Acyrthosiphon pisum AQP2: a multifunctional insect aquaglyceroporin

Annotation of the recently sequenced genome of the pea aphid (Acyrthosiphon pisum) identified a gene ApAQP2 (ACYPI009194, Gene ID: 100168499) with homology to the Major Intrinsic Protein/aquaporin superfamily of membrane channel proteins. Phylogenetic analysis suggests that ApAQP2 is a member of an insect-specific clade of this superfamily. Homology model structures of ApAQP2 showed a novel array of amino acids comprising the substrate selectivity-determining "aromatic/arginine" region of the putative transport pore. Subsequent characterization of the transport properties of ApAQP2 upon expression in Xenopus oocytes supports an unusual substrate selectivity profile. Water permeability analyses show that the ApAQP2 protein exhibits a robust mercury-insensitive aquaporin activity. However unlike the water-specific ApAQP1 protein, ApAQP2 forms a multifunctional transport channel that shows a wide permeability profile to a range of linear polyols, including the potentially biologically relevant substrates glycerol, mannitol and sorbitol. Gene expression analysis indicates that ApAQP2 is highly expressed in the insect bacteriocytes (cells bearing the symbiotic bacteria Buchnera) and the fat body. Overall the results demonstrate that ApAQP2 is a novel insect aquaglyceroporin which may be involved in water and polyol transport in support of the Buchnera symbiosis and aphid osmoregulation.