Coexpression of alpha and beta subunits of the rod cyclic GMP-gated channel restores native sensitivity to cyclic AMP: Role of D604/N1201

Coexpression of the beta wt and alpha wt subunits of the bovine rod channel restores two characteristics of the native channels: higher sensitivity to cAMP and potentiation of cGMP-induced currents by low cAMP concentrations. To test whether the increased sensitivity to cAMP is due to the uncharged nature of the asparagine residue (N1201) situated in place of aspartate D60 4 in the beta subunit as previously suggested (Varnum et al., 1995, Neuron. 15:619-625), we compared currents from wild-type (alpha wt and alpha wt/be ta wt) acid from mutated channels (alpha D604N, alpha D604N/beta wt, and al pha wt/beta N1201D). The results show that the sensitivity to cAMP and cAMP potentiation is partly but-not entirely determined by the charge of residu e 1201 in the beta subunit. The D604N mutation in the alpha subunit and, to a lesser extent, coexpression of the beta wt subunit with the alpha wt sub unit reduce the open probability for cGMP compared to that of the alpha wt channel. Interpretation of the data with the MWC allosteric model (model of Monod, Wyman, Changeux; Monod et al,, 1965, J. Mel. Biol. 12:88-118) sugge sts that the D604N mutation in the alpha subunits and coassembly of alpha a nd beta subunits alter the free energy of gating by cAMP more than that of cAMP binding.