Subunit organization of the abalone Haliotis tuberculata hemocyanin type 2 (HtH2), and the cDNA sequence encoding its functional units d, e, f, g and h.

We have developed a HPLC procedure to isolate the two different hemocyanin types (HtH1 and HtH2) of the European abalone Haliotis tuberculata. On the basis of limited proteolytic cleavage, two-dimensional immunoelectrophoresis, PAGE, N-terminal protein sequencing and cDNA sequencing, we have identified eight different 40-60-kDa functional units (FUs) in HtH2, termed HtH2-a to HtH2-h, and determined their linear arrangement within the elongated 400-kDa subunit. From a Haliotis cDNA library, we have isolated and sequenced a cDNA clone which encodes the five C-terminal FUs d, e, f, g and h of HtH2. As shown by multiple sequence alignments, defg of HtH2 correspond structurally to defg from Octopus dofleini hemocyanin. HtH2-e is the first FU of a gastropod hemocyanin to be sequenced. The new Haliotis hemocyanin sequences are compared to their counterparts in Octopus, Helix pomatia and HtH1 (from the latter, the sequences of FU-f, FU-g and FU-h have recently been determined) and discussed in relation to the recent 2.3 A X-ray structure of FU-g from Octopus hemocyanin and the 15 A three-dimensional reconstruction of the Megathura crenulata hemocyanin didecamer from electron micrographs. This data allows, for the first time, an insight into the evolution of the two functionally different hemocyanin isoforms found in marine gastropods. It appears that they evolved several hundred million years ago within the Prosobranchia, after separation of the latter from the branch leading to the Pulmonata. Moreover, as a structural explanation for the inefficiency of the type 1 hemocyanin to form multidecamers in vivo, the additional N-glycosylation sites in HtH1 compared to HtH2 are discussed.