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We have examined the role of a highly conserved arginine (R209), which flanks the M1 transmembrane segment of nAChRs, in the biogenesis and function of neuronal nAChRs. Point mutations revealed that, in alphaBgtx-sensitive neuronal alpha7 nAChRs, the conserved arginine is required for the transport of assembled receptors to the cell surface. By contrast, R209 does not play any role in the transport of assembled alpha-Bgtx-insensitive neuronal alpha3beta4 nAChRs to the cell surface. However, a basic residue at this position of alpha3 and beta4 subunits is necessary for either synthesis, folding, or assembly of alpha3beta4 receptors. Moreover, electrophysiological experiments revealed that in alpha3beta4 receptors the conserved arginine of the alpha3 subunit is involved in either coupling agonist binding to the channel or regulating single channel kinetics.

The cytosolic enzymes asymmetrical diadenosine tetraphosphate hydrolase (EC 3.6.1.17, Ap R Aase) and diadenosine triphosphate hydrolase (EC 3.6.1.29, Ap Q Aase) are inhibited competitively by suramin. Ap R Aase and Ap Q Aase were assayed in cytosolic rat brain extracts using fluorogenic analogues of the respective substrates diadenosine tetraphosphate (Ap R A) and diadenosine triphosphate (Ap Q A). K i values for suramin as inhibitor of Ap R Aase and Ap Q Aase were 5U10 3T M and 3U10 3U M, respectively. Results indicate that suramin or suramin-like derivatives may be useful tools to investigate diadenosine polyphosphate cleaving enzymes and that the intracellular diadenosine polyphosphate metabolism may be a pharmacological target of suramin with biological and clinical implications.z 1998 Federation of European Biochemical Societies.