Ligand-free open-closed transitions of periplasmic binding proteins: the case of glutamine-binding protein.
The ability to undergo large-scale domain rearrangements is essential for the substrate-binding function of periplasmic binding proteins (PBPs), which are indispensable for nutrient uptake in Gram-negative bacteria. Crystal structures indicate that PBPs typically adopt either an "open" unliganded configuration or a "closed" liganded one. However, it is not clear whether, as a general rule, PBPs remain open until ligand-induced interdomain closure or are in equilibrium with a minor population of unliganded, closed species. Evidence for the latter has been recently reported on maltose-binding protein (MBP) in aqueous solution [Tang, C., et al. (2007) Nature 449, 1078-1082] via paramagnetic relaxation enhancement (PRE), a technique able to probe lowly populated regions of conformational space. Here, we use PRE to study the unliganded open-closed transition of another PBP: glutamine-binding protein (GlnBP). Through a combination of domain structure knowledge and intermolecular and concentration dependence PRE experiments, a set of surface residues was found to be involved in intermolecular interactions. Barring such residues, PRE data on ligand-free GlnBP, paramagnetically labeled at two sites (one at a time), could be appropriately explained by the unliganded, open crystal structure in that it both yielded a good PRE fit and was not significantly affected by PRE-based refinement. Thus, contrary to MBP, our data did not particularly suggest the coexistence of a minor closed conformer. Several possibilities were explored to explain the observed differences in such closely structurally related systems; among them, a particularly interesting one arises from close inspection of the interdomain "hinge" region of various PBPs: strong hydrogen bond interactions discourage large-scale interdomain dynamics.
Bermejo
Guillermo A
GA
Laboratory of Molecular Biophysics, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892, USA.
Strub
Marie-Paule
MP
Ho
Chien
C
Tjandra
Nico
N
D002352:Carrier Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000737:chemistry / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D005973:Glutamine / Q000235:genetics / Q000378:metabolism*; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D058619:Maltose-Binding Proteins; D016297:Mutagenesis, Site-Directed; D033902:Periplasmic Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding / Q000235:genetics; D011487:Protein Conformation; D017434:Protein Structure, Tertiary / Q000235:genetics
D002352:Carrier Proteins; D029968:Escherichia coli Proteins; D008024:Ligands; D058619:Maltose-Binding Proteins; D033902:Periplasmic Binding Proteins; C037853:glutamine transport proteins; D005973:Glutamine