Elucidating the role of unique structural features of the α1D-adrenergic receptor: A tale of two tails

Abstract

G protein-coupled receptors (GPCRs) are seven transmembrane domain proteins accounting for ~4% of the human genome, and the predicted target of ~30% FDA approved therapeutics. Often, drugs compete with endogenous ligands for orthosteric binding sites, giving rise to off-target interactions and deleterious side-effects – thus there is a need to identify novel potential ligand binding sites with increased specificity. Recent advances in the field suggest that targeting protein:protein interaction networks, or post-translational modifications that regulate GPCR expression, trafficking, and/or function, may be the key to improving ligand selectivity and decreasing unwanted toxicities. To this end, I devoted my Ph.D. studies to studying two unique structural features that regulate the function of the α1D-adrenergic receptor (α1D-AR) – a GPCR vital for myriad peripheral and central nervous processes which has been implicated in the development and maintenance of hypertension, benign prostate hypertrophy, schizophrenia, and post-traumatic stress disorder – a distal C-terminal Type I PDZ (PSD95/Dlg/ZO-1) ligand and an unusual N-terminal domain. Using yeast two-hybrid and tandem affinity MS/MS assays, the Hague lab previously identified that the α1D-AR C-terminal PDZ ligand is responsible for the formation of obligate, cell-type specific, macromolecular protein complexes containing syntrophin, utrophin, α-dystrobrevin, α-catulin, liprin, phospholipase-Cβ2, and scribble. These complexes enhance receptor cell surface expression and function both in vitro and in vivo. Furthermore, a screen of 23 GPCRs containing Type I PDZ ligands revealed that these interacting partners are unique to α1D-AR. Thus, these protein:protein interaction networks represent an opportunity to develop novel medications with increased specificity for α1D-AR than currently available drugs. However, before this can come to fruition, a more thorough understanding of how these complexes are organized is necessary. Towards mapping α1D-AR complex architecture, biolayer interferometry revealed that scribble displays >8x binding affinity compared to other known α1D-AR interactors. Complementary in situ and in vitro interaction assays revealed that scribble PDZ domains 1 and 4 are high affinity α1D-AR PDZ ligand interaction sites. The development of a novel SNAP-GST pull-down assay found that scribble is able to bind multiple α1D-AR C-terminal PDZ ligands via a cooperative mechanism. Structure-function analyses identified R1110PDZ4 as a unique, critical residue dictating the α1D-AR:PDZ4 interaction. A crystal structure of a non-binding mutant, PDZ4 R1110G, predicts a spatial shift of the carboxylate-binding loop prevents docking of the C-terminal PDZ ligand. Thus, these findings identify scribble PDZ1 and 4 as high affinity α1D-AR interaction sites, and potential targets to treat diseases associated with aberrant α1D-AR function. Additionally, α1D-ARs contain two putative N-glycosylation sites within the large N-terminal domain at N65 and N82. However, determining the glycosylation state of this receptor has proven challenging. Towards understanding the role of these putative glycosylation sites, site-directed mutagenesis and lectin affinity purification identified N65 and N82 as bona fide acceptors for N-glycans. Surprisingly, it was revealed that simultaneously mutating N65 and N82 causes early termination of α1D-AR between transmembrane domain 2 and 3. Label-free dynamic mass redistribution and cell surface trafficking assays revealed that single and double glycosylation-deficient mutants display limited function with impaired plasma membrane expression. Confocal microscopy imaging analysis and SNAP-tag sucrose density fractionation assays revealed the dual glycosylation mutant α1D-AR is widely distributed throughout the cytosol and nucleus. Based on these novel findings, I propose α1D-AR transmembrane domain 2 acts as an ER localization signal during active protein biogenesis, and that α1D-AR N-terminal glycosylation is required for complete translation of nascent, functional receptor. Taken together, the results from these studies identify two promising mechanisms for the development of targeted therapeutics to treat diseases and disorders associated with α1D-AR dysfunction.