Cell Biology

The More the Merrier

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Science  30 Apr 2010:
Vol. 328, Issue 5978, pp. 549
DOI: 10.1126/science.328.5978.549-a

Cajal bodies in the zebrafish embryo.

CREDIT: MAGDALENA STRZELECKA

Many biological processes are compartmentalized within the cell by restricting the localization of proteins and other molecules. The spatial organization of these compartments, often bounded by internal membranes, can facilitate the delicate control of complex processes—for instance, the transcription of DNA into RNA, which occurs within the nucleus of eukaryotic cells. However, many non—membrane-bound intracellular compartments have known molecular compositions yet largely unknown functions. Studies attempting to identify their function by mutating or removing protein components to disrupt the compartment physically have been hard to interpret because of the apparent lack of phenotypic changes. This has led to speculation that this kind of compartmentalization may serve to increase the efficiency of an otherwise ongoing process, which may only become rate-limiting under rare physiological conditions. Strzelecka et al. have uncovered just such a critical role for coilin, which is a conserved component of Cajal bodies, during embryogenesis in zebrafish. Cajal bodies are 0.5- to 1-µm structures first described by Ramón y Cajal in 1903. They are found in the nuclei of plant and animal cells, most noticeably in those cells engaging intensely in transcription, and are composed of proteins and RNAs, such as the small nuclear ribonucleoprotein (snRNP) complexes that are important for pre-mRNA splicing. The snRNPs are complicated assemblies that undergo a multistep maturation process, which, it has been proposed, takes place in Cajal bodies. The authors depleted coilin in zebrafish embryos, which caused a disruption of Cajal bodies and the dispersal of snRNPs, before leading subsequently to cell death. These coilin morphants exhibited defective snRNP biogenesis and pre-mRNA splicing, which could be rescued by adding purified mature human snRNPs, revealing a role for coilin in snRNP biogenesis in the zebrafish embryo. The authors suggest that the formation of Cajal bodies concentrates snRNP components and promotes their faithful interaction, which is critical during early embryogenesis when transcriptional activity, and thus the need for snRNPs, is particularly high. These principles may be generally applicable to other non–membrane-bound subcellular compartments with currently unknown functions.

Nat. Struct. Mol. Biol. 17, 403 (2010).

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