Role of survival motor neuron complex components in small nuclear ribonucleoprotein assembly

Survival motor neuron (SMN) complex is essential for the biogenesis of the small nuclear ribonucleoprotein (snRNP) complex, although the complete role of each SMN complex component for the snRNP synthesis is largely unclear. We have identified an interaction between the two components Gemin2-Gemin7 using the mammalian two-hybrid system. In vitro stability assay revealed that the known SMN-Gemin7 interaction becomes stable in the presence of Gemin2 possibly via the identified Gemin2-Gemin7 interaction. Gemin7 knockdown revealed a decrease in snRNP assembly activity and a decrease in SmE protein, a component of snRNP, in the SMN complex, which was consistent with a previous discussion that the Gemin6-Gemin7 heterodimer may serve as a surrogate for the SmD3-SmB particle in forming a subcore, the intermediate complex for snRNP. Interestingly, we found that Unrip, but not Gemin8, can remove Gemin7 from the stable SMN-Gemin2-Gemin7 ternary complex. In an in vitro snRNP assembly assay using the Unrip knockdown and the untreated cell lysates, we revealed that there was a decrease in Gemin7 and increase in SmB/B' in the SMN complex observed in untreated cells during the assay, suggesting that the Gemin6-Gemin7 heterodimer in the subcore is exchanged by the SmD3-SmB particle to form snRNP. Surprisingly, these changes were not observed in the assay using the Unrip knockdown cell extracts, indicating the importance of Unrip in the formation of snRNP likely via removal of the Gemin6-Gemin7 from the SMN complex. Taken together, these results indicate that snRNP is synthesized by harmonization of the SMN complex components.