AI Summary
This article discusses how HIV-1 adapts to mutations in the capsid protein that affect capsid assembly. Specifically, viruses containing mutations in certain positively charged residues were found to acquire second-site mutations that restore nearly normal replication fitness. These mutations allow for the restoration of proper capsid assembly by compensating for the lost pentamers, enabling closed conical capsid assembly both in vitro and inside virions. The study highlights the adaptability of HIV-1 to mutations in key capsid proteins and suggests that compensatory mutations can restore lost fitness by tuning the equilibrium between hexamers and pentamers during capsid assembly.
The HIV-1 capsid is composed of capsid (CA) protein hexamers and pentamers (capsomers) that contain a central pore hypothesised to regulate capsid assembly and facilitate nucleotide import early during post-infection. These pore functions are mediated by two positively charged rings created by CA Arg-18 (R18) and Lys-25 (K25). Here we describe the forced evolution of viruses containing mutations in R18 and K25. Whilst R18 mutants fail to replicate, K25A viruses acquire compensating mutations that restore nearly wild-type replication fitness. These compensating mutations, which rescue reverse transcription and infection without reintroducing lost pore charges, map to three adaptation hot-spots located within and between capsomers. The second-site suppressor mutations act by restoring the formation of pentamers lost upon K25 mutation, enabling closed conical capsid assembly both in vitro and inside virions. These results indicate that there is no intrinsic requirement for K25 in either nucleotide import or capsid assembly. We propose that whilst HIV-1 must maintain a precise hexamer:pentamer equilibrium for proper capsid assembly, compensatory mutations can tune this equilibrium to restore fitness lost by mutation of the central pore.
HIV-1 particle assembly is driven by the Gag precursor protein (Gag). Together with viral genomic RNA and a smaller number of GagPol precursor proteins, Gag assembles to form an immature lattice on the inner leaflet of the infected cell plasma membrane. Upon