A 240 kDa protein represents the complete β subunit of the cyclic nucleotide-gated channel from rod photoreceptor

The cyclic nucleotide-gated channel from rod photoreceptors is composed of two distinct subunits (α and β). The properties of the a subunit, which can form functional channels by itself, are modified by coexpression with a homologous polypeptide, designated the β subunit. However, the α subunit from rod photoreceptor membranes copurifies with a 240 kDa protein that is significantly larger than this putative β subunit. We now demonstrate by peptide sequencing and by cloning and functional expression of cDNA that the 240 kDa protein represents the complete β 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 the previously cloned human "β subunit." Expression of the complete β subunit in HEK 293 cells results in a polypeptide with the same apparent molecular weight as the 240 kDa protein of the native rod channel. Coexpression of the α subunit with the fulllength β subunit yields hetero-oligomeric channels with properties characteristic of the native channel.