Time-resolved quantitative proteomics implicates the core snRNP protein SmB together with SMN in neural trafficking.

The biogenesis of splicing snRNPs (small nuclear ribonucleoproteins) is a complex process, beginning and ending in the nucleus of the cell but including key stages that take place in the cytoplasm. In particular, the SMN (survival motor neuron) protein complex is required for addition of the core Sm proteins to the snRNP. Insufficiency of SMN results in the inherited neurodegenerative condition, spinal muscular atrophy (SMA). Details of the physical organization of the cytoplasmic stages of snRNP biogenesis are unknown. Here, we use time-resolved quantitative proteomics to identify proteins that associate preferentially with either newly assembled or mature splicing snRNPs. We identified highly mobile SmB protein-trafficking vesicles in neural cells, which are dependent on the cellular levels of SMN and SmB for their morphology and mobility. We propose that these represent a family of related vesicles, some of which play a role in snRNP biogenesis and some that might play more diverse roles in cellular RNA metabolism.

Changes in intranuclear mobility of mature snRNPs provide a mechanism for splicing defects in spinal muscular atrophy.

It is becoming increasingly clear that defects in RNA metabolism can lead to disease. Spinal muscular atrophy (SMA), a leading genetic cause of infant mortality, results from insufficient amounts of survival motor neuron (SMN) protein. SMN is required for the biogenesis of small nuclear ribonucleoproteins (snRNPs): essential components of the spliceosome. Splicing abnormalities have been detected in models of SMA but it is unclear how lowered SMN affects the fidelity of pre-mRNA splicing. We have examined the dynamics of mature snRNPs in cells depleted of SMN and demonstrated that SMN depletion increases the mobility of mature snRNPs within the nucleus. To dissect the molecular mechanism by which SMN deficiency affects intranuclear snRNP mobility, we employed a panel of inhibitors of different stages of pre-mRNA processing. This in vivo modelling demonstrates that snRNP mobility is altered directly as a result of impaired snRNP maturation. Current models of nuclear dynamics predict that subnuclear structures, including the spliceosome, form by self-organization mediated by stochastic interactions between their molecular components. Thus, alteration of the intranuclear mobility of snRNPs provides a molecular mechanism for splicing defects in SMA.