HIV-1 Capsid Core: A Bullet to the Heart of the Target Cell

Abstract

The first step of the intracellular phase of retroviral infection is the release of the viral capsid core in the cytoplasm. This structure contains the viral genetic material that will be reverse transcribed and integrated into the genome of infected cells. Up to recent times, the role of the capsid core was considered essentially to protect this genetic material during the earlier phases of this process. However, increasing evidence demonstrates that the permanence inside the cell of the capsid as an intact, or almost intact, structure is longer than thought. This suggests its involvement in more aspects of the infectious cycle than previously foreseen, particularly in the steps of viral genomic material translocation into the nucleus and in the phases preceding integration. During the trip across the infected cell, many host factors are brought to interact with the capsid, some possessing antiviral properties, others, serving as viral cofactors. All these interactions rely on the properties of the unique component of the capsid core, the capsid protein CA. Likely, the drawback of ensuring these multiple functions is the extreme genetic fragility that has been shown to characterize this protein. Here, we recapitulate the busy agenda of an HIV-1 capsid in the infectious process, in particular in the light of the most recent findings.

Keywords: HIV-1; capsid; cellular cofactors; genetic fragility; nuclear transport; restriction factors; reverse transcription; uncoating.

FIGURE 1

Capsid forms throughout the HIV life cycle. (A) Gag and Gag-Pol precursors simplified structures. Gag precursor includes the matrix protein (MA), the capsid (CA, depicted with the NTD in green and the CTD in magenta), the spacer peptide 1 (SP1), the nucleocapsid (NC), the spacer peptide 2 (SP2), and the peptide 6 (p6). A frameshift during translation allows the production of Gag-Pol precursor, with a ratio of 1:20 with respect to the Gag precursor. In this structure the NC is fused to the protease (PR), the reverse transcriptase (RT), and the integrase (IN) domains. (B) Structure of CA monomer. CA is composed of two domains connected by a flexible linker: the NTD (in green), formed by a beta-hairpin and seven alpha-helices, and the CTD (in magenta), formed by four alpha-helices. The CypA binding loop in the NTD is indicated. PDB ID: 6WAP (Lu et al., 2020). (C) Schematic structure of the Gag precursor composed from top to bottom of MA, CA-NTD, CA-CTD, SP1, NC, SP2, and p6. (D) Schematic structure of a hexamer in the immature lattice, after the first proteolytic cleavage, which occurs between SP1 and NC. The MA are attached to the membrane through their myristoylated domain. Proceeding toward the center of the viral particle there are three hexameric structures composed by the CA-NTDs, CA-CTDs, and SP1. (E) Schematic top view of the mature capsid lattice where CA monomers are arranged in hexamers and are connected to each other through the NTDs, while the CTDs are involved in the interactions between hexamers.

FIGURE 2

Capsid core structure. (A) The mature capsid core has the shape of a fullerene cone, formed by 125 hexamers (in orange) and 12 pentamers (in yellow). Image republished with permission of Nature Publishing Group (Pornillos et al., 2011). (B) Top and lateral view of pentameric and hexameric capsid assemblies. In both structures, the NTDs (in green) are forming the inner ring while the CTDs (in magenta) are forming the external ring. The pocket present in the hexamer, at the NTD-CTD interface (involved in the interaction with host factors, see main text) is indicated. The pocket is absent in the pentamer. PDB IDs: 5MCX, 5MCY (Mattei et al., 2016).

FIGURE 3

Interaction between TRIM and the capsid. TRIM5α and TRIMCyp are represented in their dimeric form. Each monomer (in orange and in blue) is formed by the RING domain, the B-Box 2 domain, the coiled-coil domain and the C-terminal domain which is the one responsible for the interaction with the capsid core. In TRIM5α this domain is the PRYSPRY domain while in TRIMCyp is CypA.

FIGURE 4

Models for the timing of uncoating. HIV-1 enters the cell after recognition by the envelope glycoproteins of the cellular receptor CD4 (in gray) and the cellular co-receptor CXC4 or CCR5 (in black). This leads to the fusion of the cell and viral membranes and to the release of the capsid core in the cytoplasm. In the figure, the three models of uncoating covered in this review are depicted: the cytoplasmic uncoating (on the left), the uncoating at the nuclear pore complex (NPC) (in the center), and the nuclear uncoating (on the right). In each model the reverse transcription of the viral genomic RNA (vRNA) (in red) into viral DNA (vDNA) (in green) has to be completed, allowing its integration in the host genome (in blue). The reverse transcription complex (RTC) is schematically shown as the association of a molecule of reverse transcriptase (RT, in purple) to the vRNA and single-stranded vDNA. The completed vDNA forms the pre-integration complex (PIC), shown as the double-stranded vDNA bound to a tetramer of integrase (IN, in orange).

FIGURE 5

Relay race of the capsid core in the host cell. From left to right a temporal view of how CA is passed between host factors in its trip toward the nucleus. The capsid core is schematically represented as a purple triangle with two host factors binding sites highlighted: the CypA binding-loop (the circle) and the NTD-CTD pocket (the square). The first to bind to the core is CypA, which recognizes the CypA-binding domain, located in the CA-NTD. The same binding site is recognized by Nup358 and its binding anchors the capsid core at the NPC, allowing its nuclear import. Then, Nup153 binds to the NTD-CTD pocket of the assembled capsid, which is the same recognition site of CPSF6. When CPSF6 takes the place of Nup153 on the binding site it can translocate the capsid core (intact or not) to deeper nuclear regions.