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Carboxyl-Terminal Splicing Enhances Physical Interactions between the Cytoplasmic Tails of Purinergic P2X Receptors

Department of Genomic Drug Discovery Science, Graduate School of Pharmaceutical Sciences Kyoto University Faculty of Pharmaceutical Sciences, Kyoto University, Kyoto, Japan (T.K., G.T.); Endocrinology and Reproduction Research Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland (K.K., M-L.H., S.S.S.); Department of Pharmacology, University of Occupational and Environmental Health, Japan School of Medicine, Fukuoka, Japan (S.U., N.Y.); and Department of Molecular and Cell Pharmacology, National Research Institute for Child Health and Development, Tokyo, Japan (T.K., A.T.)

Purinergic P2X receptors are ion-conducting channels composed of three subunits, each having two transmembrane domains and intracellular amino (N) and carboxyl (C) termini. Although alternative splicing extensively modifies the C-terminal sequences of P2X subunits, the direct influence of such post-transcriptional modifications on receptor architecture and function remains poorly understood. In this study, we focused on mouse pituitary P2X₂ receptors. In this tissue, progressive splicing of the P2X_2a C terminus generated two functional subunit variants, P2X_2b and P2X_2e, which exhibited accelerated desensitization rates and attenuated calcium signals when the receptors were in homomeric states. To measure the intersubunit interaction in living cells, the efficient transfer of bioluminescent resonance energy between luciferase and fluorescent proteins attached to the N- or C-subunit termini of these subunits was used. The constitutive interactions between the full-length C termini of P2X_2a receptor were detected by a significant increase in fluorescence/luminescence intensity ratio compared with negative controls. Moreover, interactions between C termini and between C- and N termini of adjacent subunits were significantly enhanced in homomeric and heteromeric receptors containing P2X_2b or P2X_2e subunits. Finally, deletion of two amino acids at the splicing junction, but not at the C-terminal end of the P2X_2b receptor, resulted in the enhancement of channel desensitization and luminescence resonance energy transfer. These results indicate that C-terminal structure plays a critical role in the cytoplasmic intersubunit interactions and suggest that the extent of subunit interactions before ATP application could contribute to the subsequent channel activity and conformation changes associated with agonist-dependent desensitization.

Received for publication October 12, 2005.

Accepted for publication February 8, 2006.

Address correspondence to: Dr. Gozoh Tsujimoto, Department of Genomic Drug Discovery Science, Graduate School of Pharmaceutical Sciences, Kyoto University Faculty of Pharmaceutical Sciences, Kyoto University, Yoshida Shimoadachi-cho, Sakyo-ku, Kyoto 606-8501, Japan. E-mail: gtsuji{at}pharm.kyoto-u.ac.jp

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