Evolution, Structure, And Function Of Phenoloxidase In The Pea Aphid Acyrthosiphon Pisum

Abstract

Phenoloxidases (monophenol monooxygenase, EC 1.14.18.1; catechol oxidase, EC 1.10.3.1) are a group of enzymes with copper cofactors that produce reactive quinones and are part of the melanin synthesis pathway, both of which have important roles in immunity. The pea aphid, Acyrthosiphon pisum, which according to genome annotation is deficient in many other immune system components, codes for two phenoloxidase proteins that represent putative dimer components and possess the amino acid residues contributing to the active site. Constitutive phenoloxidase activity was detectible in the pea aphid hemolymph. It was activated by both conformational change with methanol and proteolytic cleavage of the propeptide with trypsin. Phenoloxidase activity was not significantly altered by aseptic wounding or infection studies with Escherichia coli or Micrococcus luteus. Phylogenetic analysis of insect phenoloxidases yielded a topology consistent with a lineage-specific duplication in each order (including Hemiptera). The possibility that the topology could be generated by a duplication, probably in the ancestral insect, followed by coevolution between the two phenoloxidase subunits within each order, was explored but rejected. The three-dimensional structures of the pea aphid phenoloxidases were reconstructed by homology modeling. The models of all three possible dimeric states of phenoloxidase (two homodimers and one heterodimer) did not exhibit conformational change in response to propeptide cleavage and their conformation differed from other modeled insect phenoloxidases. Taken together, these results suggest that the pea aphid has a functional phenoloxidase, but that it may be activated and function in a different way from the phenoloxidases in previously-studied insects.