Crystal Structure of tert-Butyldimethylsilyl-spiroaminooxathioledioxide-thymine (TSAO-T) in Complex with HIV-1 Reverse Transcriptase (RT) Redefines the Elastic Limits of the Non-nucleoside Inhibitor-Binding Pocket

tert-Butyldimethylsilyl-spiroaminooxathioledioxide (TSAO) compounds have an embedded thymidine-analogue backbone; however, TSAO compounds invoke non-nucleoside RT inhibitor (NNRTI) resistance mutations. Our crystal structure of RT:7 (TSAO-T) complex shows that 7 binds inside the NNRTI binding pocket, assuming a dragon shape, and interacts extensively with almost all the pocket residues. The structure also explains the structure activity relationships and resistance data for TSAO compounds. The binding of 7 causes hyper-expansion of the pocket and significant rearrangement of RT subdomains. This nonoptimal complex formation is apparently responsible (1) for the lower stability of a RT (p66/p51) dimer and (2) for the lower potency of 7 despite of its extensive interactions with RT. However, the HIV-1 RT:7 structure reveals novel design features such as (1). interactions with the conserved Tyr183 from the YMDD-motif and (2) a possible way for an NNRTI to reach the polymerase active site that may be exploited in designing new NNRTI.