ABSTRACT
A mutation in the allosteric site of the caspase 3 dimer interface of Val266 to histidine abolishes activity of the enzyme, and models predict that the mutation mimics the action of small molecule allosteric inhibitors by preventing formation of the active site. Mutations were coupled to His266 at two sites in the interface, E124A and Y197C. We present results from X-ray crystallography, enzymatic activity and molecular dynamics simulations for seven proteins, consisting of single, double and triple mutants. The results demonstrate that considering allosteric inhibition of caspase 3 as a shift between discrete 'off-state' or 'on-state' conformations is insufficient. Although His266 is accommodated in the interface, the structural defects are propagated to the active site through a helix on the protein surface. A more comprehensive view of allosteric regulation of caspase 3 requires the representation of an ensemble of inactive states and shows that subtle structural changes lead to the population of the inactive ensemble.
Subject(s)
Allosteric Site/genetics , Amino Acid Substitution , Caspase 3/chemistry , Allosteric Regulation/genetics , Caspase 3/genetics , Catalytic Domain , Crystallography, X-Ray , Humans , Hydrogen Bonding , Molecular Dynamics Simulation , Mutagenesis, Site-Directed , Protein Interaction Domains and Motifs , Protein Structure, SecondaryABSTRACT
Interactions between loops 2, 2' and 4, known as the loop bundle, stabilize the active site of caspase-3. Loop 4 (L4) is of particular interest due to its location between the active site and the dimer interface. We have disrupted a salt bridge between K242 and E246 at the base of L4 to determine its role in overall conformational stability and in maintaining the active site environment. Stability measurements show that only the K242A single mutant decreases stability of the dimer, whereas both single mutants and the double mutant demonstrate much lower activity compared to wild-type caspase-3. Structural studies of the caspase-3 variants show the involvement of K242 in hydrophobic interactions that stabilize helix 5, near the dimer interface, and the role of E246 appears to be to neutralize the positive charge of K242 within the hydrophobic cluster. Overall, the results suggest E246 and K242 are important in procaspase-3 for their interaction with neighboring residues, not with one another. Conversely, formation of the K242-E246 salt bridge in caspase-3 is needed for an accurate, stable conformation of loop L4 and proper active site formation in the mature enzyme.
Subject(s)
Caspase 3/chemistry , Caspase 3/metabolism , Static Electricity , Caspase 3/genetics , Catalytic Domain , Crystallography, X-Ray , Enzyme Stability , Hydrogen Bonding , Models, Molecular , Mutation , Protein Folding , Structure-Activity Relationship , ThermodynamicsABSTRACT
The caspase-3 zymogen has essentially zero activity until it is cleaved by initiator caspases during apoptosis. However, a mutation of V266E in the dimer interface activates the protease in the absence of chain cleavage. We show that low concentrations of the pseudo-activated procaspase-3 kill mammalian cells rapidly and, importantly, this protein is not cleaved nor is it inhibited efficiently by the endogenous regulator XIAP (X-linked inhibitor of apoptosis). The 1.63 A (1 A = 0.1 nm) structure of the variant demonstrates that the mutation is accommodated at the dimer interface to generate an enzyme with substantially the same activity and specificity as wild-type caspase-3. Structural modelling predicts that the interface mutation prevents the intersubunit linker from binding in the dimer interface, allowing the active sites to form in the procaspase in the absence of cleavage. The direct activation of procaspase-3 through a conformational switch rather than by chain cleavage may lead to novel therapeutic strategies for inducing cell death.
Subject(s)
Apoptosis , Caspase 3/metabolism , Enzyme Precursors/metabolism , X-Linked Inhibitor of Apoptosis Protein/metabolism , Amino Acid Substitution , Binding Sites/genetics , Blotting, Western , Caspase 3/chemistry , Caspase 3/genetics , Cell Line , Crystallography, X-Ray , Enzyme Activation , Enzyme Precursors/chemistry , Enzyme Precursors/genetics , Humans , Models, Molecular , Mutation , Protein Conformation , Protein Multimerization , Protein Structure, Tertiary , Structure-Activity Relationship , Transfection , X-Linked Inhibitor of Apoptosis Protein/geneticsABSTRACT
We describe here the use of several spectroscopies, such as fluorescence emission, circular dichroism, and differential quenching by acrylamide, in examining the equilibrium and kinetic folding of proteins. The first section regarding equilibrium techniques provides practical information for determining the conformational stability of a protein. In addition, several equilibrium-folding models are discussed, from two-state monomer to four-state homodimer, providing a comprehensive protocol for interpretation of folding curves. The second section focuses on the experimental design and interpretation of kinetic data, such as burst-phase analysis and exponential fits, used in elucidating kinetic folding pathways. In addition, simulation programs are used routinely to support folding models generated by kinetic experiments, and the fundamentals of simulations are covered.
