Resistance to N-peptide fusion inhibitors correlates with thermodynamic stability of the gp41 six-helix bundle but not HIV item kinetics (Retrovirology)

Background: The HIV-1 envelope glycoprotein (Env) undergoes conformational changes that mediate fusion between virus and host cell membranes. These changes involve transient exposure of two heptad-repeat domains (HR1 and HR2) in the gp41 subunit and their subsequent self-assembly into a six-helix bundle (6HB) that drives fusion. Env residues and features that influence conformational changes and the rate of virus item, however, are poorly understood. Peptides corresponding to HR1 and HR2 (N and C peptides, respectively) interrupt formation of the 6HB by binding to the heptad repeats of a fusion-intermediate conformation of Env, making the peptides valuable probes for studying Env conformational changes. Results: Using a panel of Envs that are resistant to N-peptide fusion inhibitors, we investigated relationships between virus item kinetics, 6HB stability, and resistance to peptide fusion inhibitors to elucidate how HR1 and HR2 mutations affect Env conformational changes and virus item. We found that gp41 resistance mutations increased 6HB stability without increasing item kinetics. Similarly, we show that increased 6HB thermodynamic stability does not correlate with increased item kinetics. Thus, N-peptide fusion inhibitors do not necessarily select for Envs with faster item kinetics, nor does faster item kinetics predict decreased potency of peptide fusion inhibitors. Conclusions: These findings provide new insights into the relationship between 6HB stability and viral item kinetics and mechanisms of resistance to inhibitors targeting fusion-intermediate conformations of Env. These studies further highlight how residues in HR1 and HR2 can influence virus item by altering stability of the 6HB and possibly other conformations of Env that affect rate-limiting steps in HIV item.