The research team of Rundong Zhang, a research fellow at Department of Ophthalmology / State Key Laboratory of Biotherapy, West China Hospital, Sichuan University, has published a research paper titled “Negative Cooperativity Between Gemin2 and RNA Provides Insights into RNA Selection and the SMN Complex's Release in snRNP Assembly” in Nucleic Acids Research ( IF=11.147 ). Rundong Zhang is the corresponding author. Doctoral students Hongfei Yi and Li Mu are the first authors.

Spliceosomes are the main molecular machines that cut out the non-coding regions of genes and splice the coding regions together in eukaryotic cells. Small nuclear ribonucleoprotein complexes ( snRNPs ) are the main components of spliceosomes, including U1, U2, U4 and U5 snRNP. “The assembly of snRNP cores, in which seven Sm proteins, D1/D2/F/E/G/D3/B, form a ring around the nonameric Sm site of snRNAs, is the early step of spliceosome formation and essential to eukaryotes. It is mediated by the PMRT5 and SMN complexes sequentially in vivo. SMN deficiency causes neurodegenerative disease spinal muscular atrophy ( SMA ). How the SMN complex assembles snRNP cores is largely unknown, especially how the SMN complex achieves high RNA assembly specificity and how it is released. Here we show, using crystallographic and biochemical approaches, that Gemin2 of the SMN complex enhances RNA specificity of SmD1/D2/F/E/G via a negative cooperativity between Gemin2 and RNA in binding SmD1/D2/F/E/G. Gemin2, independent of its N-tail, constrains the horseshoe-shaped SmD1/D2/F/E/G from outside in a physiologically relevant, narrow state, enabling high RNA specificity. Moreover, the assembly of RNAs inside widens SmD1/D2/F/E/G, causes the release of Gemin2/SMN allosterically and allows SmD3/B to join. The assembly of SmD3/B further facilitates the release of Gemin2/SMN. This is the first to show negative cooperativity in snRNP assembly, which provides insights into RNA selection and the SMN complex's release.” ( Abstract ) In addition, how SMN complex dissociates after helping snRNP core assembly has always been an unsolved mystery in this field.

Using crystallographic and biochemical approaches, Rundong Zhang's team analyzed the whole process of SMN / Gemin 2 combining SmD1/D2/F/E/G to form the assembly intermediate to snRNP core, and found a new mechanism for SMN complex to assemble snRNP core. “The bindings of Gemin2 and U4 snRNA to 5Sm are compatible sterically and the schematic model of Sm core assembly in vivo. Superposition of SmE ( A ) or SmD2 ( B ) of the 7SΔN complex with that of U4 snRNP core ( PDB code 4WZJ ) ( 40) reveals that Gemin2 is compatible with U4 snRNA in binding to 5Sm spatially, indicating allosteric, not competitive inhibition between Gemin2 and U4 snRNA in binding to 5Sm. The five Sm proteins in the 7SΔN complex are colored in cyan, and in the U4 snRNP core in grey. SMN_Ge2BD and Gemin2 are colored in blue and red respectively. U4 snRNA is colored in orange. ( C ) The mechanisms of Sm core assembly. The steps  (2 ) – ( 6 ) are the new mechanisms discovered in this study. The step ( 1 ) is from literature ( 12,15 ).” ( Figure 6 in the paper )

This mechanism can be applied to the core assembly of spliceosome snRNP in all eukaryotic cells. It should also be applied to the core assembly of U7 snRNP. Moreover,the elucidation of this mechanism shed light to the pathogenesis studies of spinal muscular atrophy ( SMA ).

Article link: https://academic.oup.com/nar/advance-article/doi/10.1093/nar/gkz1135/5651324