Abstract
With the importance of RNA-based regulatory pathways, the potential for targeting noncoding and coding RNAs by small molecule therapeutics is of great interest. Platinum(II) complexes including cisplatin (cis-diamminedichloroplatinum(II)) are widely prescribed anticancer compounds that form stable adducts on nucleic acids. In tumors, DNA damage from Pt(II) initiates apoptotic signaling, but this activity is not necessary for cytotoxicity (e.g., Yu et al., 2008), suggesting accumulation and consequences of Pt(II) lesions on non-DNA targets. We previously reported an azide-functionalized compound, picazoplatin, designed for post-treatment click labeling that enables detection of Pt complexes (White et al., 2013). Here, we report in-gel fluorescent detection of Pt-bound rRNA and tRNA extracted from picazoplatin-treated S. cerevisiae and labeled using Cu-free click chemistry. These data provide the first evidence that cellular tRNA is a platinum drug substrate. We assess Pt(II) binding sites within rRNA from cisplatin-treated S. cerevisiae, in regions where damage is linked to significant downstream consequences including the sarcin-ricin loop (SRL) Helix 95. Pt-RNA adducts occur on the nucleotide substrates of ribosome-inactivating proteins, as well as on the bulged-G motif critical for elongation factor recognition of the loop. At therapeutically relevant concentrations, Pt(II) also binds robustly within conserved cation-binding pockets in Domains V and VI rRNA at the peptidyl transferase center. Taken together, these results demonstrate a convenient click chemistry methodology that can be applied to identify other metal or covalent modification-based drug targets and suggest a ribotoxic mechanism for cisplatin cytotoxicity.