Abstract
Betulinic acid (BA), a pentacyclic triterpenoid natural product, has received considerable attention owing to the broad spectrum of biological activities such as anti-HIV, anti-inflammatory, and especially, antitumor. Despite numerous studies on the antitumor mechanisms over the past decade, the cellular targets in cancer cells remain obscure. Here, we developed two photoprobes, BA-1/BA-2, by attaching terminal alkyne-containing "minimalist" tetrazole/diazirine photo-crosslinkers (L3 and L7) at the C-3 and C-28 positions of BA, respectively, based on well-established structure-activity relationships. The potential cellular targets were identified via competitive proteome profiling and bioimaging approaches.