FRG1P-mediated aggregation of proteins involved in pre-mRNA processing.

FRG1 is considered a candidate gene for facioscapulohumeral muscular dystrophy (FSHD) based on its location at chromosome 4qter and its upregulation in FSHD muscle. The FRG1 protein (FRG1P) localizes to nucleoli, Cajal bodies (and speckles), and has been suggested to be a component of the human spliceosome but its exact function is unknown. Recently, transgenic mice overexpressing high levels of FRG1P in skeletal muscle were described to present with muscular dystrophy. Moreover, upregulation of FRG1P was demonstrated to correlate with missplicing of specific pre-mRNAs. In this study, we have combined colocalization studies with yeast two-hybrid screens to identify proteins that associate with FRG1P. We demonstrate that artificially induced nucleolar aggregates of VSV-FRG1P specifically sequester proteins involved in pre-mRNA processing. In addition, we have identified SMN, PABPN1, and FAM71B, a novel speckle and Cajal body protein, as binding partners of FRG1P. All these proteins are, or seem to be, involved in RNA biogenesis. Our data confirm the presence of FRG1P in protein complexes containing human spliceosomes and support a potential role of FRG1P in either splicing or another step in nuclear RNA biogenesis. Intriguingly, among FRG1P-associated proteins are SMN and PABPN1, both being involved in neuromuscular disorders, possibly through RNA biogenesis-related processes.