Relationship between HIV-1 Gag Multimerization and Membrane Binding

Abstract

HIV-1 viral particle assembly occurs specifically at the plasma membrane and is driven primarily by the viral polyprotein Gag. Selective association of Gag with the plasma membrane is a key step in the viral assembly pathway, which is traditionally attributed to the MA domain. MA regulates specific plasma membrane binding through two primary mechanisms including: (1) specific interaction of the MA highly basic region (HBR) with the plasma membrane phospholipid phosphatidylinositol (4,5) bisphosphate [PI(4,5)P₂], and (2) tRNA binding to the MA HBR, which prevents Gag association with non-PI(4,5)P₂ containing membranes. Gag multimerization, driven by both CA-CA inter-protein interactions and NC-RNA binding, also plays an essential role in viral particle assembly, mediating the establishment and growth of the immature Gag lattice on the plasma membrane. In addition to these functions, the multimerization of HIV-1 Gag has also been demonstrated to enhance its membrane binding activity through the MA domain. This review provides an overview of the mechanisms regulating Gag membrane binding through the MA domain and multimerization through the CA and NC domains, and examines how these two functions are intertwined, allowing for multimerization mediated enhancement of Gag membrane binding.

Keywords: Gag; HIV-1; capsid; genomic RNA binding; matrix; membrane binding; multimerization; nucleocapsid; particle assembly; tRNA binding.

Figure 1

Plasma membrane-specific binding of HIV-1 Gag. (A) Molecular determinants for Gag membrane binding. Top, the domain organization of HIV-1 Gag. Blue, matrix (MA); green, capsid (CA, N-terminal domain [NTD] and C-terminal domain [CTD]); red, nucleocapsid (NC); gray, p6; and black, spacer peptides 1 and 2 (SP1 and SP2). Additionally, the locations of the myristoyl moiety (Myr) and the highly basic region (HBR, light blue) are shown. The HBR amino acid sequence is shown with basic residues highlighted in blue. Bottom, host molecules that bind the MA HBR. Gag binds membranes through interactions between the HBR and acidic phospholipid headgroups. tRNA binding to the HBR prevents interaction with membranes that contain a prevalent acidic phospholipid, phosphatidylserine (PS), but allows for Gag binding to membranes containing phosphatidylinositol (4,5) bisphosphate [PI(4,5)P₂]. For clarity, the myristoyl moiety is not depicted. (B) A model that accounts for specific localization of Gag to the plasma membrane. tRNA-mediated inhibition of membrane binding prevents localization of Gag to intracellular membranes, which lack PI(4,5)P₂, but allows for binding to the plasma membrane where PI(4,5)P₂ is present.

Figure 2

Organization of MA and CA in the membrane-bound Gag lattice. Gag domains are depicted as in Figure 1 but only MA and CA are shown. Blue, MA; light blue, HBR; and green, CA. Three types of CA-CA interactions contributing to formation of immature Gag lattice are shown on the right. The two-fold, three-fold, and six-fold interaction interfaces are denoted by pink bar, triangle, and hexagon, respectively. Six-fold interactions involving the MHR and six-helix bundle (formed by CA-CTD and SP1 regions) form Gag hexamer subunits, with interactions at the two-fold and two-fold interfaces binding adjacent hexamer rings to one another. In the immature lattice, MA trimers form above the two-fold CA interaction interface, with an MA domain presumably contributed from each of three adjacent hexamer rings.

Figure 3

Immature lattice establishment and assembly progression. Side view (A) and top view (B) of lattice formation and growth. Initial Gag complexes arrive at the plasma membrane from the cytosol, potentially in complex with genomic RNA. Higher order multimerization occurs at the plasma membrane, with monomeric and dimeric Gag (from the cytosolic pool) added to the growing lattice edges. Gag domains are shown in the same color scheme as in Figure 1 and Figure 2. Pink polygons depict different CA-CA interfaces as described in Figure 2.