A 240 KDA PROTEIN REPRESENTS THE COMPLETE BETA-SUBUNIT OF THE CYCLIC NUCLEOTIDE-GATED CHANNEL FROM ROD PHOTORECEPTOR

The cyclic nucleotide-gated channel from rod photoreceptors is compose d of two distinct subunits (alpha and beta), The properties of the alp ha subunit, which can form functional channels by itself, are modified by coexpression with a homologous polypeptide, designated the beta su bunit. However, the alpha subunit from rod photoreceptor membranes cop urifies with a 240 kDa protein that is significantly larger than this putative beta subunit, We now demonstrate by peptide sequencing and by cloning and functional expression of cDNA that the 240 kDa protein re presents the complete beta subunit with an unusual bipartite structure . The N-terminal part is essentially identical to a glutamic acid-rich protein (GARP), whereas the C-terminal part is highly homologous to t he previously cloned human ''beta subunit,'' Expression of the complet e beta subunit in HEK 293 cells results in a polypeptide with the same apparent molecular weight as the 240 kDa protein of the native rod ch annel. Coexpression of the alpha subunit with the full-length beta sub unit yields hetero-oligomeric channels with properties characteristic of the native channel.