The product of an oculopharyngeal muscular dystrophy gene, poly(A)-binding protein 2, interacts with SKIP and stimulates muscle-specific gene expression

Oculopharyngeal muscular dystrophy (OPMD) is caused by short expansions of the GCG trinucleotide repeat encoding the polyalanine tract of the poly(A)-binding protein 2 (PABP2). PABP2 binds to the growing poly(A) tail, stimulating its extension during the polyadenylation process, and limits the length of the newly synthesized poly(A) tail. Whereas PABP2 is expressed ubiquitously, the clinical and pathological features of OPMD patients are restricted to the skeletal muscle. To elucidate the possible role of PABP2 in skeletal muscle, we established the stable C2 cell lines expressing human PABP2. These stable cell lines showed morphologically enhanced myotube formation accompanied by an increased expression of myogenic factors, MyoD and myogenin. In nuclear run-on assay, the transcription rate of the MyoD gene was significantly increased by PABP2 transfection. We found the N-terminal region of PABP2 was responsible for the up-regulation of these myogenic factors. Furthermore, Ski-interacting protein (SKIP) was isolated as a binding protein for PABP2 using the yeast two-hybrid system. The interaction of PABP2 and SKIP was confirmed by glutathione S-transferase-pulldown assay and immunoprecipitation. Confocal laser scanning showed PABP2 was co-localized with SKIP in nuclear speckles. The reporter assays showed that PABP2 co-operated with SKIP to synergistically activate E-box-mediated transcription through MYOD: Moreover, both PABP2 and SKIP were directly associated with MyoD to form a single complex. These findings suggest that PABP2 and SKIP directly control the expression of muscle-specific genes at the transcription level.