The ? opioid receptor (KOPR) is one of the three major types (?, ? and ?) of opioid receptors that mediate effects of opioids in vivo. Activation of KOPR produces analgesia, dysphoria, water diuresis, hypothermia and modulation of immune responses. KOPR antagonists may be potentially useful for curbing cocaine craving and as anti-depressants and anti-anxiety agents. KOPR, a member of the 7TMR family, is coupled through pertussis toxin-sensitive G proteins to a variety of effectors. For 7TMRs the capacity of agonists to modulate downstream signaling molecules depends on the availability of the receptors on cell surface. The number of cell surface 7TMRs reflects a balance between biosynthesis and endocytosis pathways. The post-activation endocytic events have been well-documented;however, regulation along the biosynthesis pathway is much less understood. The focus of this grant application is to characterize the regulation of the KOPR trafficking along the biosynthesis pathway, in particular by the proteins GEC1, sortilin and 14-3-3, which we found to interact with the KOPR and to be involved in KOPR export trafficking. The central hypothesis is that the export trafficking is regulated by molecules interacting with the KOPR.
The specific aims are as follows. (1) To delineate mechanisms underlying GEC1-promoted expression and trafficking of the KOPR. The hypothesis that GEC1 enhances KOPR expression by enhancing ATPase activity of NSF, but not by membrane association by lipid conjugation will be tested. (2) To investigate the role of 14-3-3 in the trafficking and cellular pharmacology of the KOPR. The hypothesis is that 14-3-3 proteins bind to KOPR C-terminal domain and/or i3 loop, which masks COPI binding, allowing the KOPR to sort to plasma membranes. (3) To examine the interaction of the KOPR with sortilin and its functional consequences. The hypothesis to be tested is that sortilin binding to the KOPR sorts the receptor to endosomes and lysosomes and this represents a novel post-ER quality control mechanism. We will also examine if the interactions of the KOPR with these proteins affect signaling and regulation. The proposed studies will provide better understanding of cell biology of the KOPR and provide mechanistic insights into regulation of export of the KOPR. In addition, such understanding will have important implications for other membrane bound receptors since these regulatory mechanisms of export are likely to be applicable to some other proteins.
The proposed research will provide better understanding on how cell surface receptor numbers are regulated, which play important roles in response to external stimuli, including drugs and neurotransmitters.
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