title: Thermodynamics of GTP and GDP binding to bacterial initiation factor 2 suggests two types of structural transitions.
abstract: During initiation of messenger RNA translation in bacteria, the GTPase initiation factor (IF) 2 plays major roles in the assembly of the preinitiation 30S complex and its docking to the 50S ribosomal subunit leading to the 70S initiation complex, ready to form the first peptide bond in a nascent protein. Rapid and accurate initiation of bacterial protein synthesis is driven by conformational changes in IF2, induced by GDP-GTP exchange and GTP hydrolysis. We have used isothermal titration calorimetry and linear extrapolation to characterize the thermodynamics of the binding of GDP and GTP to free IF2 in the temperature interval 4-37 degrees C. IF2 binds with about 20-fold and 2-fold higher affinity for GDP than for GTP at 4 and 37 degrees C, respectively. The binding of IF2 to both GTP and GDP is characterized by a large heat capacity change (-868+/-25 and -577+/-23 cal mol(-1) K(-1), respectively), associated with compensatory changes in binding entropy and enthalpy. From our data, we propose that GTP binding to IF2 leads to protection of hydrophobic amino acid residues from solvent by the locking of switch I and switch II loops to the gamma-phosphate of GTP, as in the case of elongation factor G. From the large heat capacity change (also upon GDP binding) not seen in the case of elongation factor G, we propose the existence of yet another type of conformational change in IF2, which is induced by GDP and GTP alike. Also, this transition is likely to protect hydrophobic groups from solvent, and its functional relevance is discussed.
authors.affiliation: Institute of Technology, University of Tartu, Nooruse Street 1, Room 425, 50411 Tartu, Estonia.
authors.forename: Vasili
authors.initials: V
authors.lastname: Hauryliuk
authors.forename: Vladimir A
authors.initials: VA
authors.lastname: Mitkevich
authors.forename: Albena
authors.initials: A
authors.lastname: Draycheva
authors.forename: Stoyan
authors.initials: S
authors.lastname: Tankov
authors.forename: Viktoriya
authors.initials: V
authors.lastname: Shyp
authors.forename: Andrey
authors.initials: A
authors.lastname: Ermakov
authors.forename: Alexandra A
authors.initials: AA
authors.lastname: Kulikova
authors.forename: Alexander A
authors.initials: AA
authors.lastname: Makarov
authors.forename: Mans
authors.initials: M
authors.lastname: Ehrenberg
mesh_terms: D000494:Allosteric Regulation; D002151:Calorimetry / Q000379:methods; D006153:Guanosine Diphosphate / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000378:metabolism*; D007700:Kinetics; D039881:Prokaryotic Initiation Factor-2 / Q000737:chemistry* / Q000378:metabolism*; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D013696:Temperature; D013816:Thermodynamics
chemical_list: D039881:Prokaryotic Initiation Factor-2; D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate