Abstract
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.
Highlights
Hexapoda comprise of the well-known class Insecta with a second one called Entognatha
Hemocyanins are highly conserved respiratory proteins in the lineage of arthropods and molluscs. They occur freely dissolved in the hemolymph and especially for arthropods, they consist of subunits of ca. 72–74 kDa which connect themselves to hexamers
6-mer and strongly reminiscent like structures in every species (Figure 1). They were characterized as 2 × 6-mer and strongly to the hemocyanins in Astacusfound leptodactylus, Homarus, americanus, Cancer reminiscent to the found hemocyanins in Astacus leptodactylus
Summary
Hexapoda comprise of the well-known class Insecta with a second one called Entognatha. Hemocyanin was identified in Collembola hemolymph by RT-PCR and RACE techniques, leading to a full-length sequence for Sinella curviseta hemocyanin (termed ScuHc1) and a partial sequence from Folsomia candida hemocyanin (FcaHc1) [19] Both were classified as hexapod hemocyanin subunit type 1 based on about 60% sequence identity with the subunit type 1 hemocyanin from the stonefly. Perla marginata [20], which was shown to be a biologically active 1 × 6-meric hemocyanin during the stonefly’s larval stadium [20]. It has not been possible, so far, to present the structure of a hexapod hemocyanin. Some ideas concerning the function of this respiratory protein in Collembola will be discussed
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