Expression and subcellular localization of two isoforms of the survival motor neuron protein in different cell types

The survival motor neuron (SMN) gene is deleted or mutated in over 98% of spinal muscular atrophy patients who show specific motoneuron loss. By performing transfection experiments with rat smn cDNA, we show that two isoforms of SMN with Mr of 32 kDa and 35 kDa are produced by the same cDNA. In cultured motoneurons, both forms colocalize in coiled bodies and not in GEMS bodies as shown for HeLa cells. Subcellular fractionation of cells acutely dissociated from rat embryonic ventral spinal cord shows that the two SMN isoforms have a different subcellular localization, namely, that the 32 kDa isoform is enriched in the cytosol, whereas the 35 kDa isoform is segregating in the microsomal fraction. We show that the 35 kDa isoform of SMN is part of an insoluble complex but is absent from the cytoplasmic membranes and from the mitochondria. Immunostaining studies show that neither SMN isoform colocalizes with Bcl-2, the mitochondrial antiapoptotic protein suggested to bind to SMN in HeLa cells. Our results show that the isoforms of SMN protein have different subcellular localization and may therefore play independent biological roles. Moreover, the absence of colocalization of SMN with Bcl-2 in motoneurons suggests that some of the interactors of SMN may vary depending on the cell type, and this underscores the importance of identifying motoneuron-specific SMN interactors.