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The RNA-binding properties of SMN: deletion analysis of the zebrafish orthologue defines domains conserved in evolution.

Spinal muscular atrophy (SMA) is a common autosomal recessive disorder that results in the degeneration of spinal motor neurons. SMA is caused by alterations of the survival motor neuron ( SMN ) gene which encodes a novel protein of hitherto unclear function. The SMN protein associates with ribonucleoprotein particles involved in RNA processing and exhibits an RNA-binding capacity. We have isolated the zebrafish Danio rerio and nematode Caenorhabditis elegans orthologues and have found that the RNA-binding capacity is conserved across species. Purified recombinant SMN proteins from both species showed selectivity to poly(G) homopolymer RNA in vitro, similar to that of the human protein. Studying deletions of the zebrafish SMN protein, we defined an RNA-binding element in exon 2a, which is highly conserved across species, and revealed that its binding activity is modulated by protein domains encoded by exon 2b and exon 3. Finally, the deleted recombinant zebrafish protein mimicking an SMA frameshift mutation showed a dramatic change in vitro in the formation of the RNA-protein complexes. These observations indicate that the RNA-binding capacity of SMN is an evolutionarily conserved function and further support the view that defects in RNA metabolism most likely account for the pathogenesis of SMA.