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. 2021 Mar 5;13(3):417. doi: 10.3390/v13030417

Figure 2.

Figure 2

HIV-1 capsid architecture and summary of binding interfaces of capsid with diverse host factors. During the maturation of HIV-1 virion, ~1500 CA monomers assemble into ~250 hexamers and exactly 12 pentamers in alignment produce fullerene cone geometry, forming capsid (Figure 2A–D). The interfaces within the capsid which bind diverse host factors are highlighted in different colors, as detailed in the Figure 2E legend. (A) Structure of the CA monomer (PDB ID: 4XZF). The left structure is shown in surface representation and the right structure as ribbon representation. CA consists of two α-helical domains, an N-terminal domain (NTD) labeled in gold and a C-terminal domain (CTD) labeled in light blue, connected by a flexible linker (grey). All α-helices (H1-H11) and key structural elements are indicated by arrows with names. (B) Structure of a CA hexamer with top view (left) and side view (right) (PDB ID: 4XZF). (C). Structure of high-order (hexagonal) CA hexamers with top view (left) and side view (right) (PDB ID: 4XZF). (D). Architecture of the capsid (PDB ID: 3J3Y). The incorporation of pentamers (orange) at either end of capsid provides the curvature necessary to close the conical structure. (E) The interfaces within capsid that can bind diverse host factors are highlighted in different colors, which are shown in Figure 2A–D. Host proteins CypA, Nup358, and TRN-1 bind to the flexible CypA-binding loop (labeled in red) exposed on the top of CA-NTD that protrudes from the capsid outer surface. Host proteins CPSF6 and Nup153 share the same phenylalanine-glycine binding pocket (labeled in green) created by the NTD-CTD interface between two neighboring CA monomers in a hexamer. The restriction factor Mx2 specifically binds to the three-fold inter-hexamer interface (labeled in purple) of capsid; moreover, it is unable to bind to a CA monomer or a single hexamer. Host protein FEZ1 and metabolite IP6 can bind to the positively charged central pore of hexamers formed by R18 (labeled in dark blue) of CA. TRIM5α can form a hexagonal network (labeled in pink, shown in Figure 2D, also called TRIM5α cage) that avidly binds the capsid shell. The extent to which the TRIM5α cage can cover the capsid and how TRIM5α directly contacts the capsid surface have not been fully established.