Evolution of an RNP assembly system: A minimal SMN‐complex facilitates formation of U snRNPs in Drosophila melanogaster

Abstract

In vertebrates, assembly of spliceosomal UsnRNPs is mediated by the SMN‐complex, a macromolecular entity composed of the proteins SMN and Gemins 2–8. Here we have studied the evolution of this machinery using complete genome assemblies of multiple model organisms. The SMN‐complex has gained complexity in evolution by a block‐wise addition of Gemins onto an ancestral core complex composed of SMN and Gemin2. In contrast to this overall evolutionary trend to more complexity in metazoans, orthologs of most Gemins are missing in dipterans. In accordance with these bioinformatic data a novel biochemical purification strategy elucidated that the dipteran Drosophila melanogaster contains a SMN‐complex of remarkable simplicity. Surprisingly, this minimal complex not only mediates the assembly reaction in a manner very similar to its vertebrate counterpart, but also prevents mis‐assembly onto non‐target RNAs. Our data suggest that only a minority of Gemins is required for the assembly reaction per se, whereas others may serve additional functions in the context of U snRNP biogenesis. The evolution of the SMN‐complex is an interesting example of how the simplification of a biochemical process contributes to genome compaction.