A Photo-Crosslinkable Biotin Derivative of the Phosphoantigen (E)-4-hydroxy-3-methylbut-2-enyl Diphosphate (hmbpp) Activates V Gamma 9v Delta 2 T Cells and Binds to the Hmbpp Site of Btn3a1

V gamma 9V delta 2 T cells play an important role in the cross talk of the innate and adaptive immune system. For their activation by phosphoantigens (PAgs), both cell surface receptors, the eponymous V gamma 9V delta 2 T cell antigen receptors (V gamma 9V delta 2 TCRs) on V gamma 9V delta 2 T cells and butyrophilin 3A1 (BTN3A1) on the phosphoantigen-"presenting" cell, are mandatory. To find yet undetected but further contributing proteins, a biotinylated, photo-crosslinkable benzophenone probe BioBP-HMBPP (2) was synthesized from a known allyl alcohol in nine steps and overall 16% yield. 2 is based on the picomolar PAg (E)-4-hydroxy-3-methylbut-2-enyl diphosphate (HMBPP, 1). Laser irradiation of 2 at 308nm initiated the photo-crosslinking reaction with proteins. When the B30.2 domain of BTN3A1, which contains a positively charged PAg-binding pocket, was exposed to increasing amounts of HMBPP (1), labeling by BioBP-HMBPP (2) was reduced significantly. Because BSA labeling was not impaired, 2 clearly binds to the same site as natural ligand 1. Thus, BioBP-HMBPP (2) is a suitable tool to identify co-ligands or receptors involved in PAg-mediated T cell activation.