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 s pinal muscular atrophy patients who show specific motoneuron loss. By perfo rming 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 cult ured motoneurons, both forms colocalize in coiled bodies and not in GEMS bo dies as shown for Hela cells. Subcellular fractionation of cells acutely di ssociated from rat embryonic ventral spinal cord shows that the two SMN iso forms have a different subcellular localization, namely, that the 32 kDa is oform 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 f rom the mitochondria. Immunostaining studies show that neither SMN isoform colocalizes with Bcl-2, the mitochondrial antiapoptotic protein suggested t o bind to SMN in HeLa cells. Our results show that the isoforms of SMN prot ein have different subcellular localization and may therefore play independ ent biological roles, Moreover, the absence of colocalization of SMN with B cl-2 in motoneurons suggests that some of the interactors of SMN may vary d epending on the cell type, and this underscores the importance of identifyi ng motoneuron-specific SMN interactors. J. Neurosci. Res. 62:346-356, 2000. (C) 2000 Wiley-Liss, Inc.