Differences in backbone dynamics of two homologous bacterial albumin-binding modules: implications for binding...

Proteins G and PAB are bacterial albumin-binding proteins expressed at the surface of group C and G streptococci and Peptostreptococcus magnus, respectively. Repeated albumin-binding domains, known as GA modules, are found in both proteins. The third GA module of protein G from the group G streptococcal strain G148 (G148-GA3) and the second GA module of protein PAB from P.magnus strain ALB8 (ALB8-GA) exhibit 59% sequence identity and both fold to form three-helix bundle structures that are very stable against thermal denaturation. ALB8-GA binds human serum albumin with higher affinity than G148-GA3, but G148-GA3 shows substantially broader albumin-binding specificity than ALB8-GA. The (15)N nuclear magnetic resonance spin relaxation measurements reported here, show that the two GA modules exhibit mobility on the picosecond-nanosecond time scale in directly corresponding regions (loops and termini). Most residues in G148-GA3 were seen to be involved in conformational exchange processes on the microsecond-millisecond time scale, whereas for ALB8-GA such motions were only identified for the beginning of helix 2 and its preceding loop. Furthermore, and more importantly, hydrogen-deuterium exchange and saturation transfer experiments reveal large differences between the two GA modules with respect to motions on the second-hour time scale. The high degree of similarity between the two GA modules with respect to sequence, structure and stability, and the observed differences in dynamics, binding affinity and binding specificity to different albumins, suggest a distinct correlation between dynamics, binding affinity and binding specificity. Finally, it is noteworthy in this context that the module G148-GA3, which has broad albumin-binding specificity, is expressed by group C and G streptococci known to infect all mammalian species, whereas P.magnus with the ALB8-GA module has been isolated only from humans.