Complexoform-restricted covalent TRMT112 ligands that allosterically agonize METTL5

Adaptors serve as hubs to regulate diverse protein complexes in cells. This multitude of functions can complicate the study of adaptors, as their genetic disruption may simultaneously impair the activities of several compositionally distinct complexes (or adaptor ‘complexoforms’). Here we describe the chemical proteomic discovery of bicyclopyrrolidine acrylamide stereoprobes that react with cysteine-100 (C100) of the methyltransferase (MT) adaptor TRMT112 in human cells. Curiously, the stereoprobes showed negligible reactivity with uncomplexed recombinant TRMT112, and we found that this interaction was restored excluively in the presence of METTL5, but not other MTs. A co-crystal structure revealed stereoprobe binding to a composite pocket proximal to C100 of TRMT112 that is templated by METTL5 and absent in other TRMT112:MT complexes. Structural rearrangements promoted by stereoprobe binding in turn lead to allosteric agonism of METTL5, thus revealing how covalent ligands targeting a pleiotropic adaptor can confer partner-specific functional effects through reactivity with a single complexoform.