Wilhelm bernhard workshop on the cell nucleus


THE MULTIFUNCTIONAL TUMOUR SUPPRESSOR ZINC FINGER PROTEIN WT1 BINDS TO TRANSCRIPTS IN VIVO



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THE MULTIFUNCTIONAL TUMOUR SUPPRESSOR ZINC FINGER PROTEIN WT1 BINDS TO TRANSCRIPTS IN VIVO

Ladomery M.1, Hastie N.1, Woolner S.1, Slight J.1 and Sommerville J.2

1MRC Human Genetics Unit, Western General Hospital, Edinburgh, Scotland, UK; 2School of Biology, Bute Medical Buildings, University of St Andrews, St Andrews, Scotland, UK

The Wilms tumour suppressor gene WT1 encodes a protein involved in urogenital development and disease. The salient feature of WT1 is the presence of four ‘Krüppel’ type C2-H2 zinc fingers in the C-terminus. Uniquely to WT1, an evolutionarily conserved alternative splicing event inserts three amino-acids (KTS) between the third and fourth zinc fingers which disrupts DNA binding. The ratio of +/-KTS isoforms is crucial for normal development. Previous work has shown that WT1 (+KTS) interacts with splice factors, and that WT1 zinc fingers (+/-KTS), particularly zinc finger one, bind to RNA in vitro. In this study we investigate the role of zinc finger one and the +KTS splice in vivo by expressing tagged proteins in mammalian cells and Xenopus oocytes. We find that both full length +/-KTS isoforms and deletion constructs that include zinc finger one co-sediment with RNP on density gradients. In Xenopus oocytes both isoforms located to the lateral loops of lampbrush chromosomes. Strikingly, only the +KTS isoform was detected in B-snurposomes, but not if co-expressed with WT1 (-KTS). However co-expression of the C-terminus (amino-acids 233-449, +KTS) resulted in snurposome staining, consistent with an in vivo interaction between isoforms via the N-terminus. Expressed WT1 was also detected in the RNA-rich granular component of nucleoli, and co-immunoprecipitated with oocyte transcripts. Full length WT1 was most stably bound to transcripts, followed by the C-terminus; whereas the least stably bound was CTF1 (C-terminus minus zinc finger one). Expression of the transcription factor EGR1, whose three zinc fingers correspond to WT1 zinc fingers 2-4, caused general chromosomal loop retraction and transcriptional shut-down. However a construct in which WT1 zinc finger one was added to EGR1 now reflected the properties of WT1 (-KTS). We suggest that in evolution, WT1 has acquired the ability to interact with transcripts and splice factors due to the modification of zinc finger one and the +KTS alternative splice.



References:

Ladomery et al., 2003, J Cell Sci. 116, pp. 1539-1549.



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