Question: 

Wilms' tumor protein, Wt1, is a zinc finger molecule, which is required for normal embryonic development. Mutations of the Wt1 gene can give rise to childhood cancer of the kidneys. Different Wt1 isoforms exist, which function either as transcription factors or have a presumed role in mRNA processing. The aim of this study was to analyze the molecular pathways along which Wt1 protein can undergo the previously reported shuttling between the cell cytoplasm and nucleus.

Methods: 

Interaction of Wt1 protein with various importin a subtypes and importin b was assessed in vitro pull-down assays and co-immunoprecipitation experiments. Nuclear localization signals (NLS) were identified by combining site-directed mutagenesis with subcellular immunodetection of the transfected Wt1 variants.

Results: 

Wt1(+/-KTS) proteins were found to interact with importin a1 and importin binvitro and in living cells in vivo. A NLS that was necessary and sufficient for nuclear import could be mapped to the third Wt1 zinc finger. Mutation of this NLS strongly weakened binding of Wt1 to importins. Furthermore, Wt1 protein was detected for the first time in insulin producing beta cells of the endocrine pancreas. In contrast to most other cell types, which contain Wt1 only in their nuclei, Wt1 was restricted to the cytoplasm of beta cells.

Conclusions: 

Nuclear translocation of the Wilms' tumor protein Wt1 involves importins a and b, and a NLS in the third zinc finger. Our results suggest a novel role of cytoplasmic Wt1 protein in insulin producing beta cells of the pancreas.