title: Determining the molecular basis for the pH-dependent interaction between the link module of human TSG-6 and hyaluronan.
abstract: TSG-6 is an inflammation-associated hyaluronan (HA)-binding protein that has anti-inflammatory and protective functions in arthritis and asthma as well as a critical role in mammalian ovulation. The interaction between TSG-6 and HA is pH-dependent, with a marked reduction in affinity on increasing the pH from 6.0 to 8.0. Here we have investigated the mechanism underlying this pH dependence using a combined approach of site-directed mutagenesis, NMR, isothermal titration calorimetry and microtiter plate assays. Analysis of single-site mutants of the TSG-6 Link module indicated that the loss in affinity above pH 6.0 is mediated by the change in ionization state of a histidine residue (His(4)) that is not within the HA-binding site. To understand this in molecular terms, the pH-dependent folding profile and the pK(a) values of charged residues within the Link module were determined using NMR. These data indicated that His(4) makes a salt bridge to one side-chain oxygen atom of a buried aspartate residue (Asp(89)), whereas the other oxygen is simultaneously hydrogen-bonded to a key HA-binding residue (Tyr(12)). This molecular network transmits the change in ionization state of His(4) to the HA-binding site, which explains the loss of affinity at high pH. In contrast, simulations of the pH affinity curves indicate that another histidine residue, His(45), is largely responsible for the gain in affinity for HA between pH 3.5 and 6.0. The pH-dependent interaction of TSG-6 with HA (and other ligands) provides a means of differentially regulating the functional activity of this protein in different tissue microenvironments.
authors.affiliation: Medical Research Council Immunochemistry Unit, University of Oxford, South Parks Road, Oxford, UK.
authors.forename: Charles D
authors.initials: CD
authors.lastname: Blundell
authors.forename: David J
authors.initials: DJ
authors.lastname: Mahoney
authors.forename: Martin R
authors.initials: MR
authors.lastname: Cordell
authors.forename: Andrew
authors.initials: A
authors.lastname: Almond
authors.forename: Jan D
authors.initials: JD
authors.lastname: Kahmann
authors.forename: András
authors.initials: A
authors.lastname: Perczel
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Taylor
authors.forename: Iain D
authors.initials: ID
authors.lastname: Campbell
authors.forename: Anthony J
authors.initials: AJ
authors.lastname: Day
mesh_terms: D000596:Amino Acids / Q000737:chemistry / Q000378:metabolism; D015815:Cell Adhesion Molecules / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D006820:Hyaluronic Acid / Q000737:chemistry* / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008024:Ligands; D011485:Protein Binding; D017510:Protein Folding; D017434:Protein Structure, Tertiary
chemical_list: D000596:Amino Acids; D015815:Cell Adhesion Molecules; D008024:Ligands; C072273:TNFAIP6 protein, human; D006820:Hyaluronic Acid