Figure 3.

The HIV-1 capsid. During maturation, proteolytic processing of the Gag polyprotein liberates the mature CA protein. (A) The monomeric CA (PDB accession 2M8N) is a predominantly α-helical protein with N- and C-terminal domains (CA-NTD and CA-CTD, respectively) connected by a flexible interdomain linker region. Individual α-helices are indicated (H1-H11) and the locations of key structural elements (e.g., the N-terminal β-hairpin, CypA-binding loop, interdomain linker, and major homology region) are labeled. (B) In the final stage of maturation, CA monomers multimerize into hexameric (orange) and pentameric (yellow) subunits that assemble into a conical structure exhibiting fullerene geometry. This protein superstructure, known as the capsid, contains approximately 250 CA hexamers and exactly 12 CA pentamers. The incorporation of pentameric subunits is concentrated at either end of capsid to provide the curvature necessary to close this structure. The CA-NTD is labeled in orange and the CA-CTD in blue. Panel B images are reprinted with permission from the authors of references [6] and [33] and from Cell Press and Nature Publishing Group.