The Spinal Muscular Atrophy disease protein Survival Motor Neuron (SMN) operates as part of a multiprotein complex whose components also include Gemins 2-8 and Unrip. The fruit fly Drosophila melanogaster is thought to have a slightly smaller SMN complex comprised of SMN, Gemin2/3/5 and, possibly, Unrip. Based upon in vivo interaction methods, we have identified novel interacting partners of the Drosophila SMN complex with homologies to Gemin4/6/7/8. The Gemin4 and Gemin8 orthologues are required for neuromuscular function and survival. The Gemin6/7/Unrip module can be recruited via the SMN-associated Gemin8, hence mirroring the human SMN complex architecture. Our findings lead us to propose that an elaborate SMN complex that is typical in metazoans is also present in Drosophila.

Spinal Muscular Atrophy (SMA) is a neuromuscular disorder that results from decreased levels of the survival motor neuron (SMN) protein. SMN is part of a multiprotein complex that also includes Gemins 2–8 and Unrip. The SMN-Gemins complex cooperates with the protein arginine methyltransferase 5 (PRMT5) complex, whose constituents include WD45, PRMT5 and pICln. Both complexes function as molecular chaperones, interacting with and assisting in the assembly of an Sm protein core onto small nuclear RNAs (snRNAs) to generate small nuclear ribonucleoproteins (snRNPs), which are the operating components of the spliceosome. Molecular and structural studies have refined our knowledge of the key events taking place within the crowded environment of cells and the numerous precautions undertaken to ensure the faithful assembly of snRNPs. Nonetheless, it remains unclear whether a loss of chaperoning in snRNP assembly, considered as a “housekeeping” activity, is responsible for the selective neuromuscular phenotype in SMA. This review thus shines light on in vivo studies that point toward disturbances in snRNP assembly and the consequential transcriptome abnormalities as the primary drivers of the progressive neuromuscular degeneration underpinning the disease. Disruption of U1 snRNP or snRNP assembly factors other than SMN induces phenotypes that mirror aspects of SMN deficiency, and splicing defects, described in numerous SMA models, can lead to a DNA damage and stress response that compromises the survival of the motor system. Restoring the correct chaperoning of snRNP assembly is therefore predicted to enhance the benefit of SMA therapeutic modalities based on augmenting SMN expression.