Molecular mechanism of coilin interaction with core snRNPs

Abstract Cajal bodies (CBs) are nuclear membrane-less organelles that accumulate various short non-coding RNAs and facilitate their biogenesis. They also function in quality control during the assembly of small nuclear ribonucleoprotein particles (snRNPs), sequestering immature or defective complexes. In this paper, we show that coilin, the key scaffolding protein of CBs, is the factor that discriminates between mature and immature snRNPs. We provide evidence that the C-terminus of coilin contains a bipartite snRNP-interaction module composed of a nonspecific RNA-binding region formed by RG repeats and a Tudor-like domain that interacts specifically with Sm proteins. The Tudor-like domain contains two conserved loops that protrude from the core barrel and bind the Sm proteins E, F, and G. Both the RNA-binding and Sm-binding regions are essential for productive interactions between coilin and core snRNPs, providing a molecular explanation for the specificity of coilin-mediated sequestration of immature snRNPs in Cajal bodies.