New clues to the function of the Cajal body

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Abstract
Small RNA molecules play major roles in a number of cellular functions. Depending on their predominant cellular location, small RNAs have been classified as nuclear (sn), nucleolar (sno) or cytoplasmic (sc). The nucleoplasmic species include the spliceosomal snRNAs (U1, U2, U4, U5 and U6) and the U7 snRNA required for histone pre‐mRNA 3′‐end formation. The snoRNAs are key players in the processing of ribosomal RNA (rRNA), and scRNAs function in protein synthesis. Most small RNAs have a complex life cycle, being synthesized as precursors that must undergo a series of processing events in order to become functionally mature. Many of these post‐transcriptional modifications are ‘guided’ by other small RNAs and occur in specific cellular compartments such as the nucleolus (Filipowicz and Pogacic, 2002). Two related papers in The EMBO Journal from the groups of Tamás Kiss and Edouard Bertrand now highlight a role for Cajal bodies in the post‐transcriptional modification of spliceosomal snRNAs and snoRNAs. Kiss and co‐workers report a new family of small RNAs that localize specifically to Cajal bodies and are involved in the maturation of spliceosomal snRNAs (Darzacq et al ., 2002), and Bertrand and co‐workers show that the final processing steps involved in the biogenesis of U3 snoRNA take place in Cajal bodies (Verheggen et al ., 2002). Despite being identified around 100 years ago by the neurocytologist Ramon y Cajal, the function of Cajal bodies has so far proved elusive. When viewed with the electron microscope, these subnuclear ‘organelles’ appear …