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 alte rations of the survival motor neuron (SMN) gene which encodes a novel prote in of hitherto unclear function, The SMN protein associates with ribonucleo protein particles involved in RNA processing and exhibits an RNA-binding ca pacity, We have isolated the zebrafish Danio rerio and nematode Caenorhabdi tis elegans orthologues and have found that the RNA-binding capacity is con served 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 defi ned an RNA-binding element in exon 2a, which is highly conserved across spe cies, and revealed that its binding activity is modulated by protein domain s encoded by exon 2b and exon 3, Finally, the deleted recombinant zebrafish protein mimicking an SMA frameshift mutation showed a dramatic change in v itro in the formation of the RNA-protein complexes. These observations indi cate that the RNA-binding capacity of SMN is an evolutionarily conserved fu nction and further support the view that defects in RNA metabolism most lik ely account for the pathogenesis of SMA.