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 pa thogenic bacteria, including Pseudomonas aeruginosa. Pilin is involved in a ttachment of the bacterium to host cells during infection, in the initiatio n of immune response, and serves as a receptor for a variety of bacteriopha ge. We have used N-15 nuclear magnetic resonance relaxation measurements to probe the backbone dynamics of an N-terminally truncated monomeric pilin f rom P. aeruginosa strain K122-4. N-15-T-1, -T-2, and {H-1}-N-15 nuclear Ove rhauser enhancement measurements were carried out at three magnetic field s trengths. The measurements were interpreted using the Lipari-Szabo model-fr ee analysis, which reveals the amplitude of spatial restriction for backbon e N-NH bond vectors with respect to nano- to picosecond time-scale motions. Regions of well-defined secondary structure exhibited consistently low-amp litude spatial fluctuations, while the terminal and loop regions showed lar ger amplitude motions in the subnano- to picosecond time-scale. Interesting ly, the C-terminal disulfide loop region that contains the receptor binding domain was found to be relatively rigid on the pico- to nanosecond time-sc ale but exhibited motion in the micro- to millisecond time-scale. It is not able that this disulfide loop displays a conserved antigenic epitope and me diates binding to the asialo-GM(1) cell surface receptor. The present study suggests that a rigid backbone scaffold mediates attachment to the host ce ll receptor, and also maintains the conformation of the conserved antigenic epitope for antibody recognition. In addition, slower millisecond time-sca le motions are likely to be crucial for conferring a range of specificity f or these interactions. Characterization of pilin dynamics will aid in devel oping 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.