A permissive geometry model for TCR-CD3 activation

The T cell antigen receptor (TCR-CD3) is the most complex receptor known to date, consisting of eight transmembrane subunits. Its activation by an antigen is the initial step in an immune response. Here, we present the permissive geometry model explaining how antigen binding initiates intracellular signalling cascades. We propose that a dimeric antigen imposes its geometry on two TCR-CD3 receptors by simultaneously binding to both. This causes the TCRalphabeta subunits to rotate with respect to each other leading to displacement of the ectodomains of the associated CD3 dimers. This results in a scissor-like movement of the CD3 dimers that opens the cytoplasmic tails for interaction with signalling proteins, thus initiating signalling cascades.     

Recruitment of Nck by CD3 epsilon reveals a ligand-induced conformational change essential for T cell receptor signaling and synapse formation

How membrane receptors initiate signal transduction upon ligand binding is a matter of intense scrutiny. The T cell receptor complex (TCR-CD3) is composed of TCR alpha/beta ligand binding subunits bound to the CD3 subunits responsible for signal transduction. Although it has long been speculated that TCR-CD3 may undergo a conformational change, confirmation is still lacking. We present strong evidence that ligand engagement of TCR-CD3 induces a conformational change that exposes a proline-rich sequence in CD3 epsilon and results in recruitment of the adaptor protein Nck. This occurs earlier than and independently of tyrosine kinase activation. Finally, by interfering with Nck-CD3 epsilon association in vivo, we demonstrate that TCR-CD3 recruitment of Nck is critical for maturation of the immune synapse and for T cell activation.     

Membrane-Mediated Regulation of the Intrinsically Disordered CD3? Cytoplasmic Tail of the TCR

The regulation of T-cell-mediated immune responses depends on the phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) on T-cell receptors. Although many details of the signaling cascades are well understood, the initial mechanism and regulation of ITAM phosphorylation remains unknown. We used molecular dynamics simulations to study the influence of different compositions of lipid bilayers on the membrane association of the CD3ϵ cytoplasmic tails of the T-cell receptors. Our results show that binding of CD3ϵ to membranes is modulated by both the presence of negatively charged lipids and the lipid order of the membrane. Free-energy calculations reveal that the protein-membrane interaction is favored by the presence of nearby basic residues and the ITAM tyrosines. Phosphorylation minimizes membrane association, rendering the ITAM motif more accessible to binding partners. In systems mimicking biological membranes, the CD3ϵ chain localization is modulated by different facilitator lipids (e.g., gangliosides or phosphoinositols), revealing a plausible regulatory effect on activation through the regulation of lipid composition in cell membranes.     

Signal transduction through the T cell receptor-CD3 complex. Evidence for heterogeneity in receptor coupling

T cell receptor-CD3 complex (TCR-CD3)-mediated signal transduction was analyzed in HPB-ALL and Jurkat T cell lines. Both cell lines express high levels of TCR-CD3 complex on the cell surface, but provide different model systems for TCR-CD3 signaling in T cells. Jurkat responds with both inositol phosphate generation and intracellular Ca2+ mobilization after triggering of TCR-CD3, whereas TCR-CD3 triggering of HPB-ALL induces Ca2+ mobilization without detectable inositol phosphate generation. By employing a permeabilized cell system, we show that the HPB-ALL line expressed normal levels of Ca2(+)-induced phospholipase C activity. However, the TCR-CD3 on this cell line seems to be uncoupled from phospholipase C activation. In agreement with this result we also show, by analysis of protein kinase C-dependent phosphorylation of three distinct substrates, that TCR-CD3 in HPB-ALL is apparently uncoupled from protein kinase C activation. These findings may have implications for understanding signal-transducing pathways in T cells at various stages of differentiation.     

The dynamic processing of CD46 intracellular domains provides a molecular rheostat for T cell activation

Background

Adequate termination of an immune response is as important as the induction of an appropriate response. CD46, a regulator of complement activity, promotes T cell activation and differentiation towards a regulatory Tr1 phenotype. This Tr1 differentiation pathway is defective in patients with MS, asthma and rheumatoid arthritis, underlying its importance in controlling T cell function and the need to understand its regulatory mechanisms. CD46 has two cytoplasmic tails, Cyt1 and Cyt2, derived from alternative splicing, which are co-expressed in all nucleated human cells. The regulation of their expression and precise functions in regulating human T cell activation has not been fully elucidated.

Methodology/Principal Findings

Here, we first report the novel role of CD46 in terminating T cell activation. Second, we demonstrate that its functions as an activator and inhibitor of T cell responses are mediated through the temporal processing of its cytoplasmic tails. Cyt1 processing is required to turn T cell activation on, while processing of Cyt2 switches T cell activation off, as demonstrated by proliferation, CD25 expression and cytokine secretion. Both tails require processing by Presenilin/γSecretase (P/γS) to exert these functions. This was confirmed by expressing wild-type Cyt1 and Cyt2 tails and uncleavable mutant tails in primary T cells. The role of CD46 tails was also demonstrated with T cells expressing CD19 ectodomain-CD46 C-Terminal Fragment (CTF) fusions, which allowed specific triggering of each tail individually.

Conclusions/Significance

We conclude that CD46 acts as a molecular rheostat to control human T cell activation through the regulation of processing of its cytoplasmic tails.     

跨膜蛋白CD3ε与磷脂酰肌醇二磷酸在脂筏域体系中蛋白质-脂质相互作用的粗粒化模拟研究

细胞膜局部区域可形成富含饱和脂质、胆固醇、鞘脂的脂筏域作为其信号转导调控平台。传统实验手段在研究脂筏及其功能时受到系统复杂度高及区域结构瞬时性强等制约。近年来,分子动力学模拟技术为细胞膜的组织原则提供了重要的理论支撑,从简单的单一组分模型到多组分系统转变,最终形成了越来越多的细胞膜仿真模型。其中,粗粒化模拟由于其简化模型,可大副拓展模拟体系的复杂程度与模拟时间,在细胞膜以及蛋白质-脂质相互作用相关研究中得到了广泛应用。本文采用MARTINI粗粒化力场模拟,构建了一种含有阴离子脂质磷脂酰肌醇二磷酸(phosphatidylinositol diphosphate, PIP2)的混合膜体系。模拟结果表明,该体系在适当温度及饱和度条件下,能自发分层形成脂筏域;膜厚度、膜组分分布、膜组分流动性等多种参数均表明,脂筏结构形成且符合其结构特征;少量PIP2添加不影响分层特性且PIP2对脂筏具有显著亲和性。此外,利用该模型以跨膜信号蛋白CD3ε为例研究了脂筏域体系中蛋白质-脂质相互作用。结果表明,PIP2-CD3ε胞内区相互作用可能是脂筏招募CD3ε的驱动力,且该过程可受钙离子调控。本工作体现了粗粒化模拟在仿真膜相关研究中的巨大优势及良好应用前景。     

Efficient Nef-mediated downmodulation of TCR-CD3 and CD28 is associated with high CD4+ T cell counts in viremic HIV-2 infection

The role of the multifunctional accessory Nef protein in the immunopathogenesis of HIV-2 infection is currently poorly understood. Here, we performed comprehensive functional analyses of 50 nef genes from 21 viremic (plasma viral load, >500 copies/ml) and 16 nonviremic (<500) HIV-2-infected individuals. On average, nef alleles from both groups were equally active in modulating CD4, TCR-CD3, CD28, MHC-I, and Ii cell surface expression and in enhancing virion infectivity. Thus, many HIV-2-infected individuals efficiently control the virus in spite of efficient Nef function. However, the potency of nef alleles in downmodulating TCR-CD3 and CD28 to suppress the activation and apoptosis of T cells correlated with high numbers of CD4(+) T cells in viremic patients. No such correlations were observed in HIV-2-infected individuals with undetectable viral load. Further functional analyses showed that the Nef-mediated downmodulation of TCR-CD3 suppressed the induction of Fas, Fas-L, PD-1, and CTLA-4 cell surface expression as well as the secretion of gamma interferon (IFN-γ) by primary CD4(+) T cells. Moreover, we identified a single naturally occurring amino acid variation (I132T) in the core domain of HIV-2 Nef that selectively disrupts its ability to downmodulate TCR-CD3 and results in functional properties highly reminiscent of HIV-1 Nef proteins. Taken together, our data suggest that the efficient Nef-mediated downmodulation of TCR-CD3 and CD28 help viremic HIV-2-infected individuals to maintain normal CD4(+) T cell homeostasis by preventing T cell activation and by suppressing the induction of death receptors that may affect the functionality and survival of both virally infected and uninfected bystander cells.     

Selective accumulation of raft-associated membrane protein LAT in T cell receptor signaling assemblies

Activation of T cell antigen receptor (TCR) induces tyrosine phosphorylations that mediate the assembly of signaling protein complexes. Moreover, cholesterol-sphingolipid raft membrane domains have been implicated to play a role in TCR signal transduction. Here, we studied the assembly of TCR with signal transduction proteins and raft markers in plasma membrane subdomains of Jurkat T leukemic cells. We employed a novel method to immunoisolate plasma membrane subfragments that were highly concentrated in activated TCR-CD3 complexes and associated signaling proteins. We found that the raft transmembrane protein linker for activation of T cells (LAT), but not a palmitoylation-deficient non-raft LAT mutant, strongly accumulated in TCR-enriched immunoisolates in a tyrosine phosphorylation-dependent manner. In contrast, other raft-associated molecules, including protein tyrosine kinases Lck and Fyn, GM1, and cholesterol, were not highly concentrated in TCR-enriched plasma membrane immunoisolates. Many downstream signaling proteins coisolated with the TCR/LAT-enriched plasma membrane fragments, suggesting that LAT/TCR assemblies form a structural scaffold for TCR signal transduction proteins. Our results indicate that TCR signaling assemblies in plasma membrane subdomains, rather than generally concentrating raft-associated membrane proteins and lipids, form by a selective protein-mediated anchoring of the raft membrane protein LAT in vicinity of TCR.     

CD3-zeta surface expression is required for CD4-p56lck-mediated upregulation of T cell antigen receptor-CD3 signaling in T cells.

It has been proposed that during T cell receptor antigen recognition, CD4- or CD8-p56lck molecules interact with the T cell antigen receptor-CD3 complex (TCR-CD3) to phosphorylate various undefined substrates, which then initiate signal transduction through the TCR-CD3 complex. The ability of CD4 to modulate the TCR-CD3-induced increase in intracellular Ca2+, [Ca2+]i, and substrate tyrosine phosphorylation was studied in mutants of the human leukemic T cell line HPB-ALL characterized by their low expression of the TCR-CD3 complex on the cell surface. In TCR-CD3low cells, in which CD3-zeta was found to be associated with the TCR-CD3 complex, cross-linking CD3 with CD4 resulted in a profile of calcium mobilization, CD3-zeta, and phospholipase C-gamma 1 tyrosine phosphorylation similar to that observed in HPB-ALL cells, although the magnitude of generalized substrate tyrosine phosphorylation appeared to be smaller, as compared with wild-type cells. Responses were weak or absent when CD3 was cross-linked alone. In contrast, in a mutant in which association of CD3-zeta 2 with the TCR-CD3 was defective, cross-linking of CD3 with CD4 had a weaker effect on any of the activation parameters tested. These experiments showed that the presence of CD3-zeta 2 in the TCR-CD3 complex is of critical importance for the ability of CD4 to enhance early transducing signals inside the cell. The data also suggest that CD4-associated protein tyrosine kinase p56lck could up-regulate defective CD3-mediated induction of phospholipase C activity by increasing tyrosine phosphorylation of phospholipase C-gamma 1.     

Intracellular redistribution of nucleolin upon interaction with the CD3epsilon chain of the T cell receptor complex

T cell activation through the antigen receptor (TCR) involves the cytoplasmic tails of the CD3 subunits CD3gamma, CD3delta, CD3epsilon, and CD3zeta. Whereas the biological significance of the cytoplasmic tails of these molecules is suggested, in part, by their evolutionarily conserved sequences, their interactions with signal transduction molecules are not completely understood. We used affinity chromatography columns of glutathione S-transferase fused to the CD3epsilon cytoplasmic tail to isolate proteins that specifically interact with this subunit. In this way, we identified the shuttling protein nucleolin as a specific CD3epsilon-interacting molecule. Using competition studies and affinity chromatography on peptide columns, we were able to identify a central proline-rich sequence as the nucleolin-interacting sequence in CD3epsilon. Transfection in COS cells of wild type CD3epsilon, but not of nonbinding mutants of CD3epsilon, resulted in redistribution of nucleolin from the nucleus and nucleoli to the cytoplasm. This property was transferred to a CD8 protein chimera by appending the cytoplasmic tail of CD3epsilon. We also found that nucleolin associated with the TCR complex. This association was increased upon TCR engagement, suggesting that the CD3epsilon/nucleolin interaction may have a role in T cell activation.     

In silico design of robust bolalipid membranes

The robustness of microorganisms used in industrial fermentations is essential for the efficiency and yield of the production process. A viable tool to increase the robustness is through engineering of the cell membrane and especially by incorporating lipids from species that survive under harsh conditions. Bolalipids are tetraether lipids found in Archaea bacteria, conferring stability to these bacteria by spanning across the cytoplasmic membrane. Here we report on in silico experiments to characterize and design optimal bolalipid membranes in terms of robustness. We use coarse-grained molecular dynamics simulations to study the structure, dynamics, and stability of membranes composed of model bolalipids, consisting of two dipalmitoylphosphatidylcholine (DPPC) lipids covalently linked together at either one or both tail ends. We find that bolalipid membranes differ substantially from a normal lipid membrane, with an increase in thickness and tail order, an increase in the gel-to-liquid crystalline phase transition temperature, and a decrease in diffusivity of the lipids. By changing the flexibility of the linker between the lipid tails, we furthermore show how the membrane properties can be controlled. A stiffer linker increases the ratio between spanning and looping conformations, rendering the membrane more rigid. Our study may help in designing artificial membranes, with tunable properties, able to function under extreme conditions. As an example, we show that incorporation of bolalipids makes the membrane more tolerant toward butanol.     

The CD3 zeta subunit contains a phosphoinositide-binding motif that is required for the stable accumulation of TCR-CD3 complex at the immunological synapse

T cell activation involves a cascade of TCR-mediated signals that are regulated by three distinct intracellular signaling motifs located within the cytoplasmic tails of the CD3 chains. Whereas all the CD3 subunits possess at least one ITAM, the CD3 ε subunit also contains a proline-rich sequence and a basic-rich stretch (BRS). The CD3 ε BRS complexes selected phosphoinositides, interactions that are required for normal cell surface expression of the TCR. The cytoplasmic domain of CD3 ζ also contains several clusters of arginine and lysine residues. In this study, we report that these basic amino acids enable CD3 ζ to complex the phosphoinositides PtdIns(3)P, PtdIns(4)P, PtdIns(5)P, PtdIns(3,5)P(2), and PtdIns(3,4,5)P(3) with high affinity. Early TCR signaling pathways were unaffected by the targeted loss of the phosphoinositide-binding functions of CD3 ζ. Instead, the elimination of the phosphoinositide-binding function of CD3 ζ significantly impaired the ability of this invariant chain to accumulate stably at the immunological synapse during T cell-APC interactions. Without its phosphoinositide-binding functions, CD3 ζ was concentrated in intracellular structures after T cell activation. Such findings demonstrate a novel functional role for CD3 ζ BRS-phosphoinositide interactions in supporting T cell activation.     

Threonine phosphorylation sites in the beta 2 and beta 7 leukocyte integrin polypeptides

The cytoplasmic domains of integrins play a key role in a variety of integrin-mediated events including adhesion, migration, and signaling. The molecular mechanisms that enhance integrin function are still incompletely understood. Because protein kinases are known to be involved in the signaling and the activation of integrins, the role of phosphorylation has been studied by several groups. The beta(2) leukocyte integrin subunit has previously been shown to become phosphorylated in leukocytes on cytoplasmic serine and functionally important threonine residues. We have now mapped the phosphorylated threonine residues in activated T cells. After phorbol ester stimulation, all three threonine residues (758-760) of the threonine triplet became phosphorylated but only two at a time. CD3 stimulation leads to a strong threonine phosphorylation of the beta(2) integrin, but differed from phorbol ester activation in that phosphorylation occurred only on threonine 758. The other leukocyte-specific integrin, beta(7), has also been shown to need the cytoplasmic domain and leukocyte-specific signal transduction elements for integrin activation. Cell activation with phorbol ester, and interestingly, through the TCR-CD3 complex, caused beta(7) integrin binding to VCAM-1. Additionally, cell activation led to increased phosphorylation of the beta(7) subunit, and phosphoamino acid analysis revealed that threonine residues became phosphorylated after cell activation. Sequence analysis by manual radiosequencing by Edman degradation established that threonine phosphorylation occurred in the same threonine triplet as in beta(2) phosphorylation.     

Dissection of thymocyte signaling pathways by in vivo expression of pertussis toxin ADP-ribosyltransferase.

Stimulation of the T lymphocyte antigen receptor-CD3 complex (TCR-CD3) causes T cell activation by a process associated with increased phosphatidylinositol-specific phospholipase C (PI-PLC) activity. Evidence exists suggesting that GTP-binding (G) proteins, particularly the pertussis toxin (PT)-sensitive Gi proteins, participate in this signal transduction pathway. To clarify the role of Gi proteins in TCR-CD3 signaling, and to investigate other possible functions of Gi molecules in T cells, we expressed the S1 subunit of PT in the thymocytes of transgenic mice using the lymphocyte-specific lck promoter. Transgenic thymocytes contained S1 activity and exhibited profound depletion of Gi protein PT substrates in a manner suggesting their inactivation by S1 in vivo. Nevertheless, treatment of transgenic thymocytes with mitogenic stimuli provoked normal increases in intracellular free Ca2+ concentrations and IL-2 secretion, indicating that Gi proteins are not required for T cell activation. These normal signaling responses notwithstanding, mature thymocytes accumulated in lck-PT mice and did not appear in secondary lymphoid organs or in the circulation. Viewed in the context of the known features of Bordetella pertussis infection, our results suggest that a PT-sensitive signaling process, probably involving Gi proteins, regulates thymocyte emigration.     

The structure of the CD3 ζζ transmembrane dimer in POPC and raft-like lipid bilayer: A molecular dynamics study

Plasma membrane lipids significantly affect assembly and activity of many signaling networks. The present work is aimed at analyzing, by molecular dynamics simulations, the structure and dynamics of the CD3 ζζ dimer in palmitoyl-oleoyl-phosphatidylcholine bilayer (POPC) and in POPC/cholesterol/sphingomyelin bilayer, which resembles the raft membrane microdomain supposed to be the site of the signal transducing machinery. Both POPC and raft-like environment produce significant alterations in structure and flexibility of the CD3 ζζ with respect to nuclear magnetic resonance (NMR) model: the dimer is more compact, its secondary structure is slightly less ordered, the arrangement of the Asp6 pair, which is important for binding to the Arg residue in the alpha chain of the T cell receptor (TCR), is stabilized by water molecules. Different interactions of charged residues with lipids at the lipid–cytoplasm boundary occur when the two environments are compared. Furthermore, in contrast to what is observed in POPC, in the raft-like environment correlated motions between transmembrane and cytoplasmic regions are observed. Altogether the data suggest that when the TCR complex resides in the raft domains, the CD3 ζζ dimer assumes a specific conformation probably necessary to the correct signal transduction.     

A Transmembrane Polar Interaction Is Involved in the Functional Regulation of Integrin αLβ2

Integrins are heterodimeric transmembrane (TM) receptors formed by noncovalent associations of α and β subunits. Each subunit contains a single α-helical TM domain. Inside-out activation of an integrin involves the separation of its cytoplasmic tails, leading to disruption of αβ TM packing. The leukocyte integrin αLβ2 is required for leukocyte adhesion, migration, proliferation, cytotoxic function, and antigen presentation. In this study, we show by mutagenesis experiments that the packing of αLβ2 TMs is consistent with that of the integrin αIIbβ3 TMs. However, molecular dynamics simulations of αLβ2 TMs in lipids predicted a polar interaction involving the side chains of αL Ser1071 and β2 Thr686 in the outer-membrane association clasp (OMC). This is supported by carbonyl vibrational shifts observed in isotope-labeled αLβ2 TM peptides that were incorporated into lipid bilayers. Molecular dynamics studies simulating the separation of αLβ2 tails showed the presence of polar interaction during the initial perturbation of the inner-membrane association clasp. When the TMs underwent further separation, the polar interaction was disrupted. OMC polar interaction is important in regulating the functions of β2 integrins because mutations that disrupt the OMC polar interaction generated constitutively activated αLβ2, αMβ2, and αXβ2 in 293T transfectants. We also show that the expression of mutant β2 Thr686Gly in β2-deficient T cells rescued cell adhesion to intercellular adhesion molecule 1, but the cells showed overt elongated morphologies in response to chemokine stromal-cell-derived factor 1α treatment as compared to wild-type β2-expressing cells. These two TM polar residues are totally conserved in other members of the β2 integrins in humans and across different species. Our results provide an example of the stabilizing effect of polar interactions within the low dielectric environment of the membrane interior and demonstrate its importance in the regulation of αLβ2 function.     

The CD3 epsilon subunit of the TCR contains endocytosis signals.

Ligand binding to TCR induces its internalization and cell surface down-modulation. These phenomena contribute to the extinction of activation signals. Due to the multicomponent nature of the TCR-CD3 complex, its internalization may be mediated by one or several of its subunits. Although it has been reported that CD3 gamma and CD3 delta contain endocytosis motifs involved in the internalization of the TCR-CD3 complex, other subunits could also be involved in this process. For instance, CD3 epsilon and CD zeta display amino acid sequences reminiscent of internalization motifs. To investigate whether CD3 epsilon bears endocytosis signals, we have analyzed the internalization capacity of a panel of deletion and point mutants of CD3 epsilon that were expressed on the cell surface independently of other TCR-CD3 subunits. Here we report that CD3 epsilon displays endocytosis determinants. These data indicate that CD3 epsilon could contribute to the internalization and cell surface down-regulation of TCR-CD3 complexes. Moreover, the existence of endocytosis signals in this polypeptide could serve to retrieve unassembled CD3 epsilon subunits or partial CD3 complexes from the plasma membrane, thus restricting the expression on the cell surface to fully functional TCR-CD3 complexes.     

Signaling through sphingolipid microdomains of the plasma membrane: The concept of signaling platform

Transmembrane signaling requires modular interactions between signaling proteins, phosphorylation or dephosphorylation of the interacting protein partners [1] and temporary elaboration of supramolecular structures [2], to convey the molecular information from the cell surface to the nucleus. Such signaling complexes at the plasma membrane are instrumental in translating the extracellular cues into intracellular signals for gene activation. In the most straightforward case, ligand binding promotes homodimerization of the transmembrane receptor which facilitates modular interactions between the receptor's cytoplasmic domains and intracellular signaling and adaptor proteins [3]. For example, most growth factor receptors contain a cytoplasmic protein tyrosine kinase (PTK) domain and ligand-mediated receptor dimerization leads to cross phosphorylation of tyrosines in the receptor's cytoplasmic domains, an event that initiates the signaling cascade [4]. In other signaling pathways where the receptors have no intrinsic kinase activity, intracellular non-receptor PTKs (i.e. Src family PTKs, JAKs) are recruited to the cytoplasmic domain of the engaged receptor. Execution of these initial phosphorylations and their translation into efficient cellular stimulation requires concomitant activation of diverse signaling pathways. Availability of stable, preassembled matrices at the plasma membrane would facilitate scaffolding of a large array of receptors, coreceptors, tyrosine kinases and other signaling and adapter proteins, as it is the case in signaling via the T cell antigen receptor [5]. The concept of the signaling platform [6] has gained usage to characterize the membrane structure where many different membrane-bound components need to be assembled in a coordinated manner to carry out signaling.The structural basis of the signaling platform lies in preferential assembly of certain classes of lipids into distinct physical and functional compartments within the plasma membrane [7,8]. These membrane microdomains or rafts (Figure 1) serve as privileged sites where receptors and proximal signaling molecules optimally interact [9]. In this review, we shall discuss first how signaling platforms are assembled and how receptors and their signaling machinery could be functionally linked in such structures. The second part of our review will deal with selected examples of raft-based signaling pathways in T lymphocytes and NK cells to illustrate the ways in which rafts may facilitate signaling.     

Nef-mediated suppression of T cell activation was lost in a lentiviral lineage that gave rise to HIV-1

High-level immune activation and T cell apoptosis represent a hallmark of HIV-1 infection that is absent from nonpathogenic SIV infections in natural primate hosts. The mechanisms causing these varying levels of immune activation are not understood. Here, we report that nef alleles from the great majority of primate lentiviruses, including HIV-2, downmodulate TCR-CD3 from infected T cells, thereby blocking their responsiveness to activation. In contrast, nef alleles from HIV-1 and a subset of closely related SIVs fail to downregulate TCR-CD3 and to inhibit cell death. Thus, Nef-mediated suppression of T cell activation is a fundamental property of primate lentiviruses that likely evolved to maintain viral persistence in the context of an intact host immune system. This function was lost during viral evolution in a lineage that gave rise to HIV-1 and may have predisposed the simian precursor of HIV-1 for greater pathogenicity in humans.     

Structural and functional dynamics of an integral membrane protein complex modulated by lipid headgroup charge

We have used membrane surface charge to modulate the structural dynamics of an integral membrane protein, phospholamban (PLB), and thereby its functional inhibition of the sarcoplasmic reticulum Ca-ATPase (SERCA). It was previously shown by electron paramagnetic resonance, in vesicles of neutral lipids, that the PLB cytoplasmic domain is in equilibrium between an ordered T state and a dynamically disordered R state and that phosphorylation of PLB increases the R state and relieves SERCA inhibition, suggesting that R is less inhibitory. Here, we sought to control the T/R equilibrium by an alternative means-varying the lipid headgroup charge, thus perturbing the electrostatic interaction of PLB's cationic cytoplasmic domain with the membrane surface. We resolved the T and R states not only by electron paramagnetic resonance in the absence of SERCA but also by time-resolved fluorescence resonance energy transfer from SERCA to PLB, thus probing directly the SERCA-PLB complex. Compared to neutral lipids, anionic lipids increased both the T population and SERCA inhibition, while cationic lipids had the opposite effects. In contrast to conventional models, decreased inhibition was not accompanied by decreased binding. We conclude that PLB binds to SERCA in two distinct structural states of the cytoplasmic domain: an inhibitory T state that interacts strongly with the membrane surface and a less inhibitory R state that interacts more strongly with the anionic SERCA cytoplasmic domain. Modulating membrane surface charge provides an effective way of investigating the correlation between structural dynamics and function of integral membrane proteins.