One gene - two proteins: The C-terminus of the prototypical M2 muscarinic receptor localizes to the mitochondria and regulates cell respiration

Muscarinic acetylcholine receptors are prototypical G protein-coupled receptors (GPCRs), members of a large family of seven transmembrane receptors that mediates a wide variety of extracellular signals. We show here, for the first time in a GPCR, that the carboxyl terminal fragment of the muscarinic M2receptor, containing the transmembrane regions 6 and 7 (M2tail), is expressed by virtue of an internal ribosome entry site located in the third intracellular loop. Single cell imaging reveals that M2tail, whose expression is upregulated in cells undergoing integrated stress response, does not follow the normal route to the plasma membrane, but is almost exclusively sorted to the mitochondria inner membrane: here it controls oxygen consumption, cell proliferation and the formation of reactive oxygen species by reducing oxidative phosphorylation. Crispr/Cas9 editing of the key methionine where cap-independent translation begins in human induced pluripotent stem cells, reveals the physiological role of this process in influencing cell proliferation and oxygen consumption at the endogenous level. The expression of the C-terminal domain of a GPCR, capable of regulating mitochondrial function, constitutes a hitherto unknown mechanism notably unrelated to its canonical signalling function as a GPCR at the plasma membrane. This work thus highlights a potential novel mechanism thar cells may use for controlling their metabolism under variable environmental conditions, notably as a negative regulator of cell respiration.