Suppression of the ecdysteroid-triggered growth arrest by a novel Drosophila membrane steroid binding protein

Ecdysteroid is a crucial steroid hormone in insects, especially during metamorphosis. Here, we show that the Drosophila membrane steroid binding protein (Dm_MSBP) is a novel structural homolog of the vertebrate membrane-bound receptor component for progesterone. Dm_MSBP exhibited binding affinity to ecdysone when expressed on the cell surface of Drosophila S2 cells. In S2 cells, the stable overexpression of Dm_MSBP suppressed the growth arrest triggered by 20-hydroxyecdysone and prevented the temporal activation of extracellular signal-regulated kinase proteins. These results suggest that Dm_MSBP is a membranous suppressor to ecdysteroid and blocks the signaling by binding it in extracellular fluid.