HUMAN IMMUNODEFICIENCY VIRUS-1 REVERSE-TRANSCRIPTASE HETERODIMER STABILITY

Structural and biochemical evidence strongly supports a heterodimeric (p66p51) active form for human immunodeficiency virus-1 reverse transc riptase (RT). Heterodimer stability was examined by sedimentation anal ysis as a function of temperature and ionic strength. Using NONLIN reg ression software, monomer-dimer-trimer and monomer-dimer-tetramer asso ciation models gave the best fit to the analytical ultracentrifuge sed imentation equilibrium data. The heterodimer is the predominant form o f RT at 5 degrees C, with a dimerization k(a), value of 5.2 x 10(5) M( -1) for both models. K-a values of 2.1 x 10(5) and 3.8 x 10(5) M(-1) w ere obtained for the respective association models at 20 degrees C. RT in 50 and 100 mM Tris, pH 7.0, completely dissociates at 37 degrees C and behaves as an ideal monomeric species. The dissociation of RT as a function of increasing temperature was also observed by measuring th e decrease in sedimentation velocity (s(w,20)) If the stabilization of the heterodimer was due primarily to hydrophobic interactions we woul d anticipate an increase in the association from 21 degrees C to 37 de grees C. The opposite temperature dependence for the association of RT suggests that electrostatic and hydrogen bond interactions play an im portant role in stabilizing heterodimers. To examine the effect of ion ic strength on p66p51 association we determined the changes in s(w,20) as a function of NaCl concentration. There is a sharp decrease in s(w ,20) between 0.10 and 0.5 M NaCl, leading to apparent complete dissoci ation. The above results support a major role for electrostatic intera ctions in the stabilization of the RT heterodimer.