The Drosophila HIFĄ homologue Sima is regulated by CRM1-dependent nuclear export

ROMERO NM; PABLO WAPPNER

Vancouver. CANADA

Simposio; Keystone Symposia on Molecular and Cellular Biology; 2008

Compartmentalization within eukaryotic cells offers another control mechanism over the elaborate array of signaling networks. The nucleocytoplasmic shuttling of transcription factors is a mechanism mediating the activation and inactivation of many transcriptional responses. A well understood mechanism of nuclear export is the one dependent on  nuclear export signals (NESs) associate with the export receptor CRM-1. In this study we used the Drosophila system to investigate the mechanism controlling Sima/HIF-a subcellular localization. It was previously reported that HIFa proteins are mainly nuclear in hypoxia and cytoplasmic in normoxia but so far, the molecular basis of this regulation is unclear. We show here that the Drosophila HIF-a protein Sima shuttles continuously between the nucleus and the cytoplasm. We identified two nuclear export signals (NESs) in the Sima bHLH domain that promote CRM1-dependent nuclear export, both in cell culture and in vivo. Site directed mutagenesis of either NES provoked Sima nuclear retention and increased its transcriptional activity, suggesting that nuclear export contributes to Sima regulation. The identified NESs are conserved and functional in the bHLH domains of several bHLH-PAS proteins. We propose that rapid nuclear export of Sima upon reoxygenation regulates the duration of cellular responses to hypoxia.