50%. Taken together, these results suggest that fixation crosslinks class II molecules to each other or to other membrane proteins into structures large enough (>500,000 kDa) to diffuse translationally with perceptibly size-dependent rates. The fixation effects on both class II rotation and lateral diffusion were half-maximal at paraformaldehyde concentrations of approximately 0.2%. Possible relations between the biological effector functions of class II and the physical sizes of fixation-induced aggregates are discussed.