A NOVEL NEUTRALIZATION EPITOPE ON THE THUMB SUBDOMAIN OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 REVERSE-TRANSCRIPTASE REVEALED BY A MONOCLONAL-ANTIBODY

We have prepared a MAb, 7C4, which inhibits the RNA-dependent DNA poly merase activity of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT); this MAb has allowed identification of a previous ly unknown neutralizing epitope of RT. Analysis of the epitope and of the mechanism of polymerase inhibition revealed that 7C4 acts by inter fering with the interaction between RT and the template-primer. 7C4 re cognizes a discontinuous epitope on the two alpha-helices, alpha H and alpha I, that make up the 'thumb' subdomain of RT. The existing cryst allographic model of HIV-1 RT suggests that the 'thumb' subdomain, tog ether with the 'fingers' and 'palm', form a nucleic-acid-binding cleft in the 66 kDa subunit of RT and that alpha H is in contact with the p rimer strand of the template-primer. The extent of inhibition of enzym e activity produced by 7C4 correlates with the reported primer-length- dependency of template-primer binding to RT. Inhibition by 7C4 was com petitive with respect to the template-primer and mixed with respect to the substrate. Binding of 7C4 to RT was prevented by preincubation of the enzyme with high concentrations of template-primer but not with s ubstrate. Thus, the 7C4 epitope apparently exists on part of the templ ate-primer binding site of the alpha H and alpha I regions of the 'thu mb' subdomain. This neutralization epitope is a logical target for the development of new types of HIV-1 RT inhibitors.