Abstract
Previous electrophysiological and behavioural studies implicate esterase 6 in the processing of the pheromone cis-vaccenyl acetate and various food odorants that affect aggregation and reproductive behaviours. Here we show esterase 6 has relatively high activity against many of the short-mid chain food esters, but negligible activity against cis-vaccenyl acetate. The crystal structure of esterase 6 confirms its substrate-binding site can accommodate many short-mid chain food esters but not cis-vaccenyl acetate. Immunohistochemical assays show esterase 6 is expressed in non-neuronal cells in the third antennal segment that could be accessory or epidermal cells surrounding numerous olfactory sensilla, including basiconics involved in food odorant detection. Esterase 6 is also produced in trichoid sensilla, but not in the same cell types as the cis-vaccenyl acetate binding protein LUSH. Our data support a model in which esterase 6 acts as a direct odorant degrading enzyme for many bioactive food esters, but not cis-vaccenyl acetate.
Highlights
Segment includes sensilla that are responsive to other odorants but not to cVA10, and further biochemical, electrophysiological and behavioral comparisons of the EST6 active and null strains indicate that the enzyme acts on various short chain fatty acid food esters[12,13]
As further evidence for pleiotropic effects of the enzyme, EST6 is known to be expressed at high levels in the male ejaculatory duct, from where it is transferred to the female reproductive tract during mating[14]
Wildtype EST6 was tested for activity against 85 bioactive ester odorants and two model substrates; 4-nitrophenyl acetate (4 NPA) and 2-naphthyl acetate (2 NA)
Summary
Segment includes sensilla that are responsive to other odorants but not to cVA10, and further biochemical, electrophysiological and behavioral comparisons of the EST6 active and null strains indicate that the enzyme acts on various short chain fatty acid food esters[12,13]. As further evidence for pleiotropic effects of the enzyme, EST6 is known to be expressed at high levels in the male ejaculatory duct, from where it is transferred to the female reproductive tract during mating[14] It is rapidly (within minutes) translocated to her hemolymph, where it remains for several days. Comparisons of females mated with null and wildtype EST6 males indicate it acts in the female to stimulate her egg-laying and delay her receptivity to re-mating[15,16] Claims that this effect was mediated by EST6 action on endogenous cVA9 have since been refuted[17], but the substrate responsible for the effect remains unknown. We present data from immunohistochemical and behavioral assays with RNAi knock-down constructs that localize the expression of EST6 to a large proportion of non-neuronal cells surrounding the olfactory neurons of almost all the olfactory sensilla, but in different cells than those producing LUSH in the trichoid sensilla
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