Alternative trans-splicing of Caenorhabditis elegans sma-9/schnurri generates a short transcript that provides tissue-specific function in BMP signaling

We demonstrate here that the shorter transcripts are expressed in vivo. Furthermore, we find that one of the short transcripts plays a tissue-specific role in sma-9 function, contributing to the patterning of male-specific sensory rays, but not to the regulation of body size. Based on previous results, we suggest that this transcript encodes a C-terminal SMA-9 isoform that may provide transcriptional activation activity, while full length isoforms may mediate transcriptional repression and/or activation in a context-dependent manner.The alternative trans-splicing of sma-9 may contribute to the diversity of functions necessary to mediate tissue-specific outputs of BMP signaling.The transforming growth factor β (TGFβ) superfamily comprises a large number of secreted peptide growth factors that have major regulatory effects on cell growth and differentiation [1,2]. Members of this superfamily include the TGFβs, the prototypes of the superfamily; the bone morphogenetic proteins (BMPs); and other members such as activin, inhibin, and Nodal. TGFβ superfamily ligands bind to a heteromeric receptor complex at the cell surface. This complex contains two related transmembrane serine/threonine kinases, the type I and type II receptors [3,4]. Signaling downstream of the receptors is mediated by the Smad proteins, which shuttle between the cytoplasm and nucleus to regulate target gene expression [5]. Type I receptors directly activate receptor-regulated Smads (R-Smads) by phosphorylation at C-terminal SXS sequences [6,7]. Phosphorylation of R-Smads promotes heterotrimeric complex formation with Co-Smads and accumulation in the nucleus to regulate gene transcription [7-11]. In mammals, five R-Smads are present: two (Smad2,3) that transduce TGFβ/activin/Nodal signals and three (Smad1,5,8) that transduce BMP signals. Strikingly, the TGFβ family ligands far outnumber the Smads available for signal transduction. Furthermore, many of these ligands are capable of eliciting diverse cont