Fussel-15, a novel Ski/Sno homolog protein, antagonizes BMP signaling.

The Ski family of nuclear oncoproteins represses transforming growth factor-beta (TGF-beta) signaling through inhibition of transcriptional activity of Smad proteins. In this study, we identified a novel gene, fussel-15 (functional smad suppressing element on chromosome 15) with high homology to the recently discovered Fussel-18 protein. Both, Fussel-15 and Fussel-18, share important structural features, significant homology and similar genomic organization with the homolog Ski family members, Ski and SnoN. Unlike Ski and SnoN, which are ubiquitously expressed in human tissues, Fussel-15 expression, like Fussel-18, is much more restricted in its expression and is principally found in the nervous system of mouse and humans. Interestingly, Fussel-15 expression is even more restricted in adulthood to Purkinje cells of human cerebellum. In contrast to Fussel-18 that interacts with Smad 2, Smad3 and Smad4 and has an inhibitory activity on TGF-beta signaling, Fussel-15 interacts with Smad1, Smad2 and Smad3 molecules and suppresses mainly BMP signaling pathway but has only minor effects on TGF-beta signaling. This new protein expands the family of Ski/Sno proto-oncoproteins and represents a novel molecular regulator of BMP signaling.