Addition of coilin (green) recruits SMN (red) to Cajal bodies. A nuclear protein is labeled in magenta.

Cajal bodies (CBs), until recently known as coiled bodies, are revealed by silver staining as prominent spots in nuclei. Their purpose has been mostly a mystery since their discovery 98 years ago. They look similar to nucleoli, the cell's ribosome factories, and often lie close to them. Small nuclear ribonucleoproteins (snRNPs), components of the pre-mRNA splicing machinery, are known to be concentrated in the bodies. Although the presence of CBs is not required for splicing, cells that are particularly active in transcription have CBs.

For the last decade, cell biologists have used the protein coilin as a marker for CBs. It is the only molecule known to be uniquely concentrated in the bodies, although it is also expressed diffusely throughout the nucleus, in all tissues. Tucker et al. (page 293) knocked out coilin in mice, and were surprised to find that at least some of the mutants are viable and appear normal.

When they studied cells derived from the mutant mice, they found what they term “residual CBs.” These foci contain some of the typical proteins found in CBs, but they fail to stain brightly when treated with silver, and lack two complexes that are normally prominent components of CBs: snRNPs and the SMN (survival motor neuron) protein complex. The authors conclude that coilin is necessary for recruiting these factors to CBs. Indeed, when they transiently expressed wild-type coilin in the mutant cells, bodies formed that contain both the previously missing factors. ▪