Studies on the mechanism of inactivation of the HIV-1 nucleocapsid protein NCp7 with 2-mercaptobenzamide thioesters

The HIV-1 nucleocapsid protein (NCp7) is a small basic protein with two CysCysHisCys zinc-binding domains that specifically recognizes the psi-site of the viral RNA. NCp7 plays a number of crucial roles in the viral lifecycle, including reverse transcription and RNA encapsidation. Several classes of potential anti-HIV compounds have been designed to inactivate NCp7 through zinc ejection, including a special class of thioester compounds. We have investigated the mechanism of action of two N-substituted-S-acyl-2-mercaptobenzamide compounds (compounds I and 2) that target NCp7. UV/Visible spectroscopy studies demonstrated that both thioesters were able to eject metal from NCp7. NMR and mass spectroscopy studies showed that the thioester compounds specifically ejected zinc from the carboxyl-terminal zinc-binding domain of NCp7 by covalent modification of Cys(39). Exposure of NCp7 to compounds 1 and 2 destroyed its ability to specifically bind RNA, whereas NCp7 already bound to RNA was protected from zinc ejection by the thioesters. The thiol component of the thioesters (compound 3, 2-mercaptobenzoyl-beta-alaninamide) did not eject zinc from NCp7, but when compound 3 was incubated with acetyl CoA prior to incubation with NCp7, we observed extensive metal ejection. Thus, the thiol released by the reaction of compounds 1 and 2 could be re-acylated in vivo by acyl CoA to form a new thioester compound that is able to react with NCp7. These studies provide a better understanding of the mechanism of action of thioester compounds, which is important for future design of anti-HIV-1 compounds that target NCp7.