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 photorecept or cyclic nucleotide-gated channel has been shown to encode a 1251-ami no acid (similar to 140 kDa) polypeptide which, like its bovine counte rpart, has an unusual bipartite structure. The C-terminal part corresp onds 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 g lutamic acid-rich protein called GARP. Western blots indicate that bot h the native and heterologously expressed human beta subunit migrate a nomalously as a 220-kDa polypeptide by SDS-gel electrophoresis. Two ot her GARP variants, full-length GARP (f-GARP) and truncated GARP (t-GAR P), are also present in human, bovine, and rat rod outer segments and migrate as 120-140- and 55-62-kDa polypeptides, respectively. The bovi ne f-GARP and t-GARP cDNAs code for proteins containing 590 amino acid s and 299 amino acids, respectively. The first 571 amino acids of f-GA RP and the first 291 amino acids of t-GARP are identical to the corres ponding N-terminal amino acid sequence of the bovine beta-subunit. The two GARP variants, themselves, are not tightly associated with the ro d channel. These results indicate that mammalian rod outer segments co ntain three alternatively spliced variants of GARP, one of which const itutes the N-terminal part of the rod channel beta-subunit.