In vivo analysis of Cajal body movement, separation, and joining in live human cells

Cajal bodies (also known as coiled bodies) are subnuclear organelles that c ontain specific nuclear antigens, including splicing small nuclear ribonucl eoproteins (snRNPs) and a subset of nucleolar proteins. Cajal bodies are lo calized in the nucleoplasm and are often found at the nucleolar periphery. We have constructed a stable HeLa cell line, HeLaGFP-coilin, that expresses the Cajal body marker protein, p80 coilin, fused to the green fluorescent protein (GFP-coilin). The localization pattern and biochemical properties o f the GFP-coilin fusion protein are identical to the endogenous p80 coilin. Time-lapse recordings on 63 nuclei of HeLaGFP-coilin cells showed that all Cajal bodies move within the nucleoplasm. Movements included translocation s through the nucleoplasm, joining of bodies to form larger structures, and separation of smaller bodies from larger Cajal bodies. Also, we observed C ajal bodies moving to and from nucleoli. The data suggest that there may be at least two classes of Cajal bodies that differ in their size, antigen co mposition, and dynamic behavior. The smaller size class shows more frequent and faster rates of movement, up to 0.9 mum/min. The GFP-coilin protein is dynamically associated with Cajal bodies as shown by changes in their fluo rescence intensity over time. This study reveals an unexpectedly high level of movement and interactions of nuclear bodies in human cells and suggests that these movements may be driven, at least in part, by regulated mechani sms.