ABSTRACT
Periplasmic-binding proteins occupy the periplasmic space of bacteria and are involved in binding and transport of various ions, siderophores, and other diverse types of solutes. These proteins may be associated with membrane transport systems or may help in activation of signal transducers. There is limited information available on Mycobacterium tuberculosis (Mtb) periplasm-inhabiting proteins. In the present study, we have performed genome-wide identification and functional annotation of periplasmic-binding proteins of Mtb on the basis of signature characteristics and their functional motifs. 37 putative periplasmic-binding proteins were identified in Mtb proteome and categorized into different classes mainly known for their association with membrane transport and signaling pathways. Conclusively, this study adds 11 completely novel proteins to the periplasmic binding proteome of Mtb, which were not annotated as PBPs earlier. This study provides an overview of the periplasmic binding proteome of Mtb, which may be involved in various important patho-physiological functions of the bacteria. These proteins may serve as novel drug targets, which may lead to better treatment strategies against this deadly pathogen.
Subject(s)
Genome, Bacterial , Mycobacterium tuberculosis/genetics , Periplasmic Binding Proteins/genetics , Proteome/genetics , Humans , Molecular Sequence Annotation , Mycobacterium tuberculosis/pathogenicity , Periplasmic Binding Proteins/chemistry , Periplasmic Binding Proteins/classificationABSTRACT
Periplasmic binding proteins (PBPs) comprise a protein superfamily that is involved in prokaryotic solute transport and chemotaxis. These proteins have been used to engineer reagentless biosensors to detect natural or non-natural ligands. There is considerable interest in obtaining very stable members of this superfamily from thermophilic bacteria to use as robust engineerable parts in biosensor development. Analysis of the recently determined genome sequence of Thermus thermophilus revealed the presence of more than 30 putative PBPs in this thermophile. One of these is annotated as a glucose binding protein (GBP) based on its genetic linkage to genes that are homologous to an ATP-binding cassette glucose transport system, although the PBP sequence is homologous to periplasmic maltose binding proteins (MBPs). Here we present the cloning, over-expression, characterization of cognate ligands, and determination of the X-ray crystal structure of this gene product. We find that it is a very stable (apo-protein Tm value is 100(+/- 2) degrees C; complexes 106(+/- 3) degrees C and 111(+/- 1) degrees C for glucose and galactose, respectively) glucose (Kd value is 0.08(+/- 0.03) microM) and galactose (Kd value is 0.94(+/- 0.04) microM) binding protein. Determination of the X-ray crystal structure revealed that this T. thermophilus glucose binding protein (ttGBP) is structurally homologous to MBPs rather than other GBPs. The di or tri-saccharide ligands in MBPs are accommodated in long relatively shallow grooves. In the ttGBP binding site, this groove is partially filled by two loops and an alpha-helix, which create a buried binding site that allows binding of only monosaccharides. Comparison of ttGBP and MBP provides a clear example of structural adaptations by which the size of ligand binding sites can be controlled in the PBP super family.
Subject(s)
Bacterial Proteins/chemistry , Disaccharides/metabolism , Glucose/metabolism , Monosaccharides/metabolism , Periplasmic Binding Proteins/chemistry , Protein Structure, Tertiary , Thermus thermophilus/chemistry , Amino Acid Sequence , Bacterial Proteins/genetics , Bacterial Proteins/metabolism , Binding Sites , Carrier Proteins/chemistry , Carrier Proteins/genetics , Crystallography, X-Ray , Hydrogen Bonding , Ligands , Maltose-Binding Proteins , Models, Molecular , Molecular Sequence Data , Periplasmic Binding Proteins/classification , Periplasmic Binding Proteins/genetics , Periplasmic Binding Proteins/metabolism , Protein Binding , Protein Denaturation , Sequence Alignment , Thermus thermophilus/metabolismABSTRACT
The family of periplasmic binding proteins (PBPs) is believed to have arisen from a common ancestor and to have differentiated into two types. At first approximation, both types of PBPs have the same fold pattern, reflecting their common origin. However, the connection between the main chains of a type 2 PBP is more complicated than a type 1 PBP's. We have been interested in the possibility that such structural changes affect the folding of PBPs. In this study, we have characterized the folding pathways of MglB (a type 1 PBP) and ArgT (a type 2 PBP) by using urea gradient gel electrophoresis, fast protein size-exclusion liquid chromatography and hydrophobic dye ANS binding assay. We found a distinct difference in folding between these two proteins. The folding of MglB followed a simple two-state transition model, whereas the folding of ArgT was more complicated.
