A sequence motif conserved in diverse nuclear proteins identifies a protein interaction domain utilised for nuclear targeting by human TFIIS.

The three structural domains of transcription elongation factor TFIIS are conserved from yeast to human. Although the N-terminal domain is not needed for transcriptional activity, a similar sequence has been identified previously in other transcription factors. We found this conserved sequence, the LW motif, in another three human proteins that are predominantly nuclear localized. We investigated two examples to determine whether the LW motif is actually a dedicated nuclear targeting signal. However, in one of the newly identified proteins, hIWS1 (human Iws1), a region containing classic nuclear localization signals (NLS) rather than the LW motif was necessary and sufficient for nuclear targeting in HeLa cells. In contrast, human TFIIS does not possess an NLS and only constructs containing the LW motif were efficiently targeted to nuclei. Moreover, mutations in the motif could cause cytoplasmic accumulation of TFIIS and enabled a structure/function assay for the domain based on the efficiency of nuclear targeting. Finally, GST pull-down assays showed that the LW motif is part of a protein-binding domain. We suggest that the targeting role the LW motif plays in TFIIS arises from its more general function as a protein interaction domain, enabling TFIIS to bind a carrier protein(s) that accomplishes nuclear import.