Mitochondrial protein-lysine N-methyltransferase that promotes chronic pain. Involved in persistent inflammatory and neuropathic pain: methyltransferase activity in the mitochondria of sensory neurons promotes chronic pain via a pathway that depends on the production of reactive oxygen species (ROS) and on the engagement of spinal cord microglia. Protein-lysine N-methyltransferase activity is dependent on S-adenosyl-L-methionine. Localizes to mitochondrial cristae. Belongs to the ANT/ATPSC lysine N-methyltransferase family.