Modification of Sm small nuclear RNAs occurs in the nucleoplasmic Cajal body following import from the cytoplasm

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Abstract
Biogenesis of functional spliceosomal small nuclear RNAs (snRNAs) includes the post‐transcriptional covalent modification of numerous internal nucleotides. We have recently demonstrated that synthesis of 2′‐ O ‐methylated nucleotides and pseudouridines in the RNA polymerase II‐synthesized Sm snRNAs is directed by sequence‐specific guide RNAs. Here, we provide evidence supporting the notion that modification of Sm snRNAs occurs in nucleoplasmic Cajal bodies (CBs), where modification guide RNAs accumulate. We show that short fragments of Sm snRNAs are correctly and efficiently modified when targeted to CBs, but not when these same fragments are targeted to the nucleolus. We also demonstrate that internal modification of the U2 snRNA occurs exclusively after nuclear import of the newly assembled Sm snRNP from the cytoplasm. Finally, we show that p80 coilin, the CB marker protein, is not required for snRNA modification. In coilin knockout cells, Sm snRNAs and their modification guide RNAs colocalize in residual CBs, which do not stockpile fibrillarin and fail to recruit the U3 small nucleolar RNA.