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.

The predominant motor neuron disease in infants and adults is spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS), respectively. SMA is caused by insufficient levels of the Survival Motor Neuron (SMN) protein, which operates as part of the multiprotein SMN complex that includes the DEAD-box RNA helicase Gemin3/DDX20/DP103. C9orf72, SOD1, TDP-43 and FUS are ranked as the four major genes causing familial ALS. Accumulating evidence has revealed a surprising molecular overlap between SMA and ALS. Here, we ask the question of whether Drosophila can also be exploited to study shared pathogenic pathways. Focusing on motor behaviour, muscle mass and survival, we show that disruption of either TBPH/TDP-43 or Caz/FUS enhance defects associated with Gemin3 loss-of-function. Gemin3-associated neuromuscular junction overgrowth was however suppressed. Sod1 depletion had a modifying effect in late adulthood. We also show that Gemin3 self-interacts and Gem3ΔN, a helicase domain deletion mutant, retains the ability to interact with its wild-type counterpart. Importantly, mutant:wild-type dimers are favoured more than wild-type:wild-type dimers. In addition to reinforcing the link between SMA and ALS, further exploration of mechanistic overlaps is now possible in a genetically tractable model organism. Notably, Gemin3 can be elevated to a candidate for modifying motor neuron degeneration.

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