Nucleocytoplasmic shuttling of IPAS by its unique nuclear import and export signals unshared with other HIF-3α splice variants.

Inhibitory Per/Arnt/Sim (PAS) domain protein (IPAS) is a splice variant of hypoxia-inducible factor (HIF)-3α, and possesses two entirely different functions. One is as a transcriptional repressor against HIF-dependent hypoxic gene activation. The other is as a pro-apoptotic factor by direct binding to the pro-survival protein Bcl-xL and its related proteins on mitochondria.