The p53-induced factor Ei24 inhibits nuclear import through an importin β-binding-like domain

The etoposide-induced protein E124 was initially identified as a p53-responsive, proapoptotic factor, but no clear function has been described. Here, we use a nonbiased. proteomics approach to identify members of the importin (IMP) family of nuclear transporters as interactors of Ei24 and characterize an IMP beta-binding-like (IBBL) domain within Ei24. We show that Ei24 can bind specifically to IMP beta 1 and IMP alpha 2, but not other IMPs, and use a mutated IMP beta l derivative to show that Ei24 binds to the same site on IMP beta 1 as the IMP alpha. IBB. Ectopic expression of Ei24 reduced the extent of IMPplor IMP alpha/beta 1-dependent nuclear protein import specifically, whereas specific alanine substitutions within the IBBL abrogated this activity. Induction of endogenous Ei24 expression through etoposide treatment similarly inhibited nuclear import in a mouse embryonic fibroblast model. Thus, Ei24 can bind specifically to IMP beta 1 and IMP alpha 2 to impede their normal role in nuclear import, shedding new light on the cellular functions of EN and its tumor suppressor role.