A new potent HIV-1 reverse transcriptase inhibitor. A synthetic peptide derived from the interface subunit domains

C/EBPbeta phosphorylation by RSK creates a functional XEXD caspase inhibitory box critical for cell survival

Upon activation by liver injury, hepatic stellate cells produce excessive fibrous tissue leading to cirrhosis. The hepatotoxin CCl(4) induced activation of RSK, phosphorylation of C/EBPbeta on Thr(217), and proliferation of stellate cells in normal mice, but caused apoptosis of these cells in C/EBPbeta-/- or C/EBPbeta-Ala(217) (a dominant-negative nonphosphorylatable mutant) transgenic mice. Both C/EBPbeta-PThr(217) and the phosphorylation mimic C/EBPbeta-Glu(217), but not C/EBPbeta-Ala(217), were associated with procaspases 1 and 8 in vivo and in vitro and inhibited their activation. Our data suggest that C/EBPbeta phosphorylation on Thr(217) creates a functional XEXD caspase substrate/inhibitor box (K-Phospho-T(217)VD) that is mimicked by C/EBPbeta-Glu(217) (KE(217)VD). C/EBPbeta-/- and C/EBPbeta-Ala(217) stellate cells were rescued from apoptosis by the cell permeant KE(217)VD tetrapeptide or C/EBPbeta-Glu(217).

Factors affecting the dimerization of the p66 form of HIV-1 reverse transcriptase

The association and dissociation of the homodimeric p66/p66 form of HIV-1 reverse transcriptase were investigated. The effects on the dimerization process of different salt concentrations, pH and the presence of a template/primer and nucleotide substrates were monitored by measuring polymerase activity and analytical size-exclusion HPLC. At submicromolar concentrations of enzyme and physiological salt concentrations, most of the enzyme exists in the inactive monomeric form. Increasing NaCl concentration from 0.05 to 1 M decreased the equilibrium dissociation constant from 2.0 to 0.34 microM. Analysis of the kinetics of the dimerization process indicated it followed a two-step mechanism, with rapid initial association of the two subunits to form an inactive homodimer followed by a slow isomerization step rendering the active enzyme form. The presence of poly(rA)/dT(20) decreased the equilibrium dissociation constant of the homodimer about 30-fold, while the addition of 5 microM dTTP had no effect. The kinetics of the process showed that the template/primer favored dimerization by binding to the inactive homodimer and promoting its isomerization to the active form. These results were confirmed by analyzing the reverse reaction, i.e. the dissociation of the enzyme, by dilution in a low-ionic-strength buffer. The results suggest that binding of immature HIV-1 reverse transcriptase to its natural template/primer may be relevant in both the dimerization process and the selection of its natural primer.

Analysis of mutations and suppressors affecting interactions between the subunits of the HIV type 1 reverse transcriptase

HIV-1 reverse transcriptase (RT) catalyzes the conversion of genomic RNA into cDNA. The enzyme is a heterodimer of p66 and p51 subunits, and the dimerization of these subunits is required for optimal enzyme activity. To analyze this process at the genetic level, we developed constructs that permit the detection of the interaction between these subunits in the yeast two-hybrid system. Genetic analysis of RT subdomains required for heterodimerization revealed that the fingers and palm of p66 were dispensable for p51 interaction. However, as little as a 26-amino acid deletion at the C terminus of p51 prevented dimerization with p66. A primer grip mutation, L234A, previously shown to inhibit RT dimerization by biochemical assays, also prevented RT dimerization in the yeast two-hybrid system. Second-site mutations that restored RT dimerization in yeast to the L234A parent were recovered in the tryptophan repeat region at the dimer interface and at the polymerase active site, suggesting the involvement of these sites in RT dimerization. In vitro binding experiments confirmed the effects of the L234A mutation and the suppressor mutations on the interaction of the two subunits. The RT two-hybrid assay should facilitate the extensive genetic analysis of RT dimerization and should make possible the rapid screening of potential inhibitors of this essential process.