Crystal Structure and Biological Implications of a Bacterial Albumin Binding Module in Complex with Human Serum Albumin

Abstract

Many bactericide species express surface proteins that interact with human serum albumin (HSA). Protein PAB from the anaerobic bacterium Finegoldia magna (formerly Peptostreptococcus magnus) represents one of these proteins. Protein PAB contains a domain of 53 amino acid residues known as the GA module. GA homologs are also found in protein G of group C and G streptococci. Here we report the crystal structure of HSA in complex with the GA module of protein PAB. The model of the complex was refined to a resolution of 2.7 Å and reveals a novel binding epitope located in domain II of the albumin molecule. The GA module is composed of a left-handed threehelix bundle, and residues from the second helix and the loops surrounding it were found to be involved in HSA binding. Furthermore, the presence of HSAbound fatty acids seems to influence HSA-GA complex formation. F. magna has a much more restricted host specificity compared with C and G streptococci, which is also reflected in the binding of different animal albumins by proteins PAB and G. The structure of the HSA-GA complex offers a molecular explanation to this unusually clear example of bacterial adaptation.

GA/GB Fold switching may modulate fatty acid transfer from human serum albumin to bacteria

“…The energy for Myr binding calculated by the AutoDock Vina force field software (25) is more than 2 kcal/ mol lower in the case of the 4b1a form with respect to the alla form (24.5 and 22.1 kcal/mol, respectively) which translates in a K d value almost two orders of magnitude lower for the 4b1a form (5.5 3 10 24 vs. 3.0 3 10 22 M for the all-a form). Interestingly, the GA regions 29-36 and 45-49, which in the docking simulations appear crucial for binding of Myr to both structural forms, correspond approximately to the entire binding interface observed in the structure of the complex between the GA domain of the bacterial PAB protein and the HSA (15).…”

“…1). Although the biological function(s) of the GA module of the bacterial PAB protein is unknown, the acquisition of the GA module adds selective advantages to the bacterium in terms of growth rate and increase in virulence, probably by providing the bacteria with FAs and, possibly, other nutrients transported by HSA (15). Despite the wealth of information available on structural and functional properties of HSA (4), nothing has been reported so far in terms of molecular mechanims modulating HSA delipidation by the GA module of the bacterial PAB protein.…”

“…The GA module of the bacterial PAB protein is shown in blue. Atomic coordinates were taken from the PDB entry 1TF0 (15). Both pictures were produced using the UCSF Chimera molecular graphics program (26).…”

“…At the opposite end of the binding interface, a second hydrogen bond network is formed between residues of the loop connecting the second and third a-helix in the GA module and residues of a-helices 2-3 present in the HSA domain IIA, this second network involving the HSA residues Glu230 and Asn267, and the GA residues Ala35 and Thr37. In the HSA-GA complex, a myristate (Myr) molecule is bound in the FA6 site, pointing with its carboxyl group toward the GA region comprising residues Phe27 and Tyr28 (15).…”

A fluorescent fatty acid probe, DAUDA, selectively displaces two myristates bound in human serum albumin

“…Protein crystals are usually frozen with mother liquor containing some cryoprotectants, typically 20% glycerol or ethylene glycol. 18,19 However, these typical cryoprotectant cannot be used for HSA. We have discovered a proper cryoprotectant (5-10% DMSO) to cryo-cool HSA crystals.…”