(A) Three-state Gaussian-distributed FRET population histogram (mean ± SD) of the BG505 IDL trimer, reflecting the conformational distribution sampled by the trimers.
(B) Conformational landscape of the BG505 IDL Env trimer (A) overlaid with those of virus-associated, full-length BG505 Env and BG505 SOSIP trimer.33
(C) Conformational distribution of the double mutant BG505 SOSIP + IDL Env trimer as in (A).
(D) Conformational landscape of the BG505 SOSIP + IDL SOSIP (C) overlaid with those of virus-associated, full-length BG505 Env and BG505 SOSIP trimer.33 Parameters and statistics for the smFRET analysis are provided in Table S2.