The essential gene product Prp45 (379 aa) of Saccharomyces cerevisiae is a highly conserved, but N-terminally abridged, ortholog of the human transcriptional coactivator SKIP, which is involved in TGFbeta, Notch, and steroid hormone signaling. We used a diploid strain harboring PRP45 deletion, which is inviable in the haploid, to test for complementation with the truncated versions of Prp45. The N-terminal half of the protein (aa 1 to 190), denoted as the SNW domain, was found sufficient to support the essential function. Interestingly, substituting the SNW motif itself with AAA was compatible with viability. GFP-tagged Prp45 was localized in nuclear "speckles" over a diffuse nuclear background. We further found that Prp45 activated the transcription of a reporter gene in S. cerevisiae when targeted to DNA. The observed effect relied in part upon the presence of conserved helical repeats and upon the highly charged C-terminal domain (pI = 11.3). Prp45, which lacks most of the binding motifs of the human ortholog, and whose N-terminal half is sufficient for supporting the growth of prp45 cells, might be helpful in elucidating the essential function of SNW/SKIP proteins.

Department of Physiology and Developmental Biology Charles University Vinicna 7 128 00 Praha 2 Czech Republic

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