Proteolytic Processing and Particle Maturation

108

127

Maturation of nascent virions, a key step in retroviral replication, involves cleavage of the Gag polyprotein by the viral protease into its matrix (MA), capsid (CA), and nucleocapsid (NC) components and their subsequent reorganization. Bevirimat (BVM) defines a new class of antiviral drugs termed maturation inhibitors. BVM acts by blocking the final cleavage event in Gag processing, the separation of CA from its C-terminal spacer peptide 1 (SP1). Prior evidence suggests that BVM binds to Gag assembled in immature virions, preventing the protease from accessing the CA-SP1 cleavage site. To investigate this hypothesis, we used cryo-electron tomography to examine the structures of (noninfectious) HIV-1 viral particles isolated from BVM-treated cells. We find that these particles contain an incomplete shell of density underlying the viral envelope, with a hexagonal honeycomb structure similar to the Gag lattice of immature HIV but lacking the innermost, NC-related, layer. We conclude that the shell represents a remnant of the immature Gag lattice that has been processed, except at the CA-SP1 sites, but has remained largely intact. We also compared BVMtreated particles with virions formed by the mutant CA5, in which cleavage between CA and SP1 is also blocked. Here, we find a thinner CA-related shell with no visible evidence of honeycomb organization, indicative of an altered conformation and further suggesting that binding of BVM stabilizes the immature lattice. In both cases, the observed failure to assemble mature capsids correlates with the loss of infectivity.

mentioning

confidence: 99%

HIV-1 Maturation Inhibitor Bevirimat Stabilizes the Immature Gag Lattice

Keller

Adamson

Heymann

et al. 2011

108

127

“…The processes of Pr55 Gag cleavage and virion maturation are believed to be highly regulated (13,14,45,46,53), although the mechanisms of this regulation are not entirely clear. Some groups have proposed that phosphorylation of retroviral Gag proteins enhances their cleavage by PR (41,55).…”

mentioning

confidence: 99%

Phosphorylation and Proteolytic Cleavage of Gag Proteins in Budded Simian Immunodeficiency Virus

Rue

Roos

Tarwater

et al. 2005

“…The 160-kDa GagPol precursor consists of MA, CA, p2, and NC, followed by the pol-encoded transframe protein (p6*) and protease (PR), reverse transcriptase (RT), and integrase (IN) (see references 3 and 44 and references therein). PR cleaves Gag and GagPol into their structural and enzymatic subunits during the late stages of viral budding, thereby ensuring maturation to infectious particles (49).…”

mentioning

confidence: 99%

Nef Binds p6* in GagPol during Replication of Human Immunodeficiency Virus Type 1

Costa

Zheng

Sabotič

et al. 2004

Human immunodeficiency virus type 1 (HIV-1), the causative agent of AIDS, encodes 16 distinct proteins that are expressed differentially during the viral replicative cycle. Initially, the early regulatory proteins Tat and Rev and the socalled negative effector (Nef) are translated from multiply spliced mRNA species. During late phases, singly spliced or unspliced transcripts direct the expression of viral accessory proteins (viral proteins R and U [Vpr and Vpu] and viral infectivity factor [Vif]), as well as structural group-specific antigen (Gag), Gag and polymerase (GagPol), and envelope (Env) polyproteins (see reference 20 and references therein).