Transcriptional coregulator SNW/SKIP: the concealed tie of dissimilar pathways

Eukaryotic gene expression requires that all the steps of messenger RNA production are regulated in concert to integrate the diverse inputs cells receive. We discuss the functioning of SNW/SKIP, an essential spliceosomal component and transcriptional coregulator, which may provide regulatory coupling of transcription initiation and splicing. SNW/SKIP potentiates the activity of important transcription factors, such as vitamin D receptor, CBF1 (RBP-Jkappa), Smad2/3, and MyoD. It synergizes with Ski in overcoming pRb-mediated cell cycle arrest, and it is targeted by the viral transactivators EBNA2 and E7. SNW/SKIP may aid in conformational transition of the gene expression machine through its avidity to nuclear matrix fractions or by recruiting foldases such as the prolyl isomerase PPIL1. The extensive list of SNW/SKIP partners, its unique primary structure, conserved from yeast to humans, and its essential character suggest a distinct function of general importance.

Cyclophilins of a novel subfamily interact with SNW/SKIP coregulator in Dictyostelium discoideum and Schizosaccharomyces pombe

We screened the Dictyostelium discoideum two-hybrid cDNA library with the SNW/SKIP transcription coregulator SnwA and identified a novel cyclophilin CypE. Independently, the Schizosaccharomyces pombe cDNA library was screened with the SnwA ortholog Snw1 and the ortholog of CypE (named Cyp2) was found. Both cyclophilins bind the respective SNW protein in their autologous systems. The interaction was localized to the N-terminal part of SnwA as well as of Snw1. CypE was confirmed in vitro to be a cyclosporin A-sensitive peptidyl-prolyl cis-trans isomerase. Remarkably, both SNW proteins bind the cyclophilins in a cyclosporin A independent manner, possibly serving as adaptors for these novel isomerases. These results are the first characterization of the members of a novel cyclophilin subfamily, which includes the human CGI-124/PPIL1 protein.

The fission yeast ortholog of the coregulator SKIP interacts with the small subunit of U2AF

The mode of action of transcriptional coregulators may involve the recruitment of spliceosome components. Using the two-hybrid screen, we examined the interaction partners of spSNW1, the S. pombe ortholog of the human coregulator SNW1/SKIP/NCoA-62, and found it to interact with the small subunit of the splicing factor U2AF (spU2AF23). The interaction involves the C-terminal parts of spU2AF23 and spSNW1. Tagged variants of both proteins were expressed in S. pombe and the interaction was proved by coprecipitation in nuclear extracts. This interaction would explain the finding of SKIP in nuclear speckles (Mintz, P. J., et al., EMBO J. 18, 4308-4320, 1999) and in reconstituted spliceosomes (Neubauer, G., et al., Nat. Genet. 20, 46-50, 1998). We deleted the spSNW1 gene in the diploid strain and demonstrated that spSNW1 is an essential gene in S. pombe.