Hexagonal Bilayer Structuring Activity of Linker Chains of an Annelid Giant Hemoglobin from the PolychaetePerinereis aibuhitensis

Preferential activity of linker chains to clamp submultiples to form hexagonal bilayer (HBL) assembly of the multisubunit hemoglobin (Hb) of the polychaetePerinereis aibuhitensis(∼3.4 MDa) was demonstrated. To understand the HBL assembly that should rely on structuring activity of each subunit, reassociation in response to combining isolated subunits was monitored using gel filtration, SDS–PAGE, and transmission electron microscopy. The isolation of each subunit L, T, and M (L, linker chains; T, disulfide-bonded trimer A-b-B; M, monomeric chain a) ofPerinereisHb was made simply by exposing Hb to pH 10.5, where Hb was completely dissociated into its subunits L, T, and M. As a result, it was concluded that (i) subunits T and M have strong affinity to form an intermediate complex, submultiple D, which is a dodecamer of globin chains, 3[a · A-b-B], (ii) addition of subunit L to submultiple D brings about the formation of whole molecule, similarly (iii) addition of subunit M to T+L forms the whole molecule, and (iv) addition of subunit T to M+L brings about the formation of the whole molecule, too. The results obtained lead us to conclude that linkers do function to clamp 12 submultiples D up to a whole molecule at the final step of formation ofPerinereisHb. In summary, linkers appeared to have high affinity for submultiple D, a little affinity for subunit T, but no affinity for subunit M at all. Thus linker chains were demonstrated to preferentially clamp submultiples D together to form the HBL disc of the whole molecule.