Mitogen-activated protein kinases (MAPKs) are key signaling enzymes that regulate cellular stress responses through the phosphorylation of specific protein substrates. Dysregulation of MAPK signaling contributes to numerous diseases, including cancer and neurodegenerative disorders. Although MAPK activation and catalytic mechanisms are well characterized, the structural basis of substrate specificity remains unknown. This project aims to address this gap by capturing atomic-level structural snapshots of substrates bound within the active site of the c-Jun N-terminal kinase 1 (JNK1). To achieve this, we will employ X-ray crystallography together with innovative nuclear magnetic resonance (NMR) methods that integrate selective methyl isotope labeling and photoactivatable catalysis. By elucidating the structural details of how substrates are recognized by the active site of JNK1, our work will open new avenues for the development of substrate-competitive inhibitors of MAPKs with enhanced selectivity and therapeutic potential.