The Hunting of the snRNP

The protein–RNA complexes known as snRNPs are the cell's editors. In 1992, researchers knew from many in vitro studies that different snRNPs band together to form the “spliceosome,” which splices premessenger RNA molecules to form functional mRNAs. But they didn't know where in the nucleus snRNPs congregated. Using a new technique, Angus Lamond (now at the University of Dundee, UK) and colleagues tracked snRNPs to two mysterious structures, the interchromatin granules and the Cajal bodies (formerly coiled bodies). Follow-up work suggests that both objects are important for snRNP production and activity.

When cell biologists first detected snRNPs using immunofluorescence, they observed glowing “speckles” strewn around the nucleus. Although this technique could reveal the presence of snRNPs, the antibodies initially available couldn't determine which kinds of snRNP mustered in a particular location. So Lamond and his team added another ingredient: antisense probes that could discriminate among different snRNPs. The combination of antisense and antibodies revealed that “not all speckles were equal,” says Lamond. Some, which the researchers dubbed “nuclear foci,” glowed brighter (Carmo-Fonseca et al., 1991a,b).

The next year, Lamond's group confirmed the existence of two kinds of speckles and pinned down their identities (Carmo-Fonseca et al., 1992). Electron microscopy studies from David Spector's group at the Cold Spring Harbor Lab in New York had suggested that antibodies against snRNPs detected the structures known as interchromatin granules. Lamond's group, then at the European Molecular Biology Laboratory, had access to one of the first confocal microscopes, allowing them to analyze combinations of labels and test this possibility. When the team tagged cells with antibodies against snRNPs and against the granules, the staining patterns corresponded, confirming that some speckles were granules.

However, the antigranule antibody didn't cling to the bright foci, suggesting that they were different. The researchers suspected that they were Cajal bodies. Another group at the Scripps Research Institute in California had just discovered antibodies in autoimmune patients that recognize a Cajal body protein called coilin (Raska et al., 1991). When Lamond's lab tagged nuclei with antisense strands and this anticoilin antibody, labeling overlapped, showing that snRNPs were loitering in the Cajal bodies. Huang and Spector (1992) obtained similar results around the same time. These findings “gave us a much higher resolution picture of what was going on [in the nucleus],” says Lamond.

But they didn't explain why snRNPs were parking in the Cajal bodies and interchromatin granules. Sleeman and Lamond (1999) provided one clue by showing that snRNPs that have just entered the nucleus gather in the Cajal bodies. The snRNPs then travel to the interchromatin granules and acquire their finishing touches, maturing into functional particles (Jády et al., 2003). The granules may serve as storage depots for inactive snRNPs, says Lamond, while active snRNPs likely bind to pre-mRNA molecules at the genes themselves. Their dispersal around the nucleus probably accounts for the diffuse glow researchers noted in some labeling experiments, says Lamond.