Cajal body proteins recover from photobleaching.

Taking advantage of the large size of Xenopus oocyte nuclear structures, Handwerger et al. (page 495) analyze in detail the trafficking between the Cajal body and the nucleoplasm. The work provides direct evidence that the Cajal body, discovered a century ago, is a dynamic organelle through which proteins and RNAs move, and not simply a storage site for proteins and RNA involved in transcription and transcript processing.The authors fluorescently labeled three Cajal body components, U7 snRNA, coilin, and TATA-binding protein, and studied their dynamics by fluorescence recovery after photobleaching. The recovery rate for all three proteins is independent of the diameter of the bleach spot, and much slower than expected based on the organelle's viscosity. A mathematical model that invokes three compartments, or kinetic states with different half-lives within the organelle, fits well with the data. The slower kinetic states might represent modification or assembly events inside Cajal bodies, a conjecture the authors hope to test in future work.

On page 505, Stanek et al. show that mammalian cell Cajal bodies concentrate SART3/p110, a protein involved in U4/U6 snRNP assembly, further suggesting that these organelles are assembly sites for RNA processing complexes. ▪