Post-Translational Modifications of the Serotonin Type 4 Receptor Heterologously Expressed in Mouse Rod Cells

G-Protein-coupled serotonin receptor type 4 (5-HT4R) is a pharmacological target implicated in a variety of gastrointestinal and nervous system disorders. As for many other integral membrane proteins, structural and functional studies of this receptor could be facilitated by its heterologous overexpression in eukaryotic systems that can perform appropriate post-translational modifications (PTMs) on the protein. We previously reported the development of an expression system that employs rhodopsin's biosynthetic machinery in rod cells of the retina to express heterologous G-protein-coupled receptors (GPCRs) in a pharmacologically functional form. In this study, we analyzed the glycosylation, phosphorylation, and palmitoylation of 5-HT4R heterologously expressed in rod cells of transgenic mice. We found that the glycosylation pattern in 5-HT4R was more complex than in murine and bovine rhodopsin. Moreover, overexpression of this exogenous GPCR in rod cells also affected the glycosylation pattern of coexisting native rhodopsin. These results highlight not only the occurrence of heterogeneous PTMs on transgenic proteins but also the complications that non-native PTMs can cause in the structural and functional characterization of both endogenous and heterologous protein targets.