Assembly of erythrocruorin from the earthworm Octolasium complanatum

The spontaneous assembly of the earthworm erythrocruorin molecule (60 S) from its 1/12 subunits (10 S) obtained by alkaline dissociation is a long debated problem, since the 60 S to 10 S dissociation step has been regarded as essentially irreversible or as only partially reversible when freshly dissociated solutions are used. Erythrocruorin from the earthworm Octolasium complanatum has been reassembled from its 10 S subunits. "Age" of the subunits, pH, and divalent cation concentration are the factors that influence the assembly reaction. Of primary importance is the age of the subunits, i.e. their exposure time to the alkaline dissociating pH. Parallel sedimentation velocity and sodium dodecyl sulfate/polyacrylamide gel electrophoresis experiments on the dissociated and reassembled solutions indicate that two processes take place at alkaline pH values: disulfide exchange and limited proteolysis. These processes, whose relative importance differs in the various preparations, might be responsible for the loss of reassociating capacity of the 10 S subunits. With freshly dissociated subunits, reassembly up to 80% may be achieved at pH 6.2 to 6.5 in the absence of divalent cations; the presence of 25 to 50 mM-Ca2+ renders the reaction essentially pH-independent in the range 6.2 to 8. The effect of Ca2+ is discussed in the light of the presence of structure-stabilizing binding sites for divalent cations at the 10 S intersubunit's contact regions.