ALG-2 interacting protein AIP1: A novel link between D1 and D3 signalling

Dopamine signalling is a critically important process in the human brain that controls mood, cognition and motor activity. In order to gain detailed insight into this signalling pathway at the molecular level, we carried out yeast two-hybrid screens with D1-like (D1, D5) and D2-like (D2, D3, D4) dopamine receptors and identified 11 dopamine receptor interacting proteins (DRIPs). Using the C-terminal domain of D1 receptor as bait, we identified AIP1 (ALG-2 interacting protein 1), a known modulator of caspase-dependent and caspase-independent cell death, including neuronal cell death, that is also part of the endosomal transport system. In a separate yeast two-hybrid screen, using the third intracellular cytoplasmic loop of D3 as bait, we again identified AIP1. The interaction of AIP1 with both D1 and D3 was confirmed in vitro and in vivo using a variety of methods, including glutathione S-transferase (GST) pull-down, blot overlay and coimmunoprecipitation from mouse brain lysates. We have also observed colocalization of D1 and D3 with AIP1 in mouse brain tissue. In addition, coexpression of AIP1 with D1 resulted in > 50% reduction in binding capacity of D1 to its antagonist. Finally, AIP1 up-regulates D1 and D3 expression and appears to be important for their stability and trafficking.