Defects in the HIV immature lattice support essential lattice remodeling within budded virions

Abstract

ABSTRACTFor HIV virions to become infectious, the immature lattice of Gag polyproteins attached to the virion membrane must be cleaved. Cleavage cannot initiate without the protease formed by the homo-dimerization of domains linked to Gag. However, only 5% of the Gag polyproteins, termed Gag-Pol, carry this protease domain, and they are embedded within the structured lattice. The mechanism of Gag-Pol dimerization is unknown. Here, we use reaction-diffusion simulations of the immature Gag lattice as derived from experimental structures, showing that dynamics of the lattice on the membrane is unavoidable due to the missing 1/3 of the spherical protein coat. These dynamics allow for Gag-Pol molecules carrying the protease domains to detach and reattach at new places within the lattice. Surprisingly, dimerization timescales of minutes or less are achievable for realistic binding energies and rates despite retaining most of the large-scale lattice structure. We derive a formula allowing extrapolation of timescales as a function of interaction free energy and binding rate, thus predicting how additional stabilization of the lattice would impact dimerization times. We further show that during assembly, dimerization of Gag-Pol occurs stochastically and therefore must be actively suppressed to prevent early activation. By direct comparison to recent biochemical measurements within budded virions, we find that only moderately stable hexamer contacts (−12kBT<ΔG<-8kBT) retain both the dynamics and lattice structures that are consistent with experiment. These dynamics are likely essential for proper maturation, and our models quantify and predict lattice dynamics and protease dimerization timescales that define a key step in understanding formation of infectious viruses.Statement of SignificanceFor retroviruses such as HIV-1, the Gag polyprotein assembles an immature lattice that ensures successful budding from the cell plasma membrane. The first step in the subsequent maturation requires a pair of protease domains embedded within the lattice to form a homodimer. We show here that this homo-dimerization can proceed within minutes despite involving a small subset of Gag monomers, due to the incompleteness of the immature lattice. Using reaction-diffusion simulations, we quantify timescales of first dimerization events between the protease domains and define a formula to extrapolate across a range of energies and rates. Our models illustrate how protein contacts can be weakened to disrupt lattice assembly or stabilized to slow the remodeling essential for viral infectivity.