Coiled bodies contain U7 small nuclear RNA and associate with specific DNA sequences in interphase human cells.

Abstract

Coiled bodies (CBs) are nuclear organelles whose structures appear to be highly conserved in evolution.In rapidly cycling cells, they are typically located in the nucleoplasm but are often found in contact with the nucleolus. The CBs in human cells contain a unique protein, called p80-coilin. Studies on amphibian oocyte nuclei have revealed a protein within the "sphere" organelle that shares significant structural similarity to p80-coilin. Eukaryotic cells can be viewed as if they were a mixture of compartments, some of which can be defined physically, some functionally. In the cytoplasmic compartment, the roles of the various organelles and the interplay between them have been extensively studied. In contrast, the domains and their interconnections within the nucleus are poorly understood. However, it is now clear that the nucleus is a highly organized structure, permeated by a proteinaceous "matrix" and composed of many different subdomains. The so-called interchromatin space of somatic cells can be characterized at the ultrastructural level by several types of structures. These are perichromatin fibrils, interchromatin granule clusters, and nuclear bodies. Small nuclear ribonucleoproteins (snRNPs) are major components of each of these three subcompartments (see refs. 1 and 2 for reviews). One type of nuclear body, the coiled body (CB), contains relatively high concentrations of small RNPs as well as other proteins (see Table 1 for a list of components reported to be either enriched or not enriched in CBs). Despite (or perhaps because of) this rather lengthy roster, the list of putative CB functions is nearly as long (see refs. 3 and 4 for reviews). Early studies on amphibian oocyte nuclei also revealed morphologically distinct structures, including: nucleoli, lampbrush chromosomes, and "spheres." The subsequent finding that spheres (which are composed of B-and C-type "snurposomes") contain numerous snRNPs and snRNP proteins caused speculation that CBs and spheres were related organelles (reviewed in ref. 5). The final pieces to the puzzle linking CBs and spheres were revealed when SPH-1, an integral protein component ofThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.C snurposomes, was shown to have significant structural homology to a CB marker protein, p80-coilin (6, 7), and when Wu et al (8) showed that p80-coilin protein is targeted to spheres when the human coilin mRNA was injected into amphibian oocytes. Thus spheres and CBs are homologous organelles that are likely to have similar functions, but as yet no functions have been demonstrated (5).Of particular interest is the fact that some of the spheres within the amphibian oocyte are attached to the so-called "sphere-organizer" regions of the lampbrush chromosomes. These sphere-organizers are located at the histone gene clusters in two classes of amphibia, Anura and Urodela (9, 10). The fa...

The C-terminal domain of coilin interacts with Sm proteins and U snRNPs

“…U7 snRNPs are strongly associated with CBs (Frey and Matera 1995;Pillai et al 2001), and although U1 snRNPs do not accumulate prominently in CBs at steady state, they likely transit through them (Sleeman and Lamond 1999).…”

“…Assembly of the U7 snRNP is mediated by a specialized SMN complex that also lacks Sm D1 and D2, but contains Lsm10 and Lsm11 instead (Pillai et al 2003). Otherwise, assembly and nuclear import of U7 snRNPs resembles the pathway for spliceosomal snRNPs (Stefanovic et al 1995), and U7 snRNPs are predominantly localized to CBs with low levels being present in the nucleoplasm (Frey and Matera 1995;Pillai et al 2001). Thus, both the spliceosomal U snRNPs and the U7 snRNP, along with many, but not all, of the snRNP-specific proteins are highly enriched within CBs.…”

The Movement of Coiled Bodies Visualized in Living Plant Cells by the Green Fluorescent Protein

“…It will be instructive to investigate whether coiled bodies move through interchromosomal channels, or along an underlying nuclear matrix, and whether specific motor proteins are required. Alternatively, as some coiled bodies have been found associated with specific chromosomal loci, including gene clusters that encode U snRNAs and histone mRNAs (Frey and Matera, 1995;Smith et al, 1995), and as there is direct evidence that chromosome movements take place in several cell types (Li et al, 1998; review by Zink and Cremer, 1998), it is possible that the movements of coiled bodies are related to underlying movements of chromosome loci to which they are attached. On the other hand, as Frey et al (1999) showed that the frequency of colocalization of coiled bodies with artificial U1 and U2 snRNA genes depends on the transcriptional activity of the gene array, the movement of a coiled body might be the result of the termination of transcription and the subsequent release of the coiled body from a particular chromosome locus.…”

Dual role for the RNA-binding domain of Xenopus laevis SLBP1 in histone pre-mRNA processing

“…In the cell, much of the U7 snRNP is found in Cajal (coiled) bodies, "spheres" in Xenopus oocytes (Wu & Gall, 1993;Wu et al+, 1996), and these nuclear organelles are located near the histone genes in oocytes (Callan et al+, 1991) and in mammalian cells (Frey & Matera, 1995;Matera, 1999), suggesting a role for the coiled bodies in histone pre-mRNA processing+ Some of the xSLBP1 in the frog oocyte is also located in the spheres (Abbott et al+, 1999)+ One possibility is that xSLBP1 may function in a distinct subnuclear compartment+ The histone genes have remained tightly clustered throughout evolution, suggesting that there is selective pressure keeping them closely linked (Wang et al+, 1996a(Wang et al+, , 1996b)+ One reason for this tight clustering may be that the genes are localized in a region of the nucleus specific for histone mRNA processing+ Ninety-five percent of the U7 snRNA in the Xenopus oocyte nucleus is present in spheres (Wu & Gall, 1993)+ Spheres are found dispersed in the nucleoplasm and some are physically attached to the chromosomes at histone gene loci in amphibian oocyte lampbrush chromosome spreads (Callan et al+, 1991)+ Recently Gall and coworkers have shown that xSLBP1 is present in the spheres of the amphibian germinal vesicle (Abbott et al+, 1999)+ A region of xSLBP1, including part of the RBD and the C-terminal region, is capable of directing the xSLBP1 to coiled bodies (Abbott et al+, 1999)+ The spheres also contain RNA polymerase II and coilin (Bellini & Gall, 1998), which has also been shown to associate with the U7 snRNP (Bellini & Gall, 1998)+ Localization of xSLBP1 to the spheres (coiled bodies) may be important for efficient histone pre-mRNA processing in vivo+ The 1-2-1 protein may be deficient in localizing to the coiled bodies or may not interact with unknown factor(s) necessary for processing in vivo, but not in vitro+ In nuclear extracts, any nuclear substructures are disrupted allowing interaction of factors that may be physically separated in the nucleus in vivo+ It is also likely that histone premRNA processing is coupled to histone gene transcription in vivo, as is polyadenylation (McCracken et al+, 1997)+ Thus in vivo there may be a defined pathway by which the histone pre-mRNA processing machinery is assembled and this pathway could be disrupted in nuclear extracts+ Differences in metabolism of the same RNA molecule transcribed from a gene and injected into the nucleoplasm are likely to be observed in many cases+ Differences in the "masking" of mRNAs have been observed in Xenopus oocytes depending on whether synthetic pre-mRNA was injected or whether the premRNA was expressed from an injected gene (Meric et al+, 1996;Matsumoto et al+, 1998)+ It is likely that many steps in nuclea...…”