Structure determination of an anti-HIV-1 Fab 447-52D–peptide complex from an epitaxially twinned data set

Separation of two individual lattices within an epitaxially twinned data set allowed the crystal structure of the V3-specific neutralizing antibody 447-52D in complex with a V3 peptide (UG1033) to be determined. The structure confirms that the neutralization breadth of Fab 447-52D is likely to be attributable to the extensive focus on main-chain hydrogen-bond interactions with the peptide that permit the recognition of a range of V3 sequences. Although antibodies against the third variable loop (V3) of the HIV-1 viral envelope glycoprotein are among the first neutralizing antibodies to be detected in infected individuals, they are normally restricted in their specificity. X-ray crystallographic studies of V3-specific antibodies have contributed to a more thorough understanding of recognition of this epitope and of conserved features in the V3 loop that could potentially aid in the design of a multi-component vaccine. The human antibody 447-52D exhibits relatively broad neutralization of primary viral isolates compared with other V3-loop antibodies. A crystal structure of Fab 447-52D in complex with a V3 peptide (UG1033) was determined at 2.1 Å resolution. The structure was determined using an epitaxially twinned data set and in-house programs to detect and remove overlapping reflections. Although the processed data have lower than desired completeness and slightly higher than normal R values for the resolution, good-quality electron-density maps were obtained that enabled structure determination. The structure revealed an extended CDR H3 loop that forms a β-sheet with the peptide, with the predominant contacts being main-chain hydrogen bonds. The V3 peptide and Fab show high structural homology with the previously reported structures of other Fab 447-52D complexes, reinforcing the idea that the V3 loop may adopt a small set of conserved structures, particularly around the crown of the β-hairpin