An exon junction complex-independent function of Barentsz in neuromuscular synapse growth

The exon junction complex controls the translation, degradation and localization of spliced mRNAs, and three of its four core subunits also play a role in splicing. Here we show that the fourth subunit, Barentsz, has distinct biological functions within and separate from the exon junction complex in neuromuscular development. Barentsz controls the distribution of mitochondria in larval muscles, a function that also depends on other subunits of the exon junction complex and that is not rescued by a transgene in which residues required for binding to the core subunit eIF4AIII are mutated. In contrast, interactions with the exon junction complex are not required for Barentsz to promote the growth of neuromuscular synapses. We found that the Activin ligand Dawdle shows reduced expression in barentsz mutants and acts downstream of Barentsz to control synapse growth. Both barentsz and dawdle are required in motor neurons, muscles and glia for normal synapse growth, and exogenous Dawdle can rescue synapse growth in the absence of barentsz. These results identify a biological function for Barentsz that is independent of the exon junction complex.