Jumping on the Edge—First Evidence for a 2 × 6-meric Hemocyanin in Springtails

Hemocyanins are respiratory dioxygen carrier proteins found in many arthropods including ancient terrestrial species such as spiders and scorpions as well as marine horseshoe crabs. As hemocyanins are highly conserved in this lineage, it is possible to observe an evolutionary descent through its subunits and their overall structure. Unfortunately, little is known about the structure and function of hexapod hemocyanins. Using recent springtail taxa (Collembola) as models for basal hexapods, and the help of electron microscopy, light scattering, SDS PAGE, and Western blot, we could demonstrate for the first time the presence of 2 × 6-meric hemocyanins in the hemolymph of hexapods. The quaternary structure is composed of at least two different subunits and looks nearly identical to the hemocyanin found in decapod crustaceans. In addition, homology modeling and western blotting suggest a close structural relationship between collembolan and crustacean hemocyanin. Such a respiratory protein was possibly helpful in the early terrestrialization process of ancient Collembola. In addition, physiological adaptations to hypoxic or temporarily anoxic conditions could be a possible explanation for the presence of this respiratory protein. Nevertheless, it has to be concluded that the primary benefit of hemocyanin for springtails remains unclear.