title: Nucleotide recognition by the initiation factor aIF5B: free energy simulations of a neoclassical GTPase.
abstract: The GTPase aIF5B is a universally conserved initiation factor that assists ribosome assembly. Crystal structures of its nucleotide complexes, X-ray(GTP) and X-ray(GDP), are similar in the nucleotide vicinity, but differ in the orientation of a distant domain IV. This has led to two, contradictory, mechanistic models. One postulates that X-ray(GTP) and X-ray(GDP) are, respectively, the active, "ON" and the inactive, "OFF" states; the other postulates that both structures are OFF, whereas the ON state is still uncharacterized. We study GTP/GDP binding using molecular dynamics and a continuum electrostatic free energy method. We predict that X-ray(GTP) has a ≈ 3 kcal/mol preference to bind GDP, apparently contradicting its assignment as ON. However, the preference arises mainly from a single, nearby residue from the switch 2 motif: Glu81, which becomes protonated upon GTP binding, with a free energy cost of about 4 kcal/mol. We then propose a different model, where Glu81 protonation/deprotonation defines the ON/OFF states. With this model, the X-ray(GTP):GTP complex, with its protonated Glu81, is ON, whereas X-ray(GTP):GDP is OFF. The model postulates that distant conformational changes such as domain IV rotation are "uncoupled" from GTP/GDP exchange and do not affect the relative GTP/GDP binding affinities. We analyze the model using a general thermodynamic framework for GTPases. It yields rather precise predictions for the nucleotide specificities of each state, and the state specificities of each nucleotide, which are roughly comparable to the homologues IF2 and aIF2, despite the lack of any conformational switching in the model.
authors.affiliation: Department of Biology, Laboratoire de Biochimie (CNRS UMR7654), Ecole Polytechnique, 91128 Palaiseau, France. thomas.simonson@polytechnique.fr
authors.forename: Thomas
authors.initials: T
authors.lastname: Simonson
authors.forename: Priyadarshi
authors.initials: P
authors.lastname: Satpati
mesh_terms: D019843:Archaeal Proteins / Q000737:chemistry* / Q000378:metabolism; D020558:GTP Phosphohydrolases / Q000737:chemistry* / Q000378:metabolism; D006153:Guanosine Diphosphate / Q000737:chemistry* / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000737:chemistry* / Q000378:metabolism; D017014:Methanobacterium / Q000737:chemistry / Q000378:metabolism; D008956:Models, Chemical; D056004:Molecular Dynamics Simulation; D010448:Peptide Initiation Factors / Q000737:chemistry* / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D013816:Thermodynamics
chemical_list: D019843:Archaeal Proteins; D010448:Peptide Initiation Factors; D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; D020558:GTP Phosphohydrolases