Importin alpha1 is involved in the nuclear localization of Zac1 and the induction of p21WAF1/CIP1 by Zac1.

Zac1, a novel seven-zinc-finger transcription factor, preferentially binds GC-rich DNA elements and has intrinsic transactivation activity. To date, the NLS (nuclear localization signal) of Zac1 has not been empirically determined. We generated a series of EGFP (enhanced green fluorescence protein)-tagged deletion mutants of Zac1 and examined their subcellular localization, from which we defined two NLSs within the DNA-binding (or zinc-finger) domain. Fusion proteins consisting of the two EGFP-tagged zinc-finger clusters (zinc finger motifs 1-3 and 4-7) were located exclusively in the nucleus, demonstrating that each of the zinc-finger clusters is sufficient for nuclear localization. Physical interactions between these two zinc-finger clusters and importin alpha1 were demonstrated using an in vitro glutathione S-transferase pull-down assay. Finally, our results indicate that the association of Zac1 with importin alpha1 is also involved in regulating the transactivation activity of Zac1 on the p21WAF1/CIP1 gene and protein expression.