The sequence of a gastropod hemocyanin (HtH1 from Haliotis tuberculata)

The eight functional units (FUs), a-h, of the hemocyanin isoform HtH1 from Haliotis tuberculata (Prosobranchia, Archaeogastropoda) have been sequenced via cDNA, which provides the first complete primary structure of a gastrop od hemocyanin subunit, With 3404 amino acids (392 kDa) it is the largest po lypeptide sequence ever obtained for a respiratory protein. The cDNA compri ses 10,758 base pairs and includes the coding regions for a short signal pe ptide, the eight different functional units, a 3'-untranslated region of 47 8 base pairs, and a poly(A) tail. The predicted protein contains 13 potenti al sites for N-linked carbohydrates (one for HtH1-a, none for HtH1-c, and t wo each for the other six functional units). Multiple sequence alignments s how that the fragment HtH1-abcdefg is structurally equivalent to the seven- FU subunit from Octopus hemocyanin, which is fundamental to our understandi ng of the quaternary structures of both hemocyanins, Using the fossil recor d of the gastropod-cephalopod split to calibrate a molecular clock, the ori gin of the molluscan hemocyanin from a single-FU protein was calculated as 753 +/- 68 million years ago. This fits recent paleontological evidence for the existence of rather large mollusc-like species in the late Precambrian .