Cellular Uptake of a Fluorescent Ligand Reveals Ghrelin O-Acyltransferase Interacts with Extracellular Peptides and Exhibits Unexpected Localization for a Secretory Pathway Enzyme

Ghrelin O-acyltransferase (GOAT) playsa centralrole in the maturation and activation of the peptide hormone ghrelin,which performs a wide range of endocrinological signaling roles. Usinga tight-binding fluorescent ghrelin-derived peptide designed for highselectivity for GOAT over the ghrelin receptor GHSR, we demonstratethat GOAT interacts with extracellular ghrelin and facilitates ligandcell internalization in both transfected cells and prostate cancercells endogenously expressing GOAT. Coupled with enzyme mutagenesis,ligand uptake studies support the interaction of the putative histidinegeneral base within GOAT with the ghrelin peptide acylation site.Our work provides a new understanding of GOAT's catalytic mechanism,establishes that GOAT can interact with ghrelin and other peptideslocated outside the cell, and raises the possibility that other peptidehormones may exhibit similar complexity in their intercellular andorganismal-level signaling pathways.