Identification of GPR119 Interacting Proteins: The Effect of Abscisic Acid on the Interactions and the Role of SERPI on Localizing GPR119 to the Plasma Membrane

The research presented in this dissertation focused on the interacting proteins of the G-protein coupled receptor 119 (GPR119), a rhodopsin-like G-protein coupled receptor (GPCR). It was hypothesized that like other GPCRs, there would be GPCR interacting proteins (GIPs) with which GPR119 signaling could be defined. The hypothesis was tested using the yeast-two-hybrid screen, with β-galactosidase as the reporter gene. Seventeen proteins produced by human brain cDNAs interacted with GPR119. The association between GPR119 and the GIPs were quantified by β-galactosidase activities. The identified GIPs were of various functions, including protein binding, calcium ion homeostasis, apoptosis, and protein translocation. Critical to most receptor-protein interactions, are ligands for the receptors. The plant hormone abscisic acid (ABA), which is also an endogenous hormone in animals, is an analogue of retinoic acid, which was one of the first identified GPR119 agonists. Therefore, as a plant biologist, I was interested in whether ABA impacted GPR119-GIP interactions. It was hypothesized that “ABA might be a potential GPR119 agonist and it promotes GPR119-GIP interactions.” Using the yeast-two-hybrid system, ABA was observed to promote the interaction between GPR119 and four of the GIPS, which suggested that ABA could regulate signaling through GPR119 and these GIPs. Stress-associated endoplasmic reticulum protein 1 (SERP1) was one of the strongest GPR119 interacting proteins identified in our screen. It is known to be involved in protein glycosylation and protein translocation. SERP1 knock-out mice displayed glucose intolerance. Therefore I hypothesized that SERP1 facilitates trafficking of GPR119 from the endoplasmic reticulum (ER) to the plasma membrane. The amount of GPR119 on the cell surface was determined in the present and absence of SERP1 by western blot and densitometry readings. The results supported the hypothesis. In conclusion, we identified seventeen GIPs of GPR119. ABA was found to increase the interaction between GPR119 and four of the GIPs. In addition, I demonstrated that SERP1 plays a role in GPR119 export to the plasma membrane