FIG 12.
SIVmac gp120 V4 loop is the target of Env-specific antibodies, leading to substitutions and in-frame deletions. A crystal structure of the SIVmac gp120 subunit (58) showing the presence of the V4 loop (orange) on the external surface of the gp120 monomer where substitutions and in-frame deletions emerge rapidly during early SIV infection in macaque hosts. The D415 adaptation leads to a novel N-linked glycosylation site in the V4 region and is found near the apex of the V4 loop. P421 is a hot spot of adaptation, with several substitutions emerging at this site. The presence of a bend near the Pro site suggests adaptation leads to a loss of rigidity within the loop that is maintained by the Pro. An R424Q change was observed in two of four animals in our cohort and occurs near the bottom of the V4 loop. The Δ symbols denote the span of in-frame multiple-amino-acid deletions that emerged to become dominant in two animals (TP1 and TP2) by 18 to 23 weeks p.i. The image was produced with PDB entry 3FUS.