Regulation of SMN Protein Stability

Abstract

Spinal muscular atrophy (SMA) is caused by mutations of the survival of motor neuron (SMN1) gene and deficiency of full-length SMN protein (FL-SMN). All SMA patients retain one or more copies of the SMN2 gene, but the principal protein product of SMN2 lacks exon 7 (SMNΔ7) and is unable to compensate for a deficiency of FL-SMN. SMN is known to oligomerize and form a multimeric protein complex; however, the mechanisms regulating stability and degradation of FL-SMN and SMNΔ7 proteins have been largely unexplored. Using pulse-chase analysis, we characterized SMN protein turnover and confirmed that SMN was ubiquitinated and degraded by the ubiquitin proteasome system (UPS). The SMNΔ7 protein had a twofold shorter half-life than FL-SMN in cells despite similar intrinsic rates of turnover by the UPS in a cell-free assay. Mutations that inhibited SMN oligomerization and complex formation reduced the FL-SMN half-life. Furthermore, recruitment of SMN into large macromolecular complexes as well as increased association with several Gemin proteins was regulated in part by protein kinase A. Together, our data indicate that SMN protein stability is modulated by complex formation. Promotion of the SMN complex formation may be an important novel therapeutic strategy for SMA.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://mcb.asm.org/ .

ACKNOWLEDGMENTS

We thank George G. Harmison for expert technical assistance. We thank Liu Yang (University of Arkansas) for the myc-tagged FL-SMN and deletion mutant cDNAs. We thank Peter Wadeson for primary dorsal root ganglion cells.

This study was supported by intramural National Institute of Neurological Disorders and Stroke (NINDS) funds. This study was also supported by a NINDS Competitive Postdoctoral Fellowship (to B.G.B.), a NINDS Career Transition Award (K22-NS0048199-01), and Families of Spinal Muscular Atrophy (to C.J.S.).