Identification and functional characterization of two novel alpha1-adrenergic receptor-interacting proteins

The alpha1-adrenergic receptors (alpha1-ARs) are canonical G protein-coupled receptors (GPCRs) that respond to the catecholamine neurotransmitters epinephrine and norepinephrine to elicit physiological and pathophysiological effects including contraction of vascular smooth muscle and promotion of cardiac hypertrophy, respectively. alpha1-ARs stimulate GTP exchange in G proteins of the Gq/11 family and promote activation of phospholipases and mobilization of intracellular Ca2+ ions, as well as stimulating mitogen-activated protein kinase (MAPK) pathways. The molecular basis for determining which pathway will be induced by catecholamines binding to the receptor is unknown, but it is clear that alpha 1-ARs signal differently in different cellular contexts.; Recent work has demonstrated the importance of a growing family of proteins that function as GPCR-interacting protein scaffolds. Binding of these proteins to receptors can promote specificity and efficiency of signal transmission to downstream effectors, or mediate activation of alternate signaling pathways. The expression of various GPCR-interacting proteins in a cell can promote agonist-mediated signaling in one pathway relative to another while utilizing the same receptor and G protein partners.; I initiated a yeast two-hybrid screen to search for proteins that interact with the intracellular tail of the alpha1d-AR in order to identify novel regulators of alpha1-AR signaling. From nearly sixty positive clones, two proteins were selected for immediate characterization: the protein kinase C (PKC)-binding protein nel-like protein 2 (NELL2) and dual leucine-zipper kinase (DLK), an upstream activator of c-jun amino-terminal kinase (JNK). The interaction between NELL2 and alpha1d-AR was confirmed in vitro and in cellulo, although no functional significance of the association was uncovered. DLK and the three alpha 1-AR isoforms (a, b, and d) were found to associate and co-localize in mammalian cells. The expression of alpha1-AR with DLK promotes translocation of the kinase to a buoyant membrane fraction on sucrose gradients in an isoform-specific manner; alpha1a- and alpha 1d-AR promote translocation while alpha1b-AR does not. In untreated cells, alpha1a-AR inhibited DLK-mediated JNK phosphorylation as well as activation, and stimulation of this receptor with norepinephrine elicited DLK-mediated JNK activation. These findings reveal a functional association between alpha1-AR and an effector that provides a novel mechanism for receptor-mediated MAPK pathway selectivity.