Immunological correlates between the multiple hemocyanin subunits of Limulus polyphemus and Tachypleus tridentatus

The 60 S hemocyanins of the horseshoe crabs, Limulus polyphemus and Tachypleus tridentatus, are of interest as models of how structurally diverse subunits interact in the assembly of high molecular weight proteins. Subunit diversity is a general characteristic in hemocyanins of the Chelicerates (horseshoe crabs, spiders, and scorpions) and is expressed to a lesser extent in hemocyanins of the crustaceans (crabs, shrimp, and lobsters). In order to better characterize the hemocyanins of Limulus and Tachypleus, their subunits were fractionated by ion-exchange and thin-layer gel chromatography. Immunological techniques were used extensively in isolation and characterization of the subunits. Both hemocyanins yield six chromatographic zones at pH 8.9. The Limulus pattern is due to the existence of seven monomers and a heterodimer. The heterodimer contains one additional monomer for a total of eight distinct subunits. Limulus monomer II_A is antigenically related to III_a but is antigenically deficient. Dissociation of Tachypleus hemocyanin at pH 8.9 yields five monomers and a heterodimer that contains two additional monomers. One monomer in the dimeric component of Limulus hemocyanin is antigenically related to a monomer of the dimeric component of Tachypleus and the other two monomers in the dimers are antigenically identical. It is proposed that dimeric components link the hexameric "building blocks" of the Chelicerate hemocyanins and thus play specific functional and structural roles.