A stable immature lattice packages IP for HIV capsid maturation.

HIV virion assembly begins with the construction of an immature lattice consisting of Gag hexamers. Upon virion release, protease-mediated Gag cleavage leads to a maturation event in which the immature lattice disassembles and the mature capsid assembles. The cellular metabolite inositiol hexakisphosphate (IP) and maturation inhibitors (MIs) both bind and stabilize immature Gag hexamers, but whereas IP promotes virus maturation, MIs inhibit it. Here we show that HIV is evolutionarily constrained to maintain an immature lattice stability that ensures IP packaging without preventing maturation. Replication-deficient mutant viruses with reduced IP recruitment display increased infectivity upon treatment with the MI PF46396 (PF96) or the acquisition of second-site compensatory mutations. Both PF96 and second-site mutations stabilise the immature lattice and restore IP incorporation, suggesting that immature lattice stability and IP binding are interdependent. This IP dependence suggests that modifying MIs to compete with IP for Gag hexamer binding could substantially improve MI antiviral potency.