title: Release of ecdysteroid-phosphates from egg yolk granules and their dephosphorylation during early embryonic development in silkworm, Bombyx mori.
abstract: Newly laid eggs of many insect species store maternal ecdysteroids as physiologically inactive phosphoric esters. In the silkworm Bombyx mori, we previously reported the presence of a specific enzyme, called ecdysteroid-phosphate phosphatase (EPPase), which catalyzes the dephosphorylation of ecdysteroid-phosphates to increase the amount of free ecdysteroids during early embryonic development. In this study, we demonstrated that (1) EPPase is found in the cytosol of yolk cells, (2) ecdysteroid-phosphates are localized in yolk granules, being bound to the yolk protein vitellin (Vn), and (3) Vn-bound ecdysteroid-phosphates are scarcely hydrolyzed by EPPase, although free ecdysteroid-phosphates are completely hydrolyzed by EPPase. Thus, we investigated the mechanism by which ecdysteroid-phosphates dissociate from the Vn-ecdysteroid-phosphate complex, and indicated that the acidification of yolk granules causes the dissociation of ecdysteroid-phosphates from the Vn-ecdysteroid-phosphate complex and thereby ecdysteroid-phosphates are released from yolk granules into the cytosol. Indeed, the presence of vacuolar-type proton-translocating ATPase in the membrane fraction of yolk granules was also verified by Western blot analysis. Our experiments revealed that Vn functions as a reservoir of maternal ovarian ecdysteroid-phosphates as well as a nutritional source during embryonic development. This is the first report showing the biochemical mechanism by which maternal Vn-bound ecdysteroid-phosphates function during early embryonic development.
authors.affiliation: Department of Life and Functional Material Science, Graduate School of Natural Sciences, Konan University, Kobe, Japan.
authors.forename: Ryouichi
authors.initials: R
authors.lastname: Yamada
authors.forename: Yumi
authors.initials: Y
authors.lastname: Yamahama
authors.forename: Haruyuki
authors.initials: H
authors.lastname: Sonobe
mesh_terms: D000818:Animals; D015153:Blotting, Western; D012831:Bombyx / Q000196:embryology* / Q000378:metabolism*; D002850:Chromatography, Gel; D002852:Chromatography, Ion Exchange; D026461:Ecdysteroids / Q000378:metabolism*; D004527:Egg Proteins / Q000378:metabolism*; D004530:Egg Yolk / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism*; D025262:Vacuolar Proton-Translocating ATPases / Q000378:metabolism; D047048:Vitellins / Q000378:metabolism
chemical_list: D026461:Ecdysteroids; D004527:Egg Proteins; D047048:Vitellins; C476613:ecdysteroid-phosphate phosphatase; D010744:Phosphoric Monoester Hydrolases; D025262:Vacuolar Proton-Translocating ATPases