A role for protein phosphatase PP1γ in SMN complex formation and subnuclear localization to Cajal bodies

The spinal muscular atrophy (SMA) gene product SMN forms with gem-associated protein 2-8 (Gemin2-8) and unrip (also known as STRAP) the ubiquitous survival motor neuron (SMN) complex, which is required for the assembly of spliceosomal small nuclear ribonucleoproteins (snRNPs), their nuclear import and their localization to subnuclear domain Cajal bodies (CBs). The concentration of the SMN complex and snRNPs in CBs is reduced upon SMN deficiency in SMA cells. Subcellular localization of the SMN complex is regulated in a phosphorylation-dependent manner and the precise mechanisms remain poorly understood. Using co-immunoprecipitation in HeLa cell extracts and in vitro protein binding assays, we show here that the SMN complex and its component Gemin8 interact directly with protein phosphatase PP1 gamma. Overexpression of Gemin8 in cells increases the number of CBs and results in targeting of PP1 gamma to CBs. Moreover, depletion of PP1 gamma by RNA interference enhances the localization of the SMN complex and snRNPs to CBs. Consequently, the interaction between SMN and Gemin8 increases in cytoplasmic and nuclear extracts of PP1 gamma-depleted cells. Two-dimensional protein gel electrophoresis revealed that SMN is hyperphosphorylated in nuclear extracts of PP1 gamma-depleted cells and expression of PP1 gamma restores these isoforms. Notably, SMN deficiency in SMA leads to the aberrant subcellular localization of Gemin8 and PP1 gamma in the atrophic skeletal muscles, suggesting that the function of PP1 gamma is likely to be affected in disease. Our findings reveal a role of PP1 gamma in the formation of the SMN complex and the maintenance of CB integrity. Finally, we propose Gemin8 interaction with PP1 gamma as a target for therapeutic intervention in SMA.