title: Thermodynamics of coupled folding in the interaction of archaeal RNase P proteins RPP21 and RPP29.
abstract: We have used isothermal titration calorimetry (ITC) to identify and describe binding-coupled equilibria in the interaction between two protein subunits of archaeal ribonuclease P (RNase P). In all three domains of life, RNase P is a ribonucleoprotein complex that is primarily responsible for catalyzing the Mg²⁺-dependent cleavage of the 5' leader sequence of precursor tRNAs during tRNA maturation. In archaea, RNase P has been shown to be composed of one catalytic RNA and up to five proteins, four of which associate in the absence of RNA as two functional heterodimers, POP5-RPP30 and RPP21-RPP29. Nuclear magnetic resonance studies of the Pyrococcus furiosus RPP21 and RPP29 proteins in their free and complexed states provided evidence of significant protein folding upon binding. ITC experiments were performed over a range of temperatures, ionic strengths, and pH values, in buffers with varying ionization potentials, and with a folding-deficient RPP21 point mutant. These experiments revealed a negative heat capacity change (ΔC(p)), nearly twice that predicted from surface accessibility calculations, a strong salt dependence for the interaction, and proton release at neutral pH, but a small net contribution from these to the excess ΔC(p). We considered potential contributions from protein folding and burial of interfacial water molecules based on structural and spectroscopic data. We conclude that binding-coupled protein folding is likely responsible for a significant portion of the excess ΔC(p). These findings provide novel structural and thermodynamic insights into coupled equilibria that allow specificity in macromolecular assemblies.
authors.affiliation: Ohio State Biochemistry Program, The Ohio State University, Columbus, Ohio 43210, United States.
authors.forename: Yiren
authors.initials: Y
authors.lastname: Xu
authors.forename: Sri Vidya
authors.initials: SV
authors.lastname: Oruganti
authors.forename: Venkat
authors.initials: V
authors.lastname: Gopalan
authors.forename: Mark P
authors.initials: MP
authors.lastname: Foster
mesh_terms: D019943:Amino Acid Substitution; D019843:Archaeal Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D002151:Calorimetry; D030562:Databases, Protein; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D046912:Multiprotein Complexes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D050505:Mutant Proteins / Q000737:chemistry / Q000378:metabolism; D019906:Nuclear Magnetic Resonance, Biomolecular; D009994:Osmolar Concentration; D017354:Point Mutation; D011487:Protein Conformation; D017510:Protein Folding; D021122:Protein Subunits / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D020027:Pyrococcus furiosus / Q000201:enzymology* / Q000378:metabolism; D043262:Ribonuclease P / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D013696:Temperature; D013816:Thermodynamics; D017186:Titrimetry
chemical_list: D019843:Archaeal Proteins; D046912:Multiprotein Complexes; D050505:Mutant Proteins; D021122:Protein Subunits; D043262:Ribonuclease P