Characterization of functional domains of the SMN protein in vivo

The Survival of Motor Neurons (SMN) is the disease gene of spinal muscular atrophy. We have previously established a genetic system based on the chick en pre-B cell line DT40, in which expression of SMN protein is regulated by tetracycline, to study the function of SMN in vivo. Depletion of SMN prote in is lethal to these cells. Here we tested the functionality of mutant SMN proteins by determining their capacity to rescue the cells after depletion of wild-type SMN. Surprisingly, all of the spinal muscular atrophy-associa ted missense mutations tested were able to support cell viability and proli feration. Deletion of the amino acids encoded by exon 7 of the SMN gene res ulted in a partial loss of function. A mutant SMN protein lacking both the tyrosine/glycine repeat (in exon 6) and exon 7 failed to sustain viability, indicating that the C terminus of the protein is critical for SMN activity . Interestingly, the Tudor domain of SMN, encoded by exon 3, does not appea r to be essential for SMN function since a mutant deleted of this domain re stored cell viability. Unexpectedly, a chicken SMN mutant (Delta N39) lacki ng the N-terminal 39 amino acids that encompass the Gemin2-binding domain a lso rescued the lethal phenotype. Moreover, the level of Gemin2 in Delta N3 9-rescued cells was significantly reduced, indicating that Gemin2 is not re quired for Delta N39 to perform the essential function of SMN in DT40 cells . These findings suggest that SMN may perform a novel function in DT40 cell s.