The β2-adrenergic receptor interacts with the Na+/H+-exchanger regulatory factor to control Na+/H+ exchange

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Abstract

Stimulation of beta2-adrenergic receptors on the cell surface by adrenaline or noradrenaline leads to alterations in the metabolism, excitability, differentiation and growth of many cell types. These effects have traditionally been thought to be mediated exclusively by receptor activation of intracellular G proteins. However, certain physiological effects of beta2-adrenergic receptor stimulation, notably the regulation of cellular pH by modulation of Na+/H+ exchanger (NHE) function, do not seem to be entirely dependent on G-protein activation. We report here a direct agonist-promoted association of the beta2-adrenergic receptor with the Na+/H+ exchanger regulatory factor (NHERF), a protein that regulates the activity of the Na+/H+ exchanger type 3 (NHE3). NHERF binds to the beta2-adrenergic receptor by means of a PDZ-domain-mediated interaction with the last few residues of the carboxy-terminal cytoplasmic domain of the receptor. Mutation of the final residue of the beta2-adrenergic receptor from leucine to alanine abolishes the receptor's interaction with NHERF and also markedly alters beta2-adrenergic receptor regulation of NHE3 in cells without altering receptor-mediated activation of adenylyl cyclase. Our findings indicate that agonist-dependent beta2-adrenergic receptor binding of NHERF plays a role in beta2-adrenergic receptor-mediated regulation of Na+/H+ exchange.

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The N-methyl-D-aspartate (NMDA) receptor subserves synaptic glutamate-induced transmission and plasticity in central neurons. The yeast two-hybrid system was used to show that the cytoplasmic tails of NMDA receptor subunits interact with a prominent postsynaptic density protein PSD-95. The second PDZ domain in PSD-95 binds to the seven-amino acid, COOH-terminal domain containing the terminal tSXV motif (where S is serine, X is any amino acid, and V is valine) common to NR2 subunits and certain NR1 splice forms. Transcripts encoding PSD-95 are expressed in a pattern similar to that of NMDA receptors, and the NR2B subunit co-localizes with PSD-95 in cultured rat hippocampal neurons. The interaction of these proteins may affect the plasticity of excitatory synapses.

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Identification of EBP50: A PDZ-containing Phosphoprotein that Associates with Members of the Ezrin-Radixin-Moesin Family

David Reczek, Mark Berryman, Anthony Bretscher (1997)

Members of the ezrin-radixin-moesin (ERM) family of membrane–cytoskeletal linking proteins have NH2- and COOH-terminal domains that associate with the plasma membrane and the actin cytoskeleton, respectively. To search for ERM binding partners potentially involved in membrane association, tissue lysates were subjected to affinity chromatography on the immobilized NH2-terminal domains of ezrin and moesin, which comprise the ezrin-radixin-moesin–association domain (N-ERMAD). A collection of polypeptides at 50–53 kD from human placenta and at 58-59 kD from bovine brain bound directly to both N-ERMADs. The 50–53-kD placental proteins migrated as a major 50-kD species after phosphatase treatment, indicating that the heterogeneity is due to different phosphorylation states. We refer to these polypeptides as ERM-binding phosphoprotein 50 (EBP50). Sequence analysis of human EBP50 was used to identify an ∼2-kb human cDNA that encodes a 357-residue polypeptide. Recombinant EBP50 binds tightly to the N-ERMADs of ezrin and moesin. Peptide sequences from the brain candidate indicated that it is closely related to EBP50. EBP50 has two PSD-95/DlgA/ZO-1–like (PDZ) domains and is most likely a homologue of rabbit protein cofactor, which is involved in the protein kinase A regulation of the renal brush border Na+/H+ exchanger. EBP50 is widely distributed in tissues, and is particularly enriched in those containing polarized epithelia. Immunofluorescence microscopy of cultured cells and tissues revealed that EBP50 colocalizes with actin and ezrin in the apical microvilli of epithelial cells, and immunoelectron microscopy demonstrated that it is specifically associated with the microvilli of the placental syncytiotrophoblast. Moreover, EBP50 and ezrin can be coimmunoprecipitated as a complex from isolated human placental microvilli. These findings show that EBP50 is a physiologically relevant ezrin binding protein. Since PDZ domains are known to mediate associations with integral membrane proteins, one mode of membrane attachment of ezrin is likely to be mediated through EBP50.