Four channels, but only one program

Cajal compartments aking 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, Stan k 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. ᭿ T Cajal body proteins recover from photobleaching. nlike organelles with single membranes, chloroplasts must import proteins through both an outer and an inner membrane. Schleiff et al., reporting on page 541, have now isolated and characterized a protein complex from the chloroplast outer membrane, revealing a trans-locon with an unusual structure. Previous work identified four proteins important for translocation across the outer chloroplast membrane, but the architecture of the translocon remained unknown. In the new work, the authors purified a core complex containing the translocon proteins Toc75, Toc34, and a fragment of Toc159, and found that this complex can recognize and translocate chloroplast precursor proteins. The absence of Toc64 in the functional complex suggests that this protein only transiently associates with the core translocon. Electron microscopy and image analysis reveals that the core complex has a toroidal shape, with a finger-like domain dividing a central cavity into four curved channels. The complex is able to bind to four precursor proteins at a time, though the curved shape of the channels suggests that they might not be able to translocate simultaneously. The structure of the complex is consistent with an earlier model, in which Toc34 is the initial receptor for precursor proteins while Toc159 …