β-Arrestin2-Mediated Internalization of Mammalian Odorant Receptors

Odorant receptors comprise the biggest subfamily of G-protein-coupled receptors. Although the endocytic mechanisms of other G-protein-coupled receptors have been characterized extensively, almost nothing is known about the intracellular trafficking of odorant receptors. The present study describes the endocytic pathway of mammalian odorant receptors, which bind β-arrestin2 with high affinity and are internalized via a clathrin-dependent mechanism. After prolonged odorant exposure, receptors are not targeted to lysosomal degradation but accumulate in recycling endosomes. Odorant-induced odorant receptor desensitization is promoted by cAMP-dependent protein kinase A phosphorylation and is dependent on serine and threonine residues within the third intracellular loop of the receptor. Moreover, β-arrestin2 is redistributed into the dendritic knobs of mouse olfactory receptor neurons after treatment with a complex odorant mixture. Prolonged odorant exposure resulted in accumulation of β-arrestin2 in intracellular vesicles. Adaptation of olfactory receptor neurons to odorants can be abolished by the inhibition of clathrin-mediated endocytosis, showing the physiological relevance of the here described mechanism of odorant receptor desensitization. A better understanding of odorant receptor trafficking and additional insight into the molecular determinants underlying the interactions of odorant receptors with β-arrestin2 and other trafficking proteins will therefore be important to fully understand the mechanisms of adaptation and sensitization in the olfactory epithelium.