Chemoproteomics and Phosphoproteomics Profiling Reveals Salvianolic Acid A as a Covalent Inhibitor of mTORC1

Salvianolic acid A (SAA), a major active ingredient of Salvia miltiorrhiza Bunge (Danshen), displays strong antiproliferative activity against cancer cells. However, their protein targets remain unknown. Here, we deconvoluted the protein targets of SAA using chemoproteomics and phosphoproteomics. By using alkynylated SAA as a probe, we discovered that SAA is a covalent ligand that can modify cellular proteins via its electrophilic α,β-unsaturated ester moiety. The subsequent chemoproteomics profiling revealed that 46 proteins were covalently modified by SAA, including Raptor, a subunit of mTORC1 for recruiting substrates for mTORC1. Although gene ontology enrichment analysis of these proteins suggested that SAA displays a promiscuous protein interaction, phosphoproteomics profiling revealed that the SAA modulated phosphoproteins were mainly enriched in the signaling pathways of PI3K-Akt-mTOR, which is closely related to cell growth and proliferation. This was confirmed by the biochemical assay with purified mTORC1, a Western blot assay with phospho-specific antibodies, and a cellular thermal shift assay. Our work discovered that SAA is a covalent ligand for protein modification and mTORC1 is one of its targets. Moreover, our work demonstrated that the integrative profiling of chemoproteomics and phosphoproteomics can be a powerful tool for target deconvolution for bioactive natural products.