NMR Studies of Bacterial Albumin-Binding Modules

Abstract: Peptostreptococcus magnus and group C/G streptococci are Gram-positive cocci that are part of the normal flora and reside in more or less the same locations in the host. Protein G of group C/G streptococci was originally described as binding IgGFc of several species and later shown to also bind human serum albumin and albumins from various animal species. From albumin-binding strains of P. magnus several albumin-binding proteins have been isolated, one of which is protein PAB (Peptostreptococcal Albumin Binding), containing a domain of 45 residues showing a high (59 %) degree of homology to the human serum albumin binding domains of protein G and which was therefore called the GA (protein G-related Albumin binding) module. It has been suggested that the GA module has been transferred from protein G into protein PAB recently and that it represents the first contemporary example of module shuffling (M. de Chateau and L. Björck, J. Biol. Chem., 269:12147-51, 1994). The GA modules of protein PAB and protein G have been studied in this thesis. They are shown to be very stable and form three-helical bundles. It has been shown that they bind to the same site on human serum albumin, but that the GA module of protein PAB binds human serum albumin with higher affinity than the GA module of protein G does. It has also been shown that the GA module of protein G shows a broader albumin-binding specificity than the GA module of protein PAB. Group C/G streptococci infect virtually all mammalian species, whereas P. magnus has been isolated only from humans. During the relatively short time period when the protein G-GA of C/G streptococci has been part of protein PAB of P. magnus, its only function has therefore most likely been to bind human serum albumin. As a result the GA module of protein PAB binds non-primate albumins with much lower affinity than primate albumins, whereas for the GA module of protein G the difference in affinity between non-primate and primate is much less pronounced. Group C/G streptococci are known to infect most, if not all, mammalian species, and nearly all strains of these bacteria bind albumin. By comparison, P. magnus has only been seen to infect humans, and although most strains of P. magnus do not bind albumin, those that do have typically been isolated from patients with deep wound infections. The greater flexibility of the GA module of protein G seen from the Nitrogen-15 spin relaxation experiments is in line with its broad species specificity and, likewise for the less flexible GA module of protein PAB, which has narrow specificity and high affinity for human serum albumin. In view of the evolutionary relationship between the two GA modules our results suggest an evolution development during which the GA module of protein PAB has adapted to its environment, which contains only human serum albumin. This adaptation to the human host has led to a loss of molecular flexibility and a more narrow albumin-binding specificity.