Importin α Protein Acts as a Negative Regulator for Snail Protein Nuclear Import

Snail, a zinc finger-containing transcriptional regulator, migrates into the nucleus where it controls gene expression. We demonstrated previously that importin beta 1 directly recognizes the zinc finger domain of Snail and transports it into the nucleus. Here, using in vitro and in vivo assays, we show that importin alpha, an adaptor protein for importin beta 1, negatively regulates the nuclear import of Snail mediated by importin beta 1. In vitro binding assays indicated that importin alpha interacted with the zinc finger domain of Snail to compete with the binding of importin beta 1 and that Snail did not form a ternary complex with importin alpha/importin beta 1. Overexpression of importin alpha in A549 cells reduced the endogenous Snail protein level, which was restored by inhibitors of the proteasome and glycogen synthase kinase 3 beta. Furthermore, knockdown of importin alpha by siRNA treatment increased the endogenous Snail protein level in several cancer cell lines. This study provides a novel regulatory mechanism of the nuclear protein import process by importin alpha and gives an implication to control Snail activity by inhibiting its nuclear localization.