Mutual conformational adaptations in antigen and antibody upon complex formation between an fab and HIV-1 capsid protein p24

Background: Elucidating the structural basis of antigen-antibody recognitio n ideally requires a structural comparison of free and complexed components . To this end we have studied a mouse monoclonal antibody, denoted 13B5, ra ised against p24, the capsid protein of HIV-1. We have previously described the first crystal structure of intact p24 as visualized in the Fab13B5-p24 complex. Here we report the structure of the uncomplexed Fab13B5 at 1.8 An gstrom resolution and analyze the Fab-p24 interface and the conformational changes occurring upon complex formation. Results: Fab13B5 recognizes a nearly continuous epitope comprising a helix- turn-helix motif in the C-terminal domain of p24. Only 4 complementarity-de termining regions (CDRs) are in contact with p24 with most interactions bei ng by the heavy chain. Comparison of the free and complexed Fab reveals tha t structural changes upon binding are localized to a few side chains of CDR -H1 and -H2 but involve a larger, concerted displacement of CDR-H3. Antigen binding is also associated with an 8 degrees relative rotation of the heav y and light chain variable regions. In p24, small conformational changes lo calized to the turn between the two helices comprising the epitope result f rom Fab binding. Conclusions: The relatively small area of contact between Fab13B5 and p24 m ay be related to the fact that the epitope is a continuous peptide rather t han a more complex protein surface and correlates with a relatively low aff inity of antigen and antibody. Despite this, a significant quaternary struc tural change occurs in the Fab upon complex formation, with additional smal ler adaptations of both antigen and antibody.