SEQUENCE OF THE OCTOPUS DOFLEINI-HEMOCYANIN SUBUNIT - STRUCTURAL AND EVOLUTIONARY IMPLICATIONS

Sequencing of the subunit of the hemocyanin of Octopus dofleini has be en completed from a cDNA library. This represents the first molluscan hemocyanin to be completely sequenced. The sequence determined is for one of the two distinguishable cDNAs which have been recognized for th is protein. The protein subunit has 2896 amino acids and contains seve n functional units, each carrying two sets of three invariant histidin e residues constituting the binding sites (A and B) for two copper ato ms. The accompanying paper identifies this site in the C-terminal func tional unit (Odg). Differences in sequence for the two cDNAs, for the region in which both are available, are concentrated in the ''linker r egions'' between functional units. The sequences of the seven units ex hibit high similarity, averaging about 40% identity, with a concentrat ion of conserved sequences in the region surrounding the copper bindin g sites. The sequences around the B-site show significant homology to the sequences of arthropod hemocyanins. Comparison of the functional u nit sequences in terms of hydrophobicity and surface exposure profiles , as well as regions of probable secondary structure, indicate that al l functional units probably have a common tertiary folding; the protei n subunit is a string of similarly folded beads. A number of putative N-linked carbohydrate binding sites can be recognized in the sequence; one of these corresponds to the carbohydrate observed in the X-ray di ffraction study of functional unit Odg as disclosed in the accompying paper. Phylogenetic analysis of the sequences of the O. dofleini funct ional units, and comparison with other available molluscan sequences i ndicates that the multi-domain subunit structure must have arisen over a relatively brief period, preceeding the differentiation of major mo lluscan types. (C) 1998 Academic Press Limited.