Backbone dynamics of receptor binding and antigenic regions of a Pseudomonas aeruginosa pilin monomer.

Pilin is the major structural protein that forms type IV pili of various pathogenic bacteria, including Pseudomonas aeruginosa. Pilin is involved in attachment of the bacterium to host cells during infection, in the initiation of immune response, and serves as a receptor for a variety of bacteriophage. We have used (15)N nuclear magnetic resonance relaxation measurements to probe the backbone dynamics of an N-terminally truncated monomeric pilin from P. aeruginosa strain K122-4. (15)N-T(1), -T(2), and [(1)H]-(15)N nuclear Overhauser enhancement measurements were carried out at three magnetic field strengths. The measurements were interpreted using the Lipari-Szabo model-free analysis, which reveals the amplitude of spatial restriction for backbone N-NH bond vectors with respect to nano- to picosecond time-scale motions. Regions of well-defined secondary structure exhibited consistently low-amplitude spatial fluctuations, while the terminal and loop regions showed larger amplitude motions in the subnano- to picosecond time-scale. Interestingly, the C-terminal disulfide loop region that contains the receptor binding domain was found to be relatively rigid on the pico- to nanosecond time-scale but exhibited motion in the micro- to millisecond time-scale. It is notable that this disulfide loop displays a conserved antigenic epitope and mediates binding to the asialo-GM(1) cell surface receptor. The present study suggests that a rigid backbone scaffold mediates attachment to the host cell receptor, and also maintains the conformation of the conserved antigenic epitope for antibody recognition. In addition, slower millisecond time-scale motions are likely to be crucial for conferring a range of specificity for these interactions. Characterization of pilin dynamics will aid in developing a detailed understanding of infection, and will facilitate the design of more efficient anti-adhesin synthetic vaccines and therapeutics against pathogenic bacteria containing type IV pili.