Hemocyanins in spiders, XV. The role of the individual subunits in the assembly of Eurypelma hemocyanin.

The role of the seven different subunits in the quaternary structure of the 24-meric (37 S) hemocyanin of the tarantula, Eurypelma californicum, was studied by reassembly experiments. Individual subunits and combinations of 2, 3, 4, etc. different subunits were incubated in a total concentration of 1-2 mg/ml overnight in Tris buffer, pH 7.5, omitting divalent cations. The reassembly mixtures were then analyzed by thinlayer gel filtration, electron microscopy, analytical ultracentrifugation, and polyacrylamide gel electrophoresis. At least at the low protein concentration employed, none of the isolated monomeric subunits (a, d, e, f, g) is capable of hexamer are likewise ineffective. The minimal number of monomers required for hexamer formation is three, but only about half of the 10 possible combinations was effective. The best results were obtained with a + f + g. Four and five monomers always yielded hexamers, although the composition of the latter is not known. The heterodimer bc is a prerequisite to go beyond the hexameric state. bc alone forms tetramers and large, strand-like aggregates. a shows a high affinity towards bc, trimers and tetramers being formed. Out of all combinations of bc and two monomers, only one (bc + a + g) produced hexamers or heptamers. With three monomers plus bc, dodecamers and larger structures (but no 24-mers) were obtained, but only if also a was present (bc + a + f + g being the best combination). If all subunits minus one monomer were combined, the hexamer/heptamer level was not exceeded if a was omitted. 24-mers were formed in appreciable yield only in one case, namely if e was absent. However, this reassembly product turned out to be unstable. To obtain stable 24-meric hemocyanin, the complete set of subunits had to be present. The roles of the different subunits are defined as follows: bc is a "core" subunit which forms the inter-hexamer link within each dodecameric half. It serves also in inter-dodecamer bonding. The artificial homodimers bb and cc are ineffective in this respect. bc must be supplemented by a which is needed for dodecamer stabilization. f is also required to link two dodecamers, while g serves to stabilize this bridge. d and e are required to finish off and to stabilize the 24-mer. A model is proposed showing some neighbourhood relationships of the subunits within the hemocyanin 24-mer, and a pathway of reassembly is discussed.