We have analyzed the unique epitope for the broadly neutralizing human 
monoclonal antibody (MAb) 2G12 on the gp120 surface glycoprotein of human 
immunodeficiency virus type 1 (HIV-1). Sequence analysis, focusing on the 
conservation of relevant residues across multiple HIV-1 isolates, refined the 
epitope that was defined previously by substitutional mutagenesis (A. Trkola, M. 
Purtscher, T. Muster, C. Ballaun, A. Buchacher, N. Sullivan, K. Srinivasan, J. 
Sodroski, J. P. Moore, and H. Katinger, J. Virol. 70:1100-1108, 1996). In a 
biochemical study, we digested recombinant gp120 with various glycosidase 
enzymes of known specificities and showed that the 2G12 epitope is lost when 
gp120 is treated with mannosidases. Computational analyses were used to position 
the epitope in the context of the virion-associated envelope glycoprotein 
complex, to determine the variability of the surrounding surface, and to 
calculate the surface accessibility of possible glycan- and polypeptide-epitope 
components. Together, these analyses suggest that the 2G12 epitope is centered 
on the high-mannose and/or hybrid glycans of residues 295, 332, and 392, with 
peripheral glycans from 386 and 448 on either flank. The epitope is mannose 
dependent and composed primarily of carbohydrate, with probably no direct 
involvement of the gp120 polypeptide surface. It resides on a face orthogonal to 
the CD4 binding face, on a surface proximal to, but distinct from, that 
implicated in coreceptor binding. Its conservation amidst an otherwise highly 
variable gp120 surface suggests a functional role for the 2G12 binding site, 
perhaps related to the mannose-dependent attachment of HIV-1 to DC-SIGN or 
related lectins that facilitate virus entry into susceptible target cells.