Figure 4.

Hypothesis for the evolution of the eight-FU molluscan hemocyanin subunit from a mono-FU precursor. Although our present results strongly suggest that the eight-FU subunit evolved from three subsequent gene duplication and fusion events, a pedigree of the latter could not be constructed from the sequence data so far. Thus, for the present scenario, recently revealed details of the quaternary structure (5) and general considerations of the evolution of extracellular oxygen carriers (29, 30) have been taken into account. It is proposed that the dimeric repeating unit as well as the di-pentameric architecture of the extant molluscan hemocyanin evolved before the eight-unit polypeptide, and that the latter grew from the C-terminal to the N-terminal FU and not vice versa. ∼ denotes the signal peptide that probably gave rise to the different linker peptides.