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 (SMN⌬7) 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 SMN⌬7 proteins have been largely unexplored. Using pulsechase analysis, we characterized SMN protein turnover and confirmed that SMN was ubiquitinated and degraded by the ubiquitin proteasome system (UPS). The SMN⌬7 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.

Baseline results of the NeuroNEXT spinal muscular atrophy infant biomarker study

“…In humans, SMN2 is present in the same genomic region and differs from SMN1 by a single‐nucleotide substitution that results in the exclusion of exon 7 in approximately 90% of SMN transcripts 5, 6. The mRNA that results, SMNΔ7, produces a truncated protein that is nonfunctional and targeted for degradation 7, 8…”

SMN deficiency in severe models of spinal muscular atrophy causes widespread intron retention and DNA damage

“…The two paralogs are nearly identical but differ by several key nucleotides within and flanking exon 7 (4). As a result, transcripts arising from the SMN2 locus predominantly splice out exon 7, producing a protein that is rapidly degraded and thought to be nonfunctional; however, a small fraction of transcripts include exon 7 and encode a protein identical to SMN1 (5). Importantly, SMN2 copy number varies across individuals and is a modifier of disease severity, with a higher copy number leading to reduced severity.…”

Identifying Splicing Regulatory Elements with de Bruijn Graphs

“…SMN stability is affected by its ability to oligomerize. Therefore, SMN mutations that prevent oligomerization lead to rapid degradation, and this may be the reason that it causes SMA (Burnett et al, 2008). It is also worth mentioning that the SMN protein is part of a large multiprotein complex (the SMN complex), which is essential for the biogenesis of small nuclear ribonucleoprotein particles (snRNPs).…”

“…It has been shown that ubiquitin has a controlling role in the splicing pathway and hence affects spliceosome assembly (Bellare et al, 2008). Moreover, according to Burnett et al (2008), ubiquitin influences the stability and degradation of the SMN protein. In humans, SMN is encoded by two genes, SMN1 and SMN2.…”

“…This attachment can signal protein degradation (Burnett et al, 2008). It has been shown that ubiquitin has a controlling role in the splicing pathway and hence affects spliceosome assembly (Bellare et al, 2008).…”