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

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.