How HIV-1 Gag assembles in cells: Putting together pieces of the puzzle

Abstract

During the late stage of the viral life cycle, HIV-1 Gag assembles into a spherical immature capsid, and undergoes budding, release, and maturation. Here we review events involved in immature capsid assembly from the perspective of five different approaches used to study this process: mutational analysis, structural studies, assembly of purified recombinant Gag, assembly of newly-translated Gag in a cell-free system, and studies in cells using biochemical and imaging techniques. We summarize key findings obtained using each approach, point out where there is consensus, and highlight unanswered questions. Particular emphasis is placed on reconciling data suggesting that Gag assembles by two different paths, depending on the assembly environment. Specifically, in assembly systems that lack cellular proteins, high concentrations of Gag can spontaneously assemble using purified nucleic acid as a scaffold. However, in the more complex intracellular environment, barriers that limit self-assembly are present in the form of cellular proteins, organelles, host defenses, and the absence of free nucleic acid. To overcome these barriers and promote efficient immature capsid formation in an unfavorable environment, Gag appears to utilize an energy-dependent, host-catalyzed, pathway of assembly intermediates in cells. Overall, we show how data obtained using a variety of techniques has led to our current understanding of HIV assembly.

Polymorphic Nature of Human T-Cell Leukemia Virus Type 1 Particle Cores as Revealed through Characterization of a Chronically Infected Cell Line

“…The MT-2 virus particle diameters were previously determined (23). majority of studies have utilized human immunodeficiency virus type 1 (HIV-1) as a model (35,36,(38)(39)(40). However, although retroviruses have structurally conserved Gag proteins and similar replication strategies, there is great morphological variability among different immature and mature retrovirus particles (including core morphology).…”

Phosphatidylinositol-(4,5)-Bisphosphate Acyl Chains Differentiate Membrane Binding of HIV-1 Gag from That of the Phospholipase Cδ1 Pleckstrin Homology Domain

“…Interestingly, one of these studies showed that MA binding to GUVs requires addition of a heterologous dimerization motif to the MA domain (35). The CA domain in Gag forms a dimerization interface, whereas the NC domain promotes higher-order Gag multimerization (38,39). To test the role of the latter, we examined a Gag-YFP derivative lacking the majority of NC (delNC), which was capable of substantial membrane binding but showed diminished Gag multimerization in cells, as evidenced by reduced Gag-CFP-Gag-YFP fluorescence resonance energy transfer and defects in virus-like particle release (28).…”

A non-cleavable hexahistidine affinity tag at the carboxyl-terminus of the HIV-1 Pr55Gag polyprotein alters nucleic acid binding properties

“…The viral RNP complex traffics to the plasma membrane where Gag multimerizes to form a spherical shell that buds from the plasma membrane. During or just after budding, Gag is proteolytically processed into the MA, CA, SP1, NC, SP2, and p6 proteins, a step that is required for the production of mature, infectious virions [1]. …”