Hemocyanin from the Australian freshwater crayfish Cherax destructor. Electron microscopy of native and reassembled molecules.

Examination and measurement of electron micrographs of negatively stained hemocyanin molecules from Cherax destructor show that the predominant aggregated forms, the 16S and 24S components, are typical structures for arthropod hexamers and dodecamers, respectively. In Cherax hemocyanin the hexamers are formed from the monomeric (Mr congruent to 75,000) subunits, M1 and M2, while the dodecamers contain in addition a dimeric (Mr congruent to 150,000) subunit, M3'. Studies of the composition of solutions of the subunits M1 and m2 to which calcium ions have been added at pH 7.8 show that, under these conditions, reassembly occurs to particles indistinguishable from native hexamers. It is noteworthy that dodecamers are not seen since this confirms the previous suggestion that incorporation of the dimeric subunit in the assembly process is necessary for their formation. The results obtained from Cherax hemocyanin are related to those of previous structural studies of arthropod hemocyanins. In particular, the possible controlling role of certain specific subunits in arthropod hemocyanin oligomers containing more than one kind of subunit is illustrated with a model for the Cherax dodecamer, in which the dimeric subunit is shared between the two halves of the molecule.