Engineering Metamorphic Chemokine Lymphotactin/XCL1 into the GAG-Binding, HIV-Inhibitory Dimer Conformation.

Unlike other chemokines, XCL1 undergoes a distinct metamorphic interconversion between a canonical monomeric chemokine fold and a unique ß-sandwich dimer. The monomeric conformation binds and activates the receptor XCR1, whereas the dimer binds extracellular matrix glycosaminoglycans and has been associated with anti-human immunodeficiency virus (HIV) activity. Functional studies of WT-XCL1 are complex, as both conformations are populated in solution. To overcome this limitation, we engineered a stabilized dimeric variant of XCL1 designated CC5. This variant features a new disulfide bond (A36C-A49C) that prevents structural interconversion by locking the chemokine into the ß-sandwich dimeric conformation, as demonstrated by NMR structural analysis and hydrogen/deuterium exchange experiments. Functional studies analyzing glycosaminoglycan binding demonstrate that CC5 binds with high affinity to heparin. In addition, CC5 exhibits potent inhibition of HIV-1 activity in primary peripheral blood mononuclear cells (PBMCs), demonstrating the importance of the dimer in blocking viral infection. Conformational variants like CC5 are valuable tools for elucidating the biological relevance of the XCL1 native-state interconversion and will assist in future antiviral and functional studies.

Interconversion between two unrelated protein folds in the lymphotactin native state.

Proteins often have multiple functional states, which might not always be accommodated by a single fold. Lymphotactin (Ltn) adopts two distinct structures in equilibrium, one corresponding to the canonical chemokine fold consisting of a monomeric three-stranded beta-sheet and carboxyl-terminal helix. The second Ltn structure solved by NMR reveals a dimeric all-beta-sheet arrangement with no similarity to other known proteins. In physiological solution conditions, both structures are significantly populated and interconvert rapidly. Interconversion replaces long-range interactions that stabilize the chemokine fold with an entirely new set of tertiary and quaternary contacts. The chemokine-like Ltn conformation is a functional XCR1 agonist, but fails to bind heparin. In contrast, the alternative structure binds glycosaminoglycans with high affinity but fails to activate XCR1. Because each structural species displays only one of the two functional properties essential for activity in vivo, the conformational equilibrium is likely to be essential for the biological activity of lymphotactin. These results demonstrate that the functional repertoire and regulation of a single naturally occurring amino acid sequence can be expanded by access to a set of highly dissimilar native-state structures.

An engineered second disulfide bond restricts lymphotactin/XCL1 to a chemokine-like conformation with XCR1 agonist activity.

Chemokines adopt a conserved tertiary structure stabilized by two disulfide bridges and direct the migration of leukocytes. Lymphotactin (Ltn) is a unique chemokine in that it contains only one disulfide and exhibits large-scale structural heterogeneity. Under physiological solution conditions (37 degrees C and 150 mM NaCl), Ltn is in equilibrium between the canonical chemokine fold (Ltn10) and a distinct four-stranded beta-sheet (Ltn40). Consequently, it has not been possible to address the biological significance of each structural species independently. To stabilize the Ltn10 structure in a manner independent of specific solution conditions, Ltn variants containing a second disulfide bridge were designed. Placement of the new cysteines was based on a sequence alignment of Ltn with either the first (Ltn-CC1) or third disulfide (Ltn-CC3) in the CC chemokine, HCC-2. NMR data demonstrate that both CC1 and CC3 retain the Ltn10 chemokine structure and no longer exhibit structural rearrangement. The ability of each mutant to activate the Ltn receptor, XCR1, has been tested using an intracellular Ca2+ flux assay. These data support the conclusion that the chemokine fold of Ltn10 is responsible for receptor activation. We also examined the role of amino- and carboxyl-terminal residues in Ltn-mediated receptor activation. In contrast to previous reports, we find that the 25 residues comprising the novel C-terminal extension do not participate in receptor activation, while the native N-terminus is absolutely required for Ltn function.

Evaluation of 3-hydroxy-3-methylglutaryl-coenzyme A lyase arginine-41 as a catalytic residue: use of acetyldithio-coenzyme A to monitor product enolization.

3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) lyase catalyzes the divalent cation-dependent cleavage of HMG-CoA to produce acetyl-CoA and acetoacetate. Arginine-41 is an invariant residue in HMG-CoA lyases. Mutation of this residue (R41Q) correlates with human HMG-CoA lyase deficiency. To evaluate the functional importance of arginine-41, R41Q and R41M recombinant mutant human HMG-CoA lyase proteins have been constructed, expressed, and purified. These mutant proteins retain structural integrity based on Mn(2+) binding and affinity labeling stoichiometry. R41Q exhibits a 10(5)-fold decrease in V(max); R41M activity is >or=10-fold lower than the activity of R41Q. Acetyldithio-CoA, an analogue of the reaction product, acetyl-CoA, has been employed to test the function of arginine-41, as well as other residues (e.g., aspartate-42 and histidine-233) implicated in catalysis. Acetyldithio-CoA supports enzyme-catalyzed exchange of the methyl protons of the acetyl group with solvent; exchange is dependent on the presence of Mg(2+) and acetoacetate. In comparison with wild-type human enzyme, D42A and H233A mutant enzymes exhibit 4-fold and 10-fold decreases, respectively, in the proton exchange rate. In contrast, R41Q and R41M mutants do not catalyze any substantial enzyme-dependent proton exchange. These results suggest a role for arginine-41 in deprotonation or enolization of acetyldithio-CoA and implicate this residue in the HMG-CoA cleavage reaction chemistry that leads to acetyl-CoA product formation. Assignment of arginine-41 as an active site residue is also supported by a homology model for HMG-CoA lyase based on the structure of 4-hydroxy-2-ketovalerate aldolase. This model suggests the proximity of arginine-41 to other amino acids (aspartate-42, glutamate-72, histidine-235) implicated as active site residues based on their function as ligands to the activator cation.

Investigation of conserved acidic residues in 3-hydroxy-3-methylglutaryl-CoA lyase: implications for human disease and for functional roles in a family of related proteins.

3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) lyase catalyzes the divalent cation-dependent cleavage of HMG-CoA to form acetyl-CoA and acetoacetate. In metal-dependent aldol and Claisen reactions, acidic residues often function either as cation ligands or as participants in general acid/base catalysis. Site-directed mutagenesis was used to produce conservative substitutions for the conserved acidic residues Glu-37, Asp-42, Glu-72, Asp-204, Glu-279, and Asp-280. HMG-CoA lyase deficiency results from a human mutation that substitutes lysine for glutamate 279. The E279K mutation has also been engineered; expression in Escherichia coli produces an unstable protein. Substitution of alanine for glutamate 279 produces a protein that is sufficiently stable for isolation and retains substantial catalytic activity. However, thermal inactivation experiments demonstrate that E279A is much less stable than wild-type enzyme. HMG-CoA lyase deficiency also results from mutations at aspartate 42. Substitutions that eliminate a carboxyl group at residue 42 perturb cation binding and substantially lower catalytic efficiency (104-105-fold decreases in specific activity for D42A, D42G, or D42H versus wild-type). Substitutions of alanine for the other conserved acidic residues indicate the importance of glutamate 72. E72A exhibits a 200-fold decrease in kcat and >103-fold decrease in kcat/Km. E72A is also characterized by inflation in the Km for activator cation (26-fold for Mg2+; >200-fold for Mn2+). Similar, but less pronounced, effects are measured for the D204A mutant. E72A and D204A mutant proteins both bind stoichiometric amounts of Mn2+, but D204A exhibits only a 2-fold inflation in KD for Mn2+, whereas E72A exhibits a 12-fold inflation in KD (23 microm) in comparison with wild-type enzyme (KD = 1.9 microm). Acidic residues corresponding to HMG-CoA lyase Asp-42 and Glu-72 are conserved in the HMG-CoA lyase protein family, which includes proteins that utilize acetyl-CoA in aldol condensations. These related reactions may require an activator cation that binds to the corresponding acidic residues in this protein family.

Investigation of the oligomeric status of the peroxisomal isoform of human 3-hydroxy-3-methylglutaryl-CoA lyase.

Unprocessed 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) lyase, retaining the mitochondrial signal sequence, has been proposed to correspond to a peroxisomal isoform. Using a modified expression plasmid and purification protocol, it is now possible to isolate substantial amounts (>10mg) of highly purified peroxisomal HMG-CoA lyase. These improvements facilitate more detailed protein chemistry approaches for characterization of the enzyme, which exhibits substantial (eightfold) dithiothreitol (DTT) stimulation of activity. The C323S mutant shows little DTT activation. Superose gel filtration chromatography data have prompted other investigators to hypothesize that the peroxisomal isoform is a monomer. This study confirms the elution properties presented in that earlier report, but also demonstrates anomalous elution up on Superose chromatography. Elution properties observed using a polyacrylamide resin (Bio-Gel P100) suggest a dimeric, rather than monomeric, enzyme. This observation has been further tested by protein chemistry experiments. The peroxisomal enzyme forms a covalently linked dimeric species upon crosslinking with dibromopropanone or o-phenylenedimaleimide or upon disulfide formation as a result of incubation with diamide. Cysteine-323 is required for intersubunit covalent crosslinking. Crosslinking efficiency is not dependent on HMG-CoA lyase protein concentration nor is it influenced by the presence of varying concentrations of an unrelated protein, such as ovalbumin. Sedimentation equilibrium analyses do not indicate a monomeric form of either human mitochondrial or human peroxisomal HMG-CoA lyase; the results suggest that these proteins are predominantly dimers. The retention of the basic N-terminal mitochondrial signal sequence in the peroxisomal HMG-CoA lyase isoform may influence elution from Superose gel filtration media but does not alter the oligomeric status of the enzyme.