HIV’s capsid is very unusual. The capsid is made of around 1200 identical CA proteins (p24). These CA proteins first assemble into either pentamers or hexamers, which assemble into a fullerene like polyhedron containing 12 pentamers at the vertices and about 200 hexamers filling the bulk. Compared to other viral capsid, HIV’s capsid is unusually big (with an equivalent triangulation number of T=1200/60=30) and highly asymmetric. Instead of a perfect icosahedron made of 20 equilateral triangles, the edges are of different lengths (as well as the h and k numbers https://viralzone.expasy.org/8577) in HIV’s capsids, and even two HIV can have slightly different capsids.
It should be difficult for such complex structures to be assembled correctly because the structural environment is very different among the subunits. For viruses with small and symmetrical capsids, the symmetry is guaranteed by the fixed binding angles between subunits (because the structural environment is equivalent everywhere). More complex viruses can encode multiple types of capsid proteins taking different positions. For example, the capsid of poliovirus can be modeled as a soccer ball, with each pentagon made of 5 VP1 and each hexagon made of 3 VP2 and 3 VP3. However, HIV’s capsid is solely made of CA, which means the very same CA need to adapt to the highly variable structural context. Some complex viruses like herpes viruses encode scaffold proteins to guide the capsid assembly, which is unlikely true for HIV as HIV’s capsid is self-assembled. Despite its irregularities, HIV’s capsid is perfectly sealed off, which is unlike murine leukemia virus with large gaps in its capsid (https://www.pnas.org/doi/abs/10.1073/pnas.1811580115?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub%20%200pubmed). How can the seemingly simple CA proteins assemble into such complex structures accurately?