Deciphering the assembly pathway of Sm‐class U snRNPs

The assembly of the Sm‐class of uridine‐rich small nuclear ribonucleoproteins (U snRNPs), albeit spontaneous in vitro, has recently been shown to be dependent on the aid of a large number of assisting factors in vivo. These factors are organized in two interacting units termed survival motor neuron (SMN)‐ and protein arginine methyltransferase 5 (PRMT5)‐complexes, respectively. While the PRMT5‐complex acts early in the assembly pathway by activating common proteins of U snRNPs, the SMN‐complex functions to join proteins and RNA in a highly ordered, apparently regulated manner. Here, we summarize recent progress in the understanding of this process and discuss the influence exerted by the aforementioned trans‐acting factors