Presentation:
HIV-1 particle assembly is driven by the viral Gag polyprotein precursor, which initiates assembly by forming an immature Gag lattice at the inner leaflet of the infected cell plasma membrane. Following completion of immature particle assembly and virus budding, the viral protease (PR) cleaves the Gag precursor at a number of sites to generate the mature Gag proteins matrix (MA), capsid (CA), nucleocapsid (NC), p6, and two small spacer peptides SP1 and SP1. PR-mediated Gag cleavage triggers a morphological transformation of the nascently released virion (known as maturation) during which the newly liberated CA protein assembles to form the viral capsid, into which are packaged the viral RNA genome and the viral enzymes reverse transcriptase (RT) and integrase (IN). Our work and that of others has demonstrated that the finely tuned stability of the immature Gag lattice is essential for particle assembly and subsequent maturation, and proper capsid stability is essential for early post-entry events. The stability of immature and mature Gag lattices is modulated by the cellular polyanion inositol hexakisphosphate (IP6). From a translational perspective, two classes of HIV-1 inhibitors – maturation inhibitors and capsid inhibitors (including the recently FDA-approved drug lenacapavir) – act by tipping the stability/instability balance of the immature Gag lattice and the mature capsid, respectively. Thus, elucidating the determinants of Gag complex stability is crucial for both achieving a basic understanding of HIV-1 replication and also for moving forward drug discovery efforts that target assembly, maturation, or capsid-mediated post-entry events, including nuclear import. In a separate line of investigation, our recent work has shown that the lipid composition of the HIV-1 virion plays a key role in HIV-1 maturation, as disrupting the cellular lipid biosynthesis enzyme neutral sphingomyelinase 2 (nSMase2) blocks Gag processing, particle maturation, and virus replication. In all of the above-described studies, drug resistance selections and forced evolution experiments have provided key mechanistic insights.