Cajal body finds its function

Abstract

The Cajal body (CB, formerly coiled body) is a prominent nuclear structure, yet its function remains obscure. Now, Xavier Darzacq, Tamas Kiss (CNRS, Universite Paul Sabatier, Toulouse, France), and colleagues speculate that the CB is where small nuclear RNAs go for modification. Figure Novel guide RNAs localize to CBs. Last year, Kiss's team identified a small guide RNA, U85, that directs both 2'-O- methylation and pseudouridylation of the spliceosomal component U5 snRNA. This year, they've identified six more RNAs, U87-U92, dubbed small Cajal body–specific RNAs, or scaRNAs. The new scaRNAs resemble U85 in structure, and they contain sequences complimentary to pol II–transcribed UsnRNAs, suggesting that the scaRNAs may be guides for posttranscriptional modification of the UsnRNAs. The scaRNAs are immunoprecipitated by antibodies against fibrillarin and GAR, proteins that are likely to be involved in these modification reactions. Most important, says Kiss, is the fact that these RNAs localize entirely to the CB, even when heavily overexpressed, making them unique in the world of CB biology. Previously, the best marker for the structure was p80-coilin, but only a fraction is in the CB—the rest floats free in the nucleoplasm. This implies that the scaRNAs are functioning in the CB, not just being stored there. Add in the fact that both snRNAs and snoRNAs seem to transit through the CB, and Kiss thinks he's found the elusive CB function. “This strongly, strongly indicates that at least one function of Cajal bodies is the posttranscriptional modification of the snRNAs,” he says. ▪ Reference: Darzacq, X., et al. 2002. EMBO J. 21:2746–2756. [PubMed]