Involvement of a Go-type G-protein coupled to guanylate cyclase in the phototransduction cGMP cascade of molluscan simple photoreceptors.
Simple photoreceptors, namely photoresponsive neurons, designated as A-P-1, Es-1, Ip-2 and Ip-1, exist in the sea slug Onchidium ganglion. Previous works has shown that, of these, Ip-2 and Ip-1 respond to light with a hyperpolarizing receptor potential, caused by the opening of light-dependent, cGMP-gated K+ channels, whereas A-P-1 and Es-1 are depolarized by light, owing to the closing of the same K+ channels. The present study of Ip-2 or Ip-1 cells was undertaken to identify the G-proteins that couple light to the activation of guanylate cyclase (GC), thereby leading to the opening of K+ channels and the consequent hyperpolarizing photocurrents. The specific channel blocker, 4-aminopyridine (4-AP), and a GC inhibitor, LY-83583, both suppressed this hyperpolarizing photocurrent. N-ethylmaleimide and GDP-beta-S also inhibited this photocurrent, consistent with the involvement of G-proteins. Mastoparan an activator of both Go- and Gi-type G-proteins, induced an outward current. Furthermore, benzalkonium chloride (C(16)BAC), a selective activator of Go, dose-dependently generated an outward current similar to that induced by mastoparan. Both of these outward currents were susceptible to 4-AP, LY-83583 and N-ethylmaleimide. Taken together, these results suggest that phototransduction in Ip-2 or Ip-1 cells is triggered by a Go-type G-protein coupled to GC. Thus, this new cGMP cascade contrasts with the conventional phototransduction cGMP cascade mediated by the Gt-type G-protein coupled to phosphodiesterase, seen in the vertebrate photoreceptors and the above A-P-1 or Es-1 cells.
Gotow
Tsukasa
T
Department of Neurology, Kagoshima University Graduate School of Medical and Dental Sciences, 8-35-1 Sakuragaoka Kagoshima 890-8520, Japan. tsukasa@m.kufm.kagoshima-u.ac.jp
Nishi
Takako
T
D000818:Animals; D006152:Cyclic GMP / Q000378:metabolism*; D004347:Drug Interactions; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005033:Ethylmaleimide / Q000494:pharmacology; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000819:agonists / Q000037:antagonists & inhibitors / Q000378:metabolism*; D017952:Ganglia, Invertebrate / Q000166:cytology; D006162:Guanylate Cyclase / Q000378:metabolism*; D066298:In Vitro Techniques; D036341:Intercellular Signaling Peptides and Proteins; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008974:Mollusca; D018408:Patch-Clamp Techniques; D010455:Peptides / Q000494:pharmacology; D010775:Photic Stimulation / Q000379:methods; D010786:Photoreceptor Cells / Q000502:physiology*; D014785:Vision, Ocular / Q000187:drug effects / Q000502:physiology*; D014862:Wasp Venoms / Q000494:pharmacology
D004791:Enzyme Inhibitors; D036341:Intercellular Signaling Peptides and Proteins; D010455:Peptides; D014862:Wasp Venoms; C037500:mastoparan; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; D006162:Guanylate Cyclase; D006152:Cyclic GMP; D005033:Ethylmaleimide