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 recognition ideally requires a structural comparison of free and complexed components. To this end we have studied a mouse monoclonal antibody, denoted 13B5, raised 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 A 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-determining regions (CDRs) are in contact with p24 with most interactions being by the heavy chain. Comparison of the free and complexed Fab reveals that 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 heavy and light chain variable regions. In p24, small conformational changes localized to the turn between the two helices comprising the epitope result from Fab binding.


CONCLUSIONS
The relatively small area of contact between Fab13B5 and p24 may be related to the fact that the epitope is a continuous peptide rather than a more complex protein surface and correlates with a relatively low affinity of antigen and antibody. Despite this, a significant quaternary structural change occurs in the Fab upon complex formation, with additional smaller adaptations of both antigen and antibody.
Monaco-Malbet
S
S
European Molecular Laboratory Biology Grenoble Outstation B.P. 156X F-38042 Cedex 9, Grenoble, France.
Berthet-Colominas
C
C
Novelli
A
A
Battaï
N
N
Piga
N
N
Cheynet
V
V
Mallet
F
F
Cusack
S
S
D000595:Amino Acid Sequence; D000818:Animals; D000911:Antibodies, Monoclonal / Q000737:chemistry / Q000276:immunology; D000915:Antibody Affinity; D000936:Antigen-Antibody Complex / Q000737:chemistry* / Q000276:immunology; D001666:Binding Sites, Antibody; D018360:Crystallography, X-Ray; D000939:Epitopes / Q000737:chemistry / Q000276:immunology; D016655:HIV Core Protein p24 / Q000737:chemistry* / Q000276:immunology*; D015497:HIV-1 / Q000276:immunology*; D019077:Helix-Turn-Helix Motifs; D006801:Humans; D007140:Immunoglobulin Fab Fragments / Q000737:chemistry* / Q000276:immunology*; D007135:Immunoglobulin Variable Region / Q000737:chemistry / Q000276:immunology; D007700:Kinetics; D051379:Mice; D008958:Models, Molecular; D008969:Molecular Sequence Data; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary
D000911:Antibodies, Monoclonal; D000936:Antigen-Antibody Complex; D000939:Epitopes; D016655:HIV Core Protein p24; D007140:Immunoglobulin Fab Fragments; D007135:Immunoglobulin Variable Region