HIV-1 Gag as an Antiviral Target: Development of Assembly and Maturation Inhibitors

Abstract

HIV-1 Gag is the master orchestrator of particle assembly. The central role of Gag at multiple stages of the HIV lifecycle has led to efforts to develop drugs that directly target Gag and prevent the formation and release of infectious particles. Until recently, however, only the catalytic site protease inhibitors have been available to inhibit late stages of HIV replication. This review summarizes the current state of development of antivirals that target Gag or disrupt late events in the retrovirus lifecycle such as maturation of the viral capsid. Maturation inhibitors represent an exciting new series of antiviral compounds, including those that specifically target CA-SP1 cleavage and the allosteric integrase inhibitors that inhibit maturation by a completely different mechanism. Numerous small molecules and peptides targeting CA have been studied in attempts to disrupt steps in assembly. Efforts to target CA have recently gained considerable momentum from the development of small molecules that bind CA and alter capsid stability at the post-entry stage of the lifecycle. Efforts to develop antivirals that inhibit incorporation of genomic RNA or to inhibit late budding events remain in preliminary stages of development. Overall, the development of novel antivirals targeting Gag and the late stages in HIV replication appears much closer to success than ever, with the new maturation inhibitors leading the way.

Fig. (1). HIV-1 Gag and Basics of Particle Assembly

The HIV Gag polyprotein is represented at the top of the figure, using structures of individual regions of the protein. Proteolytic cleavage sites are indicated by red arrows. The structures represented are from [, –161] with PDB IDs from N- to C-terminus: 2HMX (MA), 2GOL (CA NTD), 1A8O (CA CTD), 1U57 (SP1), 1F6U (NC). Structures were prepared using CN3d version 4.3. Below a cell is represented to illustrate Gag protein formation on cytosolic ribosomes and assembly and budding at the plasma membrane. The process of maturation of the core occurs during or immediately following particle budding.

Fig. (2). Distinct mechanism of action of maturation inhibitors

Bevirimat (top) inhibits CA-SP1 cleavage, binding to a pocket that is not fully elucidated but includes key residues in SP1 and in the CA CTD surrounding the CA-SP1 cleavage site. MHR = major homology region. Disruption of CA-SP1 cleavage results in aberrant particle formation with a membrane crescent of Gag and an acentric core. The ALLINIs act by a very distinct mechanism, binding to the LEDGF binding region at the dimer interface of the IN catalytic core domains (CCDs) and promoting aberrant multimerization. Particles treated with these inhibitors display a displaced density (presumably the viral RNA) and an empty core. Structures shown are from [51, 105, 161] with PDB IDs of 4USN (CA), 1U57 (SP1), and 4GW6 (IN + ALLINI).

Fig. (3). CA inhibitors and binding pockets

Binding sites for distinct inhibitors are shown. A) CAP-1 binding pocket on the CA NTD. B) Benzodiazepine (BD) inhibitor is shown also binding in the CAP-1 binding pocket of the NTD; note the NTD orientation is changed from (A) to emphasize the inhibitor binding pocket. C) CAI is a peptide binding to an interface on the CA CTD. D) PF74 binds to a distinct pocket on the CA NTD; orientation of the NTD is similar to that shown in (A). Structures are from [, , –163] with PDB IDs of 2JPR (CAP-1), 4E91 (BD), 3DS1 (CAI), and 2XDE (PF74).