Showing papers on "Conformational change published in 2010"
TL;DR: Using a range of biophysical techniques, it is shown that the endocytic cargo binding of AP2 is driven by its interaction with PtdIns4,5P2-containing membranes.
315 citations
TL;DR: An “entropy meter” is calibrated employing an experimental dynamical proxy based on NMR relaxation and it is shown that changes in the conformational entropy of calmodulin are a significant component of the energetics of binding and the distribution of motion at the interface between the target domain and cal modulin are surprisingly non-complementary.
Abstract: NMR-measured order parameters of methyl groups can be used to quantitate the entropy of protein conformational change associated with calmodulin-peptide ligand interactions. This conformational entropy is a major contributor to the affinity of calmodulin interactions.
257 citations
TL;DR: A “ratchet” mechanism involved in the unidirectional translocation is demonstrated and the step size of NS3h is defined as one base per nucleotide hydrolysis cycle, suggesting feasible strategies for developing specific inhibitors to block the action of this attractive, yet largely unexplored drug target.
Abstract: A virally encoded superfamily-2 (SF2) helicase (NS3h) is essential for the replication of hepatitis C virus, a leading cause of liver disease worldwide. Efforts to elucidate the function of NS3h and to develop inhibitors against it, however, have been hampered by limited understanding of its molecular mechanism. Here we show x-ray crystal structures for a set of NS3h complexes, including ground-state and transition-state ternary complexes captured with ATP mimics (ADP·BeF3 and ). These structures provide, for the first time, three conformational snapshots demonstrating the molecular basis of action for a SF2 helicase. Upon nucleotide binding, overall domain rotation along with structural transitions in motif V and the bound DNA leads to the release of one base from the substrate base-stacking row and the loss of several interactions between NS3h and the 3′ DNA segment. As nucleotide hydrolysis proceeds into the transition state, stretching of a “spring” helix and another overall conformational change couples rearrangement of the (d)NTPase active site to additional hydrogen-bonding between NS3h and DNA. Together with biochemistry, these results demonstrate a “ratchet” mechanism involved in the unidirectional translocation and define the step size of NS3h as one base per nucleotide hydrolysis cycle. These findings suggest feasible strategies for developing specific inhibitors to block the action of this attractive, yet largely unexplored drug target.
226 citations
TL;DR: PRS demonstrates that several relevant modes can be induced simultaneously by perturbing a single select residue on the protein, and illustrates the biological relevance of applying PRS to the GroEL, adenylate kinase, myosin, and kinesin structures in detail.
146 citations
TL;DR: These results provide the first insight into the molecular mechanism of a phosphate transceptor, and indicate that signaling does not require complete transport of the substrate.
Abstract: A novel concept in eukaryotic signal transduction is the use of nutrient transporters and closely related proteins as nutrient sensors The action mechanism of these “transceptors” is unclear The Pho84 phosphate transceptor in yeast transports phosphate and mediates rapid phosphate activation of the protein kinase A (PKA) pathway during growth induction We have now identified several phosphate-containing compounds that act as nontransported signaling agonists of Pho84 This indicates that signaling does not require complete transport of the substrate For the nontransported agonist glycerol-3-phosphate (Gly3P), we show that it is transported by two other carriers, Git1 and Pho91, without triggering signaling Gly3P is a competitive inhibitor of transport through Pho84, indicating direct interaction with its phosphate-binding site We also identified phosphonoacetic acid as a competitive inhibitor of transport without agonist function for signaling This indicates that binding of a compound into the phosphate-binding site of Pho84 is not enough to trigger signaling Apparently, signaling requires a specific conformational change that may be part of, but does not require, the complete transport cycle Using Substituted Cysteine Accessibility Method (SCAM) we identified Phe160 in TMD IV and Val392 in TMD VIII as residues exposed with their side chain into the phosphate-binding site of Pho84 Inhibition of both transport and signaling by covalent modification of Pho84F160C or Pho84V392C showed that the same binding site is used for transport of phosphate and for signaling with both phosphate and Gly3P Our results provide to the best of our knowledge the first insight into the molecular mechanism of a phosphate transceptor
139 citations
TL;DR: It is proposed that IBMPFD mutations alter the timing of the transition between nucleotide states by destabilizing the ADP‐bound form and consequently interfere with the interactions between the N‐domains and their substrates.
Abstract: Mutations in p97, a major cytosolic AAA (ATPases associated with a variety of cellular activities) chaperone, cause inclusion body myopathy associated with Paget's disease of the bone and frontotemporal dementia (IBMPFD). IBMPFD mutants have single amino-acid substitutions at the interface between the N-terminal domain (N-domain) and the adjacent AAA domain (D1), resulting in a reduced affinity for ADP. The structures of p97 N–D1 fragments bearing IBMPFD mutations adopt an atypical N-domain conformation in the presence of Mg2+·ATPγS, which is reversible by ADP, showing for the first time the nucleotide-dependent conformational change of the N-domain. The transition from the ADP- to the ATPγS-bound state is accompanied by a loop-to-helix conversion in the N–D1 linker and by an apparent re-ordering in the N-terminal region of p97. X-ray scattering experiments suggest that wild-type p97 subunits undergo a similar nucleotide-dependent N-domain conformational change. We propose that IBMPFD mutations alter the timing of the transition between nucleotide states by destabilizing the ADP-bound form and consequently interfere with the interactions between the N-domains and their substrates.
136 citations
TL;DR: The nucleation of the early oligomers is concentration-dependent and accompanied by a conformational change that precedes beta-structure formation, and can be visualized using fluorescence resonance energy transfer between the donor- labeled N-terminus and the acceptor-labeled cysteine in the mutant A140C.
134 citations
TL;DR: The X-ray crystal structure of recombinant epi-isozizaene synthase (EIZS), a sesquiterpene cyclase from Streptomyces coelicolor A3(2), reveals an alternative binding orientation of BTAC; alternative binding orientations of a carbocation intermediate could lead to the formation of alternative products.
Abstract: The X-ray crystal structure of recombinant epi-isozizaene synthase (EIZS), a sesquiterpene cyclase from Streptomyces coelicolor A3(2), has been determined at 1.60 A resolution. Specifically, the structure of wild-type EIZS is that of its closed conformation in complex with three Mg(2+) ions, inorganic pyrophosphate (PP(i)), and the benzyltriethylammonium cation (BTAC). Additionally, the structure of D99N EIZS has been determined in an open, ligand-free conformation at 1.90 A resolution. Comparison of these two structures provides the first view of conformational changes required for substrate binding and catalysis in a bacterial terpenoid cyclase. Moreover, the binding interactions of BTAC may mimic those of a carbocation intermediate in catalysis. Accordingly, the aromatic rings of F95, F96, and F198 appear to be well-oriented to stabilize carbocation intermediates in the cyclization cascade through cation-pi interactions. Mutagenesis of aromatic residues in the enzyme active site results in the production of alternative sesquiterpene product arrays due to altered modes of stabilization of carbocation intermediates as well as altered templates for the cyclization of farnesyl diphosphate. Accordingly, the 1.64 A resolution crystal structure of F198A EIZS in a complex with three Mg(2+) ions, PP(i), and BTAC reveals an alternative binding orientation of BTAC; alternative binding orientations of a carbocation intermediate could lead to the formation of alternative products. Finally, the crystal structure of wild-type EIZS in a complex with four Hg(2+) ions has been determined at 1.90 A resolution, showing that metal binding triggers a significant conformational change of helix G to cap the active site.
132 citations
TL;DR: Peptides bind virions before attachment and are carried with virions into endosomes, the compartment in which acidification initiates fusion, and suggest strategies for inhibiting flavivirus entry.
Abstract: The mechanism of membrane fusion by “class II” viral fusion proteins follows a pathway that involves large-scale domain rearrangements of the envelope glycoprotein (E) and a transition from dimers to trimers. The rearrangement is believed to proceed by an outward rotation of the E ectodomain after loss of the dimer interface, followed by a reassociation into extended trimers. The ∼55-aa-residue, membrane proximal “stem” can then zip up along domain II, bringing together the transmembrane segments of the C-terminus and the fusion loops at the tip of domain II. We find that peptides derived from the stem of dengue-virus E bind stem-less E trimer, which models a conformational intermediate. In vitro assays demonstrate that these peptides specifically block viral fusion. The peptides inhibit infectivity with potency proportional to their affinity for the conformational intermediate, even when free peptide is removed from a preincubated inoculum before infecting cells. We conclude that peptides bind virions before attachment and are carried with virions into endosomes, the compartment in which acidification initiates fusion. Binding depends on particle dynamics, as there is no inhibition of infectivity if preincubation and separation are at 4°C rather than 37°C. We propose a two-step model for the mechanism of fusion inhibition. Targeting a viral entry pathway can be an effective way to block infection. Our data, which support and extend proposed mechanisms for how the E conformational change promotes membrane fusion, suggest strategies for inhibiting flavivirus entry.
126 citations
TL;DR: The findings reveal that the two tested activators can extract P-TEFb from the 7SK snRNP and a dramatic conformational change occurred in 7SK concomitant with the ejection of HEXIM1.
Abstract: Background
The positive transcription elongation factor, P-TEFb, is required for the production of mRNAs, however the majority of the factor is present in the 7SK snRNP where it is inactivated by HEXIM1. Expression of HIV-1 Tat leads to release of P-TEFb and HEXIM1 from the 7SK snRNP in vivo, but the release mechanisms are unclear.
Methodology/Principal Findings
We developed an in vitro P-TEFb release assay in which the 7SK snRNP immunoprecipitated from HeLa cell lysates using antibodies to LARP7 was incubated with potential release factors. We found that P-TEFb was directly released from the 7SK snRNP by HIV-1 Tat or the P-TEFb binding region of the cellular activator Brd4. Glycerol gradient sedimentation analysis was used to demonstrate that the same Brd4 protein transfected into HeLa cells caused the release of P-TEFb and HEXIM1 from the 7SK snRNP in vivo. Although HEXIM1 binds tightly to 7SK RNA in vitro, release of P-TEFb from the 7SK snRNP is accompanied by the loss of HEXIM1. Using a chemical modification method, we determined that concomitant with the release of HEXIM1, 7SK underwent a major conformational change that blocks re-association of HEXIM1.
Conclusions/Significance
Given that promoter proximally paused polymerases are present on most human genes, understanding how activators recruit P-TEFb to those genes is critical. Our findings reveal that the two tested activators can extract P-TEFb from the 7SK snRNP. Importantly, we found that after P-TEFb is extracted a dramatic conformational change occurred in 7SK concomitant with the ejection of HEXIM1. Based on our findings, we hypothesize that reincorporation of HEXIM1 into the 7SK snRNP is likely the regulated step of reassembly of the 7SK snRNP containing P-TEFb.
117 citations
TL;DR: An atomic model of the L-type straight filament is reported by electron cryomicroscopy and helical image analysis that shows interesting features: an orientation change of the outer core domains against the inner core domains showing almost invariant orientation and packing, a conformational switching within domain D1, and the conformational flexibility of domains D0 and D1 with their spoke-like connection for tight molecular packing.
Abstract: Bacterial flagellar protofilaments can adopt 2 distinct conformations (L- or R-type), resulting in different functional states (the bacteria swim or tumble). The R-type protofilament was characterized previously by cryo-EM; now the same analysis of the L-type conformation provides insight into the conformational changes involved in this switch. The bacterial flagellar filament is a helical propeller rotated by the flagellar motor for bacterial locomotion. The filament is a supercoiled assembly of a single protein, flagellin, and is formed by 11 protofilaments. For bacterial taxis, the reversal of motor rotation switches the supercoil between left- and right-handed, both of which arise from combinations of two distinct conformations and packing interactions of the L-type and R-type protofilaments. Here we report an atomic model of the L-type straight filament by electron cryomicroscopy and helical image analysis. Comparison with the R-type structure shows interesting features: an orientation change of the outer core domains (D1) against the inner core domains (D0) showing almost invariant orientation and packing, a conformational switching within domain D1, and the conformational flexibility of domains D0 and D1 with their spoke-like connection for tight molecular packing.
TL;DR: It is shown by using a combination of biophysical and mutagenic approaches that the interconversion between open and closed states of the ATP-binding subdomain involves partial subdomain unfolding/refolding in an otherwise folded enzyme.
Abstract: Enzyme function is often dependent on fluctuations between inactive and active structural ensembles. Adenylate kinase isolated from Escherichia coli (AK(e)) is a small phosphotransfer enzyme in whi ...
TL;DR: Detailed quantitative information is provided about binding of sodium ions in the D2 receptor and a possibly important sodium-induced conformational change for modulation of D2 receptors function is reported.
Abstract: G-protein coupled receptors, the largest family of proteins in the human genome, are involved in many complex signal transduction pathways, typically activated by orthosteric ligand binding and subject to allosteric modulation. Dopaminergic receptors, belonging to the class A family of G-protein coupled receptors, are known to be modulated by sodium ions from an allosteric binding site, although the details of sodium effects on the receptor have not yet been described. In an effort to understand these effects, we performed microsecond scale all-atom molecular dynamics simulations on the dopaminergic D2 receptor, finding that sodium ions enter the receptor from the extracellular side and bind at a deep allosteric site (Asp2.50). Remarkably, the presence of a sodium ion at this allosteric site induces a conformational change of the rotamer toggle switch Trp6.48 which locks in a conformation identical to the one found in the partially inactive state of the crystallized human β2 adrenergic receptor. This study provides detailed quantitative information about binding of sodium ions in the D2 receptor and reports a possibly important sodium-induced conformational change for modulation of D2 receptor function.
TL;DR: Investigation of the conformational and dynamic effects of thrombin ligation at the active site, exosite I and the Na+-binding site in solution, using modern multidimensional NMR techniques reveal that apo throm bin exists in a highly dynamic zymogen-like state, and relies on ligation to achieve a fully active conformation.
Abstract: The serine protease thrombin is generated from its zymogen prothrombin at the end of the coagulation cascade. Thrombin functions as the effector enzyme of blood clotting by cleaving several procoagulant targets, but also plays a key role in attenuating the hemostatic response by activating protein C. These activities all depend on the engagement of exosites on thrombin, either through direct interaction with a substrate, as with fibrinogen, or by binding to cofactors such as thrombomodulin. How thrombin specificity is controlled is of central importance to understanding normal hemostasis and how dysregulation causes bleeding or thrombosis. The binding of ligands to thrombin via exosite I and the coordination of Na + have been associated with changes in thrombin conformation and activity. This phenomenon has become known as thrombin allostery, although direct evidence of conformational change, identification of the regions involved, and the functional consequences remain unclear. Here we investigate the conformational and dynamic effects of thrombin ligation at the active site, exosite I and the Na + -binding site in solution, using modern multidimensional NMR techniques. We obtained full resonance assignments for thrombin in seven differently liganded states, including fully unliganded apo thrombin, and have created a detailed map of residues that change environment, conformation, or dynamic state in response to each relevant single or multiple ligation event. These studies reveal that apo thrombin exists in a highly dynamic zymogen-like state, and relies on ligation to achieve a fully active conformation. Conformational plasticity confers upon thrombin the ability to be at once selective and promiscuous.
TL;DR: The conformational changes of alcohol dehydrogenase demonstrate the importance of protein dynamics in catalysis and the impact of chemical or mutational substitutions on catalytic turnover.
TL;DR: It is shown that not all myristate ligands are functional inhibitors, but that the conformational state of C-terminal helix_I is a structural determinant for functional activity, and that allosteric agonists are kinase agonists.
Abstract: Allosteric inhibitors of Bcr-Abl have emerged as a novel therapeutic option for the treatment of CML. Using fragment-based screening, a search for novel Abl inhibitors that bind to the myristate pocket was carried out. Here we show that not all myristate ligands are functional inhibitors, but that the conformational state of C-terminal helix_I is a structural determinant for functional activity. We present an NMR-based conformational assay to monitor the conformation of this crucial helix_I and show that myristate ligands that bend helix_I are functional antagonists, whereas ligands that bind to the myristate pocket but do not induce this conformational change are kinase agonists. Activation of c-Abl by allosteric agonists has been confirmed in a biochemical assay.
TL;DR: The open and closed inward-facing forms of BCRP were generated by homology modeling, representing the substrate-free and substrate-bound conformations in the absence of nucleotide, respectively, which are consistent with the experimentally observed conformational change upon substrate binding.
TL;DR: DCTP and 3TC-TP bind with nearly equal affinities, but the binding of dCTP never reaches equilibrium, and previously conflicting reports suggesting that nucleotide analogs bind tighter than normal nucleotides are reconciled.
Abstract: Single turnover studies on HIV reverse transcriptase suggest that nucleoside analogs bind more tightly to the enzyme than normal substrates, contrary to rational structural predictions. Here we resolve these controversies by monitoring the kinetics of nucleotide-induced changes in enzyme structure. We show that the specificity constant for incorporation of a normal nucleotide (dCTP) is determined solely by the rate of binding (including isomerization) because isomerization to the closed complex commits the substrate to react. In contrast, a nucleoside analog (3TC-TP, triphosphate form of lamivudine) is incorporated slowly, allowing the conformational change to come to equilibrium and revealing tight nucleotide binding. Our data reconcile previously conflicting reports suggesting that nucleotide analogs bind tighter than normal nucleotides. Rather, dCTP and 3TC-TP bind with nearly equal affinities, but the binding of dCTP never reaches equilibrium. Discrimination against 3TC-TP is based on the slower rate of incorporation due to misalignment of the substrate and/or catalytic residues.
TL;DR: Mutation of the proline residues to alanine accelerates the aggregation kinetics ofα-syn while all proline mutants formed fibrils similar to His-WT α-syn, as visualized via transmission electron microscopy.
Abstract: The neuronal protein α-synuclein (α-syn) plays a central role in Parkinson's disease (PD). The pathological features of PD are the loss of dopaminergic neurons in the substantia nigra pars compacta and the presence of Lewy bodies. The C-terminal domain of α-syn is characterized by the presence of 15 acidic amino acids and all five proline residues of the protein (P108, P117, P120, P128, and P138). The aggregation of this natively unfolded protein is accelerated in vitro by FK506 binding proteins (FKBPs) showing peptidyl-prolyl cis-trans isomerase activity. These proteins catalyze the cis-trans conformational change of the X-Pro peptide bond, often a rate-limiting step in protein folding. The acceleration of the folding of α-syn by FKBPs may accelerate disease-associated aggregation. To further elucidate the role of the proline residues in the conformation and aggregation of α-syn, we constructed several mutants of α-syn in which one or more proline residues are mutated to alanine via site-directed mutagenesis. For this purpose, we produced and purified His-WT α-syn, a recombinant α-syn with a polyhistidine tag (six His residues) and a linker, and a number of Pro-to-Ala mutants. The aggregation kinetics of these mutants and His-WT α-syn were studied by turbidity, thioflavin T fluorescence, and CD measurements. We can conclude that mutation of the proline residues to alanine accelerates the aggregation kinetics of α-syn while all proline mutants formed fibrils similar to His-WT α-syn, as visualized via transmission electron microscopy. We also demonstrate that the accelerating effect of hFKBP12 is abolished via removal of the proline residues from the C-terminus. Finally, we show that the mutant of His α-syn with all five proline residues mutated to alanine is more structured (more α-helix) than His-WT α-syn, indicating the role of the Pro residues as potential helix breakers in the inhibitory conformation of the C-terminus.
TL;DR: It is directly demonstrated from seven structures of Aeromonus punctata PEP that the mechanism is instead induced fit: the native enzyme exists in a conformationally flexible opened state with a large interdomain opening between the β-propeller and α/β-hydrolase domains.
TL;DR: It is found that the local conformational change associated with the closing of active site precedes the global conformational changes leading to mechanical motions, which supports the importance of local motions at active site to drive or gate various conformational transitions underlying the workings of a diverse range of biological nanomachines.
Abstract: The decryption of sequence of structural events during protein conformational transitions is essential to a detailed understanding of molecular functions of various biological nanomachines. Coarse-grained models have proven useful by allowing highly efficient simulations of protein conformational dynamics. By combining two coarse-grained elastic network models constructed based on the beginning and end conformations of a transition, we have developed an interpolated elastic network model to generate a transition pathway between the two protein conformations. For validation, we have predicted the order of local and global conformational changes during key ATP-driven transitions in three important biological nanomachines (myosin, F(1) ATPase and chaperonin GroEL). We have found that the local conformational change associated with the closing of active site precedes the global conformational change leading to mechanical motions. Our finding is in good agreement with the distribution of intermediate experimental structures, and it supports the importance of local motions at active site to drive or gate various conformational transitions underlying the workings of a diverse range of biological nanomachines.
TL;DR: It is demonstrated that mildly acidic pH triggers specific conformational changes in herpes simplex virus gB, and proposed that intracellular low pH induces alterations in gB conformation that, together with additional triggers such as receptor binding, are essential for virion-cell fusion during herpesviral entry by endocytosis.
Abstract: Herpesviruses can enter host cells using pH-dependent endocytosis pathways in a cell-specific manner. Envelope glycoprotein B (gB) is conserved among all herpesviruses and is a critical component of the complex that mediates membrane fusion and entry. Here we demonstrate that mildly acidic pH triggers specific conformational changes in herpes simplex virus (HSV) gB. The antigenic structure of gB was specifically altered by exposure to low pH both in vitro and during entry into host cells. The oligomeric conformation of gB was altered at a similar pH range. Exposure to acid pH appeared to convert virion gB into a lower-order oligomer. The detected conformational changes were reversible, similar to those in other class III fusion proteins. Exposure of purified, recombinant gB to mildly acidic pH resulted in similar changes in conformation and caused gB to become more hydrophobic, suggesting that low pH directly affects gB. We propose that intracellular low pH induces alterations in gB conformation that, together with additional triggers such as receptor binding, are essential for virion-cell fusion during herpesviral entry by endocytosis.
TL;DR: This analysis suggests a mechanism by which the bound cation coordinates conformational change and ATP‐hydrolysis, and invites comparison with the current F‐actin model in order to understand the structural implications for actin polymerization.
Abstract: Heat shock proteins act as cytoplasmic chaperones to ensure correct protein folding and prevent protein aggregation. The presence of stoichiometric amounts of one such heat shock protein, Hsp27, in supersaturated solutions of unmodified G-actin leads to crystallization, in preference to polymerization, of the actin. Hsp27 is not evident in the resulting crystal structure. Thus, for the first time, we present the structure of G-actin in a form that is devoid of polymerization-deterring chemical modifications or binding partners, either of which may alter its conformation. The structure contains a calcium ion and ATP within a closed nucleotide-binding cleft, and the D-loop is disordered. This native G-actin structure invites comparison with the current F-actin model in order to understand the structural implications for actin polymerization. In particular, this analysis suggests a mechanism by which the bound cation coordinates conformational change and ATP-hydrolysis.
TL;DR: Thermodynamic analysis displays hydrophobic, electrostatic and hydrogen bonds interactions are the major acting forces in stabilizing the HSA–BM complex.
TL;DR: In this article, the authors describe atomistic molecular simulations that demonstrate that both Ras and Rho superfamily members harbor an intrinsic susceptibility to sample multiple conformational states in the absence of nucleotide ligand.
TL;DR: The vibrational modes of both TR-bound and RNA-free coat protein dimers using an all-atom normal-mode analysis suggest that asymmetric contacts between the A-duplex RNA phosphodiester backbone and the EF-loop in one coat protein subunit result in the FG-loop of that subunit becoming more dynamic, whilst the equivalent loop on the other monomer decreases its mobility.
TL;DR: The effects of local unfolding of an important hinge near Pro(177) on the closed-to-open transition of AK is characterized and a novel mechanism by which hinge unfolding modulates protein conformational change is identified.
TL;DR: Eight mutant Msh2-Msh6 complexes with defective responses to nucleotide binding and/or mispair binding are studied and properties of the different mutant complexes suggest that distinct conformational states mediated by communication between the Msh 2 and Msh6 nucleotide-binding sites are required for the formation of ternary complexes and sliding clamps.
TL;DR: The formation of gp120 dimers was disrupted by a single amino acid change in the inner domain, and was reduced by removal of the V1/V2 variable loops or the N and C termini, and should assist interpretation of studies of this key viral protein.
TL;DR: It is proposed that VP5*, the rotavirus protein believed to interact with the membrane bilayer, binds lipid bilayers in an intermediate conformational state, analogous to the extended intermediate conformation of enveloped-virus fusion proteins.
Abstract: During rotavirus entry, a virion penetrates a host cell membrane, sheds its outer capsid proteins, and releases a transcriptionally active subviral particle into the cytoplasm. VP5*, the rotavirus protein believed to interact with the membrane bilayer, is a tryptic cleavage product of the outer capsid spike protein, VP4. When a rotavirus particle uncoats, VP5* folds back, in a rearrangement that resembles the fusogenic conformational changes in enveloped-virus fusion proteins. We present direct experimental evidence that this rearrangement leads to membrane binding. VP5* does not associate with liposomes when mounted as part of the trypsin-primed spikes on intact virions, nor does it do so after it has folded back into a stably trimeric, low-energy state. But it does bind liposomes when they are added to virions before uncoating, and VP5* rearrangement is then triggered by addition of EDTA. The presence of liposomes during the rearrangement enhances the otherwise inefficient VP5* conformational change. A VP5* fragment, VP5CT, produced from monomeric recombinant VP4 by successive treatments with chymotrypsin and trypsin, also binds liposomes only when the proteolysis proceeds in their presence. A monoclonal antibody that neutralizes infectivity by blocking a postattachment entry event also blocks VP5* liposome association. We propose that VP5* binds lipid bilayers in an intermediate conformational state, analogous to the extended intermediate conformation of enveloped-virus fusion proteins.