Cloning and nucleotide sequence of cDNA for rhodopsin of the squid Todarodes pacificus

Abstract

A cDNA for rhodopsin was isolated from a library constructed from poly(A)⁺RNA of the squid (Todarodes pacificus) retina. One positive clone with the longest insert of cDNA (3.1 kb) was selected by employing a PCR‐amplified cDNA fragment as a probe. The nucleotide sequence of the cDNA revealed a single open reading frame of 1,344 bp encoding a polypeptide (M _r, 49,833), which covered a complete sequence for the squid opsin. This clone had a very long 3'‐non‐coding region (1.7 kb) including multiple polyadenylation signals, AATAAA, resembling the clones for Todarodes retinochrome and retinal‐binding protein (RALBP). The analysis of hydropathicity demonstrated the presence of seven transmembrane spanning domains, and a possible retinal‐binding site, Lys‐305, was found in the 7th domain. Todarodes rhodopsin contained characteristic sequences of PPQGY repeated in the C‐terminal region, as reported in Loligo and octopus rhodopsins. Structural comparison of those cephalopod rhodopsins is also discussed.