Primary structure and expression of the human beta-subunit and related proteins of the rod photoreceptor cGMP-gated channel.

The full-length cDNA for the beta-subunit of the human rod photoreceptor cyclic nucleotide-gated channel has been shown to encode a 1251-amino acid ( approximately 140 kDa) polypeptide which, like its bovine counterpart, has an unusual bipartite structure. The C-terminal part corresponds to the previously reported "subunit 2" of the human rod channel and contains the structural features of other cyclic nucleotide-gated channel subunits including six putative membrane spanning segments, a cyclic nucleotide binding domain, a voltage-sensor motif, and a pore region. The N-terminal part contains the human homolog of the bovine glutamic acid-rich protein called GARP. Western blots indicate that both the native and heterologously expressed human beta-subunit migrate anomalously as a 220-kDa polypeptide by SDS-gel electrophoresis. Two other GARP variants, full-length GARP (f-GARP) and truncated GARP (t-GARP), are also present in human, bovine, and rat rod outer segments and migrate as 120-140- and 55-62-kDa polypeptides, respectively. The bovine f-GARP and t-GARP cDNAs code for proteins containing 590 amino acids and 299 amino acids, respectively. The first 571 amino acids of f-GARP and the first 291 amino acids of t-GARP are identical to the corresponding N-terminal amino acid sequence of the bovine beta-subunit. The two GARP variants, themselves, are not tightly associated with the rod channel. These results indicate that mammalian rod outer segments contain three alternatively spliced variants of GARP, one of which constitutes the N-terminal part of the rod channel beta-subunit.