Fig. 6. A model for the temporal sequence of CA-CA interactions during assembly of HIV-1 Gag in cells.

Critical residues in CA that are required for assembly (Class 3, 4, and 5 mutants in Fig. 3) are identical to, or in close proximity with, residues that form CA-CA interfaces in a recent high-resolution structure of the immature capsid formed by MPMV CA-NC (Bharat et al., 2012; Robinson et al., 2014). If one assumes that each interface is formed at the step when the relevant residues are required during assembly, the blockade data from Fig. 3 can be used to generate a map of when key CA-CA interfaces might form during cells. The diagram shows such a model, adapted from (Robinson et al., 2014), which proposes that formation of the inter-hexameric CA-CTD dimer interface (involving residues in CA helix 9) is required for the ∼80S intermediate to stably target to membranes; and that formation of the CA-CTD intra-hexameric interface (involving residues in the CA-CTD crystallographic base) may be required for Gag to progress past the ∼80S membrane-associated intermediate. These inter- and intra-hexameric CA-CTD contacts may be formed in repeated rounds, as additional Gag is added to the growing capsid during multimerization at the PM. Finally, for a stable ∼500S assembly intermediate to be produced, formation of the inter-hexameric CA-NTD interface may be required (using residues in CA helices 4 – 6).