Modeled FE gp120 trimers. a) Model of unliganded FE gp120 trimer containing the truncated V1/V2 stem loop in a “closed” conformation (Upper and lower panels). Top and front views showing the closed conformation of an HIV-1 FE gp120 trimer. We were unable to model the recently solved complete V1/V2 domain onto the unliganded trimer model due to steric clashes; however, a portion of the truncated V1/V2 domain, the V1/V2 stem loop (dark blue), can be seen in a “closed” conformation near the center of the trimer. More structural information is needed regarding the unliganded full-length gp120 trimer to determine precisely where the complete V1/V2 domain is positioned in this conformation; however, owing to its flexibility, the V1/V2 domain will likely also be positioned near the center of the trimer. This is also consistent with cryo-electron tomography studies [26, 28]. b) Model of CD4 bound FE gp120 trimer containing the complete V1/V2 domain in an “open” conformation. Top and front views showing the open conformation of the trimer upon binding the CD4 receptor. We were able to graft the complete V1/V2 domain (dark blue) onto the gp120 core. Upon CD4 binding, the V1/V2 domain rotates outwardly in an “open” orientation, consistent with cryo-electron tomography studies [26, 28].