Department of Therapeutic Discovery, Amgen Inc., South San Francisco, CA 94080, USA.
Protein folding, topogenesis and intracellular targeting of G protein-coupled receptors (GPCRs) must be precisely coordinated to ensure correct receptor localization. To elucidate how different steps of GPCR biosynthesis work together, we investigated the process of membrane topology determination and how it relates to the acquisition of cell surface trafficking competence in human GPR34. By monitoring a fused FLAG-tag and a conformation-sensitive native epitope during the expression of GPR34 mutant panel, a tri-basic motif in the first intracellular loop was identified as the key topogenic signal that dictates the orientation of transmembrane domain-1 (TM1). Charge disruption of the motif perturbed topogenic processes and resulted in the conformational epitope loss, post-translational processing alteration, and trafficking arrest in the Golgi. The placement of a cleavable N-terminal signal sequence as a surrogate topogenic determinant overcame the effects of tri-basic motif mutations and rectified the TM1 orientation; thereby restored the conformational epitope, post-translational modifications, and cell surface trafficking altogether. Progressive N-tail truncation and site-directed mutagenesis revealed that a proline-rich segment of the N-tail and all four cysteines individually located in the four separate extracellular regions must simultaneously reside in the ER lumen to muster the conformational epitope. Oxidation of all four cysteines was necessary for the epitope formation, but the cysteine residues themselves were not required for the trafficking event. The underlying biochemical properties of the conformational epitope was therefore the key to understand mechanistic processes propelled by positive-inside rule that simultaneously regulate the topogenesis and intracellular trafficking of GPR34.
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