The Role of the N-Terminal of the cGMP-Gated Channel from Vertebrate Rods

Abstract

Cyclic nucleotide (CNG) channels (Kaupp et al., 1989) play a fundamental role in phototransduction and chemotransduction (Torre, Ashmore, Lamb and Menini, 1995). Native CNG channels are composed by at least two subunits, usually referred to as alpha and beta subunit (Chen, Peng, Dhallan, Ahamed, Reed and Yau, 1993; Körschen et al., 1995). The alpha subunit is a polypeptide of 690 amino acids, while the full beta subunit is much longer and composed by 1394 amino acids. When heterologously expressed in Xenopus laevis oocytes, the alpha subunit, here referred to as the w.t. channel, forms functional channels, which are activated by cyclic nucleotides. The functional properties of this channel are similar but not identical to those of the native CNG channel. When the alpha and beta subunits are coexpressed in oocytes, ionic channels appear, with properties almost identical to those of the native channel. So far, it has not been possible to have functional channels formed by the beta subunit only. The amino acid sequence of these two polypeptides has a significant degree of homology with those forming voltage gated channels (Jan and Jan, 1990; Heginbotham, Abramson and MacKinnon, 1992; Guy, Durell, Warmke, Drysdale and Ganetzki, 1991; Gouldings et al., 1992; Bonigk et al., 1993; Henn, Baumann and Kaupp, 1995). This structural analogy between voltage gated channels and CNG channels has suggested to analyse the role of glutamate in position 363 of the w.t. channel (Root and MacKinnon, 1993; Eismann, Muller, Heinemann and Kaupp, 1994). Indeed this amino acid controls several features of ionic permeation, such as the sensitivity to external divalent cations, the multi-ion nature of the channel, the single channel conductance and the size of the narrowest section of the pore (Root and MacKinnon, 1993; Eismann et al., 1994; Sesti, Kaupp, Eismann, Nizzari and Torre, 1995; Bucossi et al., 1996).