Fig. 2.

HIV Env primary structure. The Env primary structure is presented schematically, with the immature polyprotein labelled gp160 and the mature, cleaved protein labelled gp120 and gp41. Protein domains are labelled within each mature protein. In gp120 the constant regions are labelled C1–C5 and the variable domains are labelled V1–V5. The shading in the variable regions indicates the relative plasticity of variable region length, with V1/V2 ranging from 50 to 90 aa, V4 from 19 to 44 and V5 from 14 to 36 aa, while V3 does not vary appreciably in size (reviewed by Checkley et al., 2011). In the gp41 ectodomain, the heptad repeat regions (also called the N- and C-heptad repeats) are labelled HR1 and HR2, respectively, and the membrane proximal external region is labelled MPER. In the gp41 CTT, the Kennedy epitope is labelled KE and the lentivirus lytic peptide sequences are labelled LLP1, LLP2 and LLP3. Functional endocytic motifs are labelled YXXΦ (near the N terminus) and LL (at the C terminus).