Wild-type Domain Research Articles

Focal Adhesion Kinase (FAK) Binds RET Kinase via Its FERM Domain, Priming a Direct and Reciprocal RET-FAK Transactivation Mechanism

Whether RET is able to directly phosphorylate and activate downstream targets independently of the binding of proteins that contain Src homology 2 or phosphotyrosine binding domains and whether mechanisms in trans by cytoplasmic kinases can modulate RET function and signaling remain largely unexplored. In this study, oligopeptide arrays were used to screen substrates directly phosphorylated by purified recombinant wild-type and oncogenic RET kinase domain in the presence or absence of small molecule inhibitors. The results of the peptide array were validated by enzyme kinetics, in vitro kinase, and cell-based experiments. The identification of focal adhesion kinase (FAK) as a direct substrate for RET kinase revealed (i) a RET-FAK transactivation mechanism consisting of direct phosphorylation of FAK Tyr-576/577 by RET and a reciprocal phosphorylation of RET by FAK, which crucially is able to rescue the kinase-impaired RET K758M mutant and (ii) that FAK binds RET via its FERM domain. Interestingly, this interaction is abolished upon RET phosphorylation, indicating that RET binding to the FERM domain of FAK is a priming step for RET-FAK transactivation. Finally, our data indicate that FAK inhibitors could be used as potential therapeutic agents for patients with multiple endocrine neoplasia type 2 tumors because both, treatment with the FAK kinase inhibitor NVP-TAE226 and FAK down-regulation by siRNA reduced RET phosphorylation and signaling as well as the proliferation and survival of tumor and transfected cell lines expressing oncogenic RET.

A Large Mutational Study in Pachyonychia Congenita

Pachyonychia congenita (PC) is a rare autosomal dominant skin disorder characterized predominantly by nail dystrophy and painful palmoplantar keratoderma. Additional clinical features include oral leukokeratosis, follicular keratosis, and cysts (steatocysts and pilosebaceous cysts). PC is due to heterozygous mutations in one of four keratin genes, namely, KRT6A, KRT6B, KRT16, or KRT17. Here, we report genetic analysis of 90 new families with PC in which we identified mutations in KRT6A, KRT6B, KRT16, or KRT17, thereby confirming their clinical diagnosis. A total of 21 previously unreported and 22 known mutations were found. Approximately half of the kindreds had mutations in KRT6A (52%), 28% had mutations in KRT16, 17% in KRT17, and 3% of families had mutations in KRT6B. Most of the mutations were heterozygous missense or small in-frame insertion/deletion mutations occurring within one of the helix boundary motif regions of the keratin polypeptide. More unusual mutations included heterozygous splice site mutations, nonsense mutations, and a 1-bp insertion mutation, leading to a frameshift and premature termination codon. This study, together with previously reported mutations, identifies mutation hotspot codons that may be useful in the development of personalized medicine for PC.

Structural basis for endosomal recruitment of ESCRT-I by ESCRT-0 in yeast

The ESCRT-0 and ESCRT-I complexes coordinate the clustering of ubiquitinated cargo with intralumenal budding of the endosomal membrane, two essential steps in vacuolar/lysosomal protein sorting from yeast to humans. The 1.85-Å crystal structure of interacting regions of the yeast ESCRT-0 and ESCRT-I complexes reveals that PSDP motifs of the Vps27 ESCRT-0 subunit bind to a novel electropositive N-terminal site on the UEV domain of the ESCRT-I subunit Vps23 centred on Trp16. This novel site is completely different from the C-terminal part of the human UEV domain that binds to P(S/T)AP motifs of human ESCRT-0 and HIV-1 Gag. Disruption of the novel PSDP-binding site eliminates the interaction in vitro and blocks enrichment of Vps23 in endosome-related class E compartments in yeast cells. However, this site is non-essential for sorting of the ESCRT cargo Cps1. Taken together, these results show how a conserved motif/domain pair can evolve to use strikingly different binding modes in different organisms.

Abstract 2327: Structural and kinetic characterization of crizotinib with wild-type and mutant anaplastic lymphoma kinase

Abstract Dysregulation of Anaplastic Lymphoma Kinase (ALK), primarily through gene translocations, has been shown to be involved in a variety of cancers. Crizotinib, an orally available small molecule inhibitor of the ALK tyrosine kinase has demonstrated marked efficacy in clinical trials of NSCLC patients harboring the EML4-ALK oncogenic gene rearrangement. Mutation of some residues within the ALK kinase domain have been reported to confer acquired or de novo resistance to crizotinib. To understand the binding of crizotinib to ALK and the mechanism of resistance of specific mutations we generated and kinetically characterized wild-type (WT) and mutant ALK kinase domains (KD). Additionally, ALK KD crystal structures were determined of the WT nonphosphorylated apoenzyme and complexes with crizotinib bound to WT and a L1196M gatekeeper mutation. No large protein conformational changes are necessary for crizotinib to bind to unliganded ALK. The interactions which crizotinib makes with ALK are similar to its binding to c-Met with the exception of notable differences in the position of the activation loop between ALK and c-Met. Mutation of the L1196 gatekeeper residue to methionine results in a ∼8-fold increase in catalytic efficiency of phosphorylation of an activation loop peptide and also more rapid enzyme auto-phosphorylation. In addition, inhibition of L1196M ALK by crizotinib was reduced ∼9-fold, compared to wild-type enzyme, from Ki determinations. The crystal structures show that L1196 or M1196 make direct contact with crizotinib. The diminished activity of crizotinib against L1196M ALK is therefore likely due to both higher intrinsic kinase activity and a subtle change in the ALK-crizotinib binding interactions. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2327. doi:10.1158/1538-7445.AM2011-2327

Effect of computational methodology on the conformational dynamics of the protein photosensor LOV1 from Chlamydomonas reinhardtii

LOV domains are the light-sensitive protein domains of plant phototropins and bacteria. They photochemically form a covalent bond between a flavin mononucleotide (FMN) chromophore and a cysteine, attached to the apo-protein, upon irradiation with blue light, which triggers a signal in the adjacent kinase. Although their signaling state has been well characterized through experimental means, their signal transduction pathway as well as dark-state activity are generally only poorly understood. Here we show results from molecular dynamics simulations where we investigated the effect of thermostating and long-range electrostatics on the solution structure and dynamical behavior of the wild-type LOV1 domain from the green algae Chlamydomonas reinhardtii in the dark. We demonstrate that these computational issues can dramatically affect the conformational fluctuations of such protein domains by suppressing configurations far from equilibrium or destabilizing local configurations, leading to artificial changes of the protein secondary structure as well as the H-bond network formed by the amino acids and the FMN. By comparing our calculation results with recent experimental data, we show that the non-invasive thermostating strategy, where the protein solute is only indirectly coupled to the thermostat via the solvent, in conjunction with the particle-mesh Ewald technique, provides dark-state conformers, which are in consistency with experimental observations. Moreover, our calculations indicate that the LOV1 domains can alter the intersystem crossing rate and rate of adduct formation by adjusting the population distribution of these dark-state conformers. This might permit them to function as a modulator of the signal intensity under low light conditions. The online version of this article (doi:10.1007/s12154-011-0060-z) contains supplementary material, which is available to authorized users.

Structure and function of a HECT domain ubiquitin‐binding site

The Rsp5 ubiquitin ligase contains a non-covalent binding site for ubiquitin within the amino-terminal lobe (N-lobe) of the HECT domain, and the X-ray crystal structure of the HECT-ubiquitin complex has been determined. Hydrophobic patch residues of ubiquitin (L8, I44, V70) were crucial for interaction with Rsp5, and amino-acid alterations at the Rsp5-binding interface resulted in defects in polyubiquitination. Our results support a model in which the N-lobe-binding site acts to localize and orient the distal end of the ubiquitin chain to promote conjugation of the next ubiquitin molecule.

Synergistic tumor growth-inhibitory effect of the prostate-specific antigen-activated fusion peptide BSD352 for prostate cancer therapy

Prostate-specific antigen (PSA), a serine protease, is a promising target for the development of prodrugs in prostate cancer treatment. In this study, we designed a novel fusion peptide, BSD352, containing three functional domains: a protein transduction domain from HIV transactivating regulatory protein (TAT) followed by the BH3 domain of the p53 upregulated modulator of apoptosis (TAT-BH3), an anti-vascular endothelial growth factor peptide (SP5.2), and an anti-basic fibroblast growth factor peptide (DG2). These different domains in BSD352 were linked together by a linker sequence corresponding to a PSA hydrolytic substrate peptide. The BSD352 fusion peptide could be selectively cleaved by PSA in PSA-producing LNCaP prostate cancer cells. Furthermore, the BSD352 fusion peptide was efficiently transduced into tumor cells both in vitro and in vivo, and the BH3 domain was found to induce tumor cell apoptosis by elevating the expression of Bax, cytochrome C release, and caspase-9 cleavage. Moreover, the SP5.2 and DG2 domains in the BSD352 fusion peptide also exhibited in-vitro endothelial cell growth inhibition and in-vivo antiangiogenic activities. Direct injection of BSD352 into an established LNCaP xenograft tumor in mice inhibited tumor growth, whereas a synergistic effect was observed with the combined use of wild-type BH3, SP5.2, and DG2 functional domains. These results suggest that BSD352 could be beneficial for the treatment of accessible prostate tumors and may provide a complementary strategy for prostate cancer therapy.

Redox State-Dependent Interaction of HMGB1 and Cisplatin-Modified DNA

HMGB1, one of the most abundant nuclear proteins, has a strong binding affinity for cisplatin-modified DNA. It has been proposed that HMGB1 enhances the anticancer efficacy of cisplatin by shielding platinated DNA lesions from repair. Two cysteine residues in HMGB1 domain A form a reversible disulfide bond under mildly oxidizing conditions. The reduced domain A protein binds to a 25-bp DNA probe containing a central 1,2-d(GpG) intrastrand cross-link, the major platinum-DNA adduct, with a 10-fold greater binding affinity than the oxidized domain A. The binding affinities of singly and doubly mutated HMGB1 domain A, respectively deficient in one or both cysteine residues that form the disulfide bond, are unaffected by changes in external redox conditions. The redox-dependent nature of the binding of HMGB1 domain A to cisplatin-modified DNA suggests that formation of the intradomain disulfide bond induces a conformational change that disfavors binding to cisplatin-modified DNA. Hydroxyl radical footprinting analyses of wild-type domain A bound to platinated DNA under different redox conditions revealed identical cleavage patterns, implying that the asymmetric binding mode of the protein across from the platinated lesion is conserved irrespective of the redox state. The results of this study reveal that the cellular redox environment can influence the interaction of HMGB1 with the platinated DNA and suggest that the redox state of the A domain is a potential factor in regulating the role of the protein in modulating the activity of cisplatin as an anticancer drug.

Shallow Boomerang-Shaped Influenza Hemagglutinin G13A Mutant Structure Promotes Leaky Membrane Fusion

Our previous studies showed that an angled boomerang-shaped structure of the influenza hemagglutinin (HA) fusion domain is critical for virus entry into host cells by membrane fusion. Since the acute angle of ∼105 degree of the wild-type fusion domain promotes efficient non-leaky membrane fusion, we asked whether different angles would still support fusion and thus facilitate virus entry. Here, we show that the G13A fusion domain mutant produces a new leaky fusion phenotype. The mutant fusion domain structure was solved by NMR spectroscopy in a lipid environment at fusion pH. The mutant adopts a similar boomerang structure as wild-type, but with a shallower kink angle of ∼150 degree. G13A perturbs the structure of model membranes to a lesser degree than wild-type, but to a greater degree than non-fusogenic fusion domain mutants. The strength of G13A binding to lipid bilayers is also intermediate between that of wild-type and non-fusogenic mutants. These membrane interactions provide a clear link between structure and function of influenza fusion domains: An acute angle is required to promote clean non-leaky fusion suitable for virus entry, presumably by interaction of the fusion domain with the transmembrane domain deep in the lipid bilayer. A shallower angle perturbs the bilayer of the target membrane so that it becomes leaky and unable to form a clean fusion pore. Mutants with no fixed boomerang angle interact with bilayers weakly and do not promote any fusion or membrane perturbation.

Effect of Human RyR2 CPVT Mutations on Interaction with Calmodulin

The ryanodine receptor (RyR) is a member of a family of intracellular calcium release channels that regulate calcium efflux from intracellular stores. The RyR2 isoform is most abundant in the heart and plays a key role in cardiac muscle excitation-contraction coupling. Clusters of mutations associated with the inherited arrhythmogenic disorder, catecholaminergic polymorphic ventricular tachycardia (CPVT), have been found in specific regions throughout RyR2, a large protein of ∼5000 amino acids. Many of these CPVT mutations (total >100) are thought to occur in significant functional domains and result in the dysregulation of RyR channel function.One such region of RyR2 is believed to comprise a calmodulin (CaM) interaction site and two EF hand motifs. The calcium-sensitive binding of CaM has been shown to regulate the opening of RyR. Hence, examining the RyR2 interaction with CaM and the potential effects of CPVT mutations on this binding may help reveal mutation-dependent mechanisms of channel dysfunction.We have prepared bacterial expression plasmid constructs containing the wild-type human RyR2 CaM-interacting domain and introduced a series of CPVT mutations that have previously been identified to occur within this region. Expression and purification of the corresponding recombinant fusion proteins has enabled calcium-dependent binding of CaM to be determined with all these constructs. Examining the distinct functional role of calcium concentration on CaM binding kinetics and further comparative structural analyses of the wild-type and mutant RyR2 domains may help reveal the specific effect(s) that CaM-mediated regulation may have in mediating CPVT-linked arrhythmogenesis.

A Novel Non-SET Domain Multi-subunit Methyltransferase Required for Sequential Nucleosomal Histone H3 Methylation by the Mixed Lineage Leukemia Protein-1 (MLL1) Core Complex

Gene expression within the context of eukaryotic chromatin is regulated by enzymes that catalyze histone lysine methylation. Histone lysine methyltransferases that have been identified to date possess the evolutionarily conserved SET or Dot1-like domains. We previously reported the identification of a new multi-subunit histone H3 lysine 4 methyltransferase lacking homology to the SET or Dot1 family of histone lysine methyltransferases. This enzymatic activity requires a complex that includes WRAD (WDR5, RbBP5, Ash2L, and DPY-30), a complex that is part of the MLL1 (mixed lineage leukemia protein-1) core complex but that also exists independently of MLL1 in the cell. Here, we report that the minimal complex required for WRAD enzymatic activity includes WDR5, RbBP5, and Ash2L and that DPY-30, although not required for enzymatic activity, increases the histone substrate specificity of the WRAD complex. We also show that WRAD requires zinc for catalytic activity, displays Michaelis-Menten kinetics, and is inhibited by S-adenosyl-homocysteine. In addition, we demonstrate that WRAD preferentially methylates lysine 4 of histone H3 within the context of the H3/H4 tetramer but does not methylate nucleosomal histone H3 on its own. In contrast, we find that MLL1 and WRAD are required for nucleosomal histone H3 methylation, and we provide evidence suggesting that each plays distinct structural and catalytic roles in the recognition and methylation of a nucleosome substrate. Our results indicate that WRAD is a new H3K4 methyltransferase with functions that include regulating the substrate and product specificities of the MLL1 core complex.

Substrate Specificity and Catalysis by the Editing Active Site of Alanyl-tRNA Synthetase from Escherichia coli

Aminoacyl-tRNA synthetases (ARSs) enhance the fidelity of protein synthesis through multiple mechanisms, including hydrolysis of the adenylate and cleavage of misacylated tRNA. Alanyl-tRNA synthetase (AlaRS) limits misacylation with glycine and serine by use of a dedicated editing domain, and a mutation in this activity has been genetically linked to a mouse model of a progressive neurodegenerative disease. Using the free-standing Pyrococcus horikoshii AlaX editing domain complexed with serine as a model and both Ser-tRNA(Ala) and Ala-tRNA(Ala) as substrates, the deacylation activities of the wild type and five different Escherichia coli AlaRS editing site substitution mutants were characterized. The wild-type AlaRS editing domain deacylated Ser-tRNA(Ala) with a k(cat)/K(M) of 6.6 × 10(5) M(-1) s(-1), equivalent to a rate enhancement of 6000 over the rate of enzyme-independent deacylation but only 12.2-fold greater than the rate with Ala-tRNA(Ala). While the E664A and T567G substitutions only minimally decreased k(cat)/K(M,) Q584H, I667E, and C666A AlaRS were more compromised in activity, with decreases in k(cat)/K(M) in the range of 6-, 6.6-, and 15-fold. C666A AlaRS was 1.7-fold more active on Ala-tRNA(Ala) relative to Ser-tRNA(Ala), providing the only example of a true reversal of substrate specificity and highlighting a potential role of the coordinated zinc in editing substrate specificity. Along with the potentially serious physiological consequences of serine misincorporation, the relatively modest specificity of the AlaRS editing domain may provide a rationale for the widespread phylogenetic distribution of AlaX free-standing editing domains, thereby contributing a further mechanism to lower concentrations of misacylated tRNA(Ala).

In Vitro Enhanced Sensitivity to Cisplatin in D67Y BRCA1 RING Domain Protein

BRCA1 is a tumor suppressor protein involved in maintaining genomic integrity through multiple functions in DNA damage repair, transcriptional regulation, cell cycle checkpoint, and protein ubiquitination. The BRCA1-BARD1 RING complex has an E3 ubiquitin ligase function that plays essential roles in response to DNA damage repair. BRCA1-associated cancers have been shown to confer a hypersensitivity to chemotherapeutic agents. Here, we have studied the functional consequence of the in vitro E3 ubiquitin ligase activity and cisplatin sensitivity of the missense mutation D67Y BRCA1 RING domain. The D67Y BRCA1 RING domain protein exhibited the reduced ubiquitination function, and was more susceptible to the drug than the D67E or wild-type BRCA1 RING domain protein. This evidence emphasized the potential of using the BRCA1 dysfunction as an important determinant of chemotherapy responses in breast cancer.

Basic Residues in the Nucleocapsid Domain of Gag Are Critical for Late Events of HIV-1 Budding

The p6 region of HIV-1 Gag contains two late (L) domains, PTAP and LYPXnL, that bind the cellular proteins Tsg101 and Alix, respectively. These interactions are thought to recruit members of the host fission machinery (ESCRT) to facilitate HIV-1 release. Here we report a new role for the p6-adjacent nucleocapsid (NC) domain in HIV-1 release. The mutation of basic residues in NC caused a pronounced decrease in virus release from 293T cells, although NC mutant Gag proteins retained the ability to interact with cellular membranes and RNAs. Remarkably, electron microscopy analyses of these mutants revealed arrested budding particles at the plasma membrane, analogous to those seen following the disruption of the PTAP motif. This result indicated that the basic residues in NC are important for virus budding. When analyzed in physiologically more relevant T-cell lines (Jurkat and CEM), NC mutant viruses remained tethered to the plasma membrane or to each other by a membranous stalk, suggesting membrane fission impairment. Remarkably, NC mutant release defects were alleviated by the coexpression of a Gag protein carrying a wild-type (WT) NC domain but devoid of all L domain motifs and by providing alternative access to the ESCRT pathway, through the in trans expression of the ubiquitin ligase Nedd4.2s. Since NC mutant Gag proteins retained the interaction with Tsg101, we concluded that NC mutant budding arrests might have resulted from the inability of Gag to recruit or utilize members of the host ESCRT machinery that act downstream of Tsg101. Together, these data support a model in which NC plays a critical role in HIV-1 budding.

C-IAP1 and UbcH5 promote K11-linked polyubiquitination of RIP1 in TNF signalling

Ubiquitin ligases are critical components of the ubiquitination process that determine substrate specificity and, in collaboration with E2 ubiquitin-conjugating enzymes, regulate the nature of polyubiquitin chains assembled on their substrates. Cellular inhibitor of apoptosis (c-IAP1 and c-IAP2) proteins are recruited to TNFR1-associated signalling complexes where they regulate receptor-stimulated NF-κB activation through their RING domain ubiquitin ligase activity. Using a directed yeast two-hybrid screen, we found several novel and previously identified E2 partners of IAP RING domains. Among these, the UbcH5 family of E2 enzymes are critical regulators of the stability of c-IAP1 protein following destabilizing stimuli such as TWEAK or CD40 signalling or IAP antagonists. We demonstrate that c-IAP1 and UbcH5 family promote K11-linked polyubiquitination of receptor-interacting protein 1 (RIP1) in vitro and in vivo. We further show that TNFα-stimulated NF-κB activation involves endogenous K11-linked ubiquitination of RIP1 within the TNFR1 signalling complex that is c-IAP1 and UbcH5 dependent. Lastly, NF-κB essential modifier efficiently binds K11-linked ubiquitin chains, suggesting that this ubiquitin linkage may have a signalling role in the activation of proliferative cellular pathways.

Structural Basis for the Properties of Two Single‐Site Proline Mutants of CYP102A1 (P450BM3)

The crystal structures of the haem domains of Ala330Pro and Ile401Pro, two single-site proline variants of CYP102A1 (P450(BM3)) from Bacillus megaterium, have been solved. In the A330P structure, the active site is constricted by the relocation of the Pro329 side chain into the substrate access channel, providing a basis for the distinctive C-H bond oxidation profiles given by the variant and the enhanced activity with small molecules. I401P, which is exceptionally active towards non-natural substrates, displays a number of structural similarities to substrate-bound forms of the wild-type enzyme, notably an off-axial water ligand, a drop in the proximal loop, and the positioning of two I-helix residues, Gly265 and His266, the reorientation of which prevents the formation of several intrahelical hydrogen bonds. Second-generation I401P variants gave high in vitro oxidation rates with non-natural substrates as varied as fluorene and propane, towards which the wild-type enzyme is essentially inactive. The substrate-free I401P haem domain had a reduction potential slightly more oxidising than the palmitate-bound wild-type haem domain, and a first electron transfer rate that was about 10 % faster. The electronic properties of A330P were, by contrast, similar to those of the substrate-free wild-type enzyme.

Creating Designed Zinc-Finger Nucleases with Minimal Cytotoxicity

Zinc-finger nucleases (ZFNs) have emerged as powerful tools for delivering a targeted genomic double-strand break (DSB) to either stimulate local homologous recombination with investigator-provided donor DNA or induce gene mutations at the site of cleavage in the absence of a donor by nonhomologous end joining both in plant cells and in mammalian cells, including human cells. ZFNs are formed by fusing zinc-finger proteins to the nonspecific cleavage domain of the FokI restriction enzyme. ZFN-mediated gene targeting yields high gene modification efficiencies (>10%) in a variety of cells and cell types by delivering a recombinogenic DSB to the targeted chromosomal locus, using two designed ZFNs. The mechanism of DSB by ZFNs requires (1) two ZFN monomers to bind to their adjacent cognate sites on DNA and (2) the FokI nuclease domains to dimerize to form the active catalytic center for the induction of the DSB. In the case of ZFNs fused to wild-type FokI cleavage domains, homodimers may also form; this could limit the efficacy and safety of ZFNs by inducing off-target cleavage. In this article, we report further refinements to obligate heterodimer variants of the FokI cleavage domain for the creation of custom ZFNs with minimal cellular toxicity. The efficacy and efficiency of the reengineered obligate heterodimer variants of the FokI cleavage domain were tested using the green fluorescent protein gene targeting reporter system. The three-finger and four-finger zinc-finger protein fusions to the REL_DKK pair among the newly generated FokI nuclease domain variants appear to eliminate or greatly reduce the toxicity of designer ZFNs to human cells.

110 Targeting focal adhesion kinase (FAK) in primary human schwannoma

Activating mutations in the tyrosine kinase domain of receptor tyrosine kinases (RTKs) cause cancer and skeletal disorders. Comparison of the crystal structures of unphosphorylated and phosphorylated wild-type FGFR2 kinase domains with those of seven unphosphorylated pathogenic mutants reveals an autoinhibitory “molecular brake” mediated by a triad of residues in the kinase hinge region of all FGFRs. Structural analysis shows that many other RTKs, including PDGFRs, VEGFRs, KIT, CSF1R, FLT3, TEK, and TIE, are also subject to regulation by this brake. Pathogenic mutations activate FGFRs and other RTKs by disengaging the brake either directly or indirectly.

Shallow Boomerang-shaped Influenza Hemagglutinin G13A Mutant Structure Promotes Leaky Membrane Fusion

Our previous studies showed that an angled boomerang-shaped structure of the influenza hemagglutinin (HA) fusion domain is critical for virus entry into host cells by membrane fusion. Because the acute angle of ∼105° of the wild-type fusion domain promotes efficient non-leaky membrane fusion, we asked whether different angles would still support fusion and thus facilitate virus entry. Here, we show that the G13A fusion domain mutant produces a new leaky fusion phenotype. The mutant fusion domain structure was solved by NMR spectroscopy in a lipid environment at fusion pH. The mutant adopted a boomerang structure similar to that of wild type but with a shallower kink angle of ∼150°. G13A perturbed the structure of model membranes to a lesser degree than wild type but to a greater degree than non-fusogenic fusion domain mutants. The strength of G13A binding to lipid bilayers was also intermediate between that of wild type and non-fusogenic mutants. These membrane interactions provide a clear link between structure and function of influenza fusion domains: an acute angle is required to promote clean non-leaky fusion suitable for virus entry presumably by interaction of the fusion domain with the transmembrane domain deep in the lipid bilayer. A shallower angle perturbs the bilayer of the target membrane so that it becomes leaky and unable to form a clean fusion pore. Mutants with no fixed boomerang angle interacted with bilayers weakly and did not promote any fusion or membrane perturbation.

In Vitro and in Vivo Analysis of the Binding of the C Terminus of the HDL Receptor Scavenger Receptor Class B, Type I (SR-BI), to the PDZ1 Domain of Its Adaptor Protein PDZK1

The PDZ1 domain of the four PDZ domain-containing protein PDZK1 has been reported to bind the C terminus of the HDL receptor scavenger receptor class B, type I (SR-BI), and to control hepatic SR-BI expression and function. We generated wild-type (WT) and mutant murine PDZ1 domains, the mutants bearing single amino acid substitutions in their carboxylate binding loop (Lys(14)-Xaa(4)-Asn(19)-Tyr-Gly-Phe-Phe-Leu(24)), and measured their binding affinity for a 7-residue peptide corresponding to the C terminus of SR-BI ((503)VLQEAKL(509)). The Y20A and G21Y substitutions abrogated all binding activity. Surprisingly, binding affinities (K(d)) of the K14A and F22A mutants were 3.2 and 4.0 μM, respectively, similar to 2.6 μM measured for the WT PDZ1. To understand these findings, we determined the high resolution structure of WT PDZ1 bound to a 5-residue sequence from the C-terminal SR-BI ((505)QEAKL(509)) using x-ray crystallography. In addition, we incorporated the K14A and Y20A substitutions into full-length PDZK1 liver-specific transgenes and expressed them in WT and PDZK1 knock-out mice. In WT mice, the transgenes did not alter endogenous hepatic SR-BI protein expression (intracellular distribution or amount) or lipoprotein metabolism (total plasma cholesterol, lipoprotein size distribution). In PDZK1 knock-out mice, as expected, the K14A mutant behaved like wild-type PDZK1 and completely corrected their hepatic SR-BI and plasma lipoprotein abnormalities. Unexpectedly, the 10-20-fold overexpressed Y20A mutant also substantially, but not completely, corrected these abnormalities. The results suggest that there may be an additional site(s) within PDZK1 that bind(s) SR-BI and mediate(s) productive SR-BI-PDZK1 interaction previously attributed exclusively to the canonical binding of the C-terminal SR-BI to PDZ1.