Abstract
We have determined the solution structure of an albumin binding domain of protein G, a surface protein of group C and G streptococci. We find that it folds into a left handed three-helix bundle similar to the albumin binding domain of protein PAB from Peptostreptococcus magnus. The two domains share 59% sequence identity, are thermally very stable, and bind to the same site on human serum albumin. The albumin binding site, the first determined for this structural motif known as the GA module, comprises residues spanning the first loop to the beginning of the third helix and includes the most conserved region of GA modules. The two GA modules have different affinities for albumin from different species, and their albumin binding patterns correspond directly to the host specificity of C/G streptococci and P. magnus, respectively. These studies of the evolution, structure, and binding properties of the GA module emphasize the power of bacterial adaptation and underline ecological and medical problems connected with the use of antibiotics.
Highlights
In the complex molecular interplay between a pathogen and its human host, protein-protein interactions play important roles
We have determined the solution structure of an albumin binding domain of protein G, a surface protein of group C and G streptococci. We find that it folds into a left handed three-helix bundle similar to the albumin binding domain of protein PAB from Peptostreptococcus magnus
The solution structure of the GA module of protein PAB originating from protein G, designated ALB8-GA, has been reported previously [13]
Summary
Expression and Purification—Expression and purification of ALB8-GA will be described elsewhere. A peptide corresponding to a truncated ALB8-GA lacking the six N-terminal residues was purchased from Clinical Chemistry, Malmo General Hospital, where it was synthesized and subsequently purified using high performance liquid chromatography. Initial structures were obtained by distance geometry embedding using all atoms without metrization. These structures were thereafter refined using restrained simulated annealing. The binding of radiolabeled ALB8-GA to HSA-beads was blocked with unlabeled ALB8-GA, G148-GA3, or a synthetic peptide corresponding to a truncated ALB8-GA lacking the six N-terminal residues, respectively, as described previously [25]. The binding of radiolabeled ALB8-GA and radiolabeled G148-GA3 to HSA-beads, respectively, was blocked with purified albumin and serum from different mammalian species (all from Sigma). Two-dimensional 15N-1H heteronuclear single quantum coherence (HSQC) spectra at different albumin concentrations were recorded at a temperature of 37 and/or 47 °C using a Varian Unity 600 MHz NMR spectrometer
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