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The eukaryotic nucleolus is involved in ribosome biogenesis and a wide range of other RNA metabolism and cellular functions. An important step in the functional analysis of the nucleolus is to determine the complement of proteins of this nuclear compartment. Here, we describe the first proteomic analysis of plant (Arabidopsis thaliana) nucleoli, in which we have identified 217 proteins. This allows a direct comparison of the proteomes of an important nuclear structure between two widely divergent species: human and Arabidopsis. The comparison identified many common proteins, plant-specific proteins, proteins of unknown function found in both proteomes, and proteins that were nucleolar in plants but nonnucleolar in human. Seventy-two proteins were expressed as GFP fusions and 87% showed nucleolar or nucleolar-associated localization. In a striking and unexpected finding, we have identified six components of the postsplicing exon-junction complex (EJC) involved in mRNA export and nonsense-mediated decay (NMD)/mRNA surveillance. This association was confirmed by GFP-fusion protein localization. These results raise the possibility that in plants, nucleoli may have additional functions in mRNA export or surveillance.

The cell wall is an important determinant of plant cell form. Here we define a class of Arabidopsis root hair mutants with defective cell walls. Plants homozygous for kojak (kjk) mutations initiate root hairs that rupture at their tip soon after initiation. The KJK gene was isolated by positional cloning, and its identity was confirmed by the molecular complementation of the Kjk − phenotype and the sequence of three kjk mutant alleles. KOJAK encodes a cellulose synthase-like protein, AtCSLD3. KOJAK/AtCSLD3 is the first member of this subfamily of proteins to be shown to have a function in cell growth. Subcellular localization of the KOJAK/AtCSLD3 protein using a GFP fusion shows that KOJAK/AtCSLD3 is located on the endoplasmic reticulum, indicating that KOJAK/AtCSLD3 is required for the synthesis of a noncellulosic wall polysaccharide. Consistent with the cell specific defect in the roots of kjk mutants, KOJAK/AtCSDL3 is preferentially expressed in hair cells of the epidermis. The Kjk − phenotype and the pattern of KOJAK/ AtCSLD3 expression suggest that this gene acts early in the process of root hair outgrowth. These results suggest that KOJAK/AtCSLD3 is involved in the biosynthesis of ␤-glucan-containing polysaccharides that are required during root hair elongation.

The nucleolus is the site within the eukaryotic nucleus of transcription of rDNA, of processing of the rDNA transcripts, and of the formation of pre-ribosomal particles. We review current ideas for the molecular organization of these processes. The earliest transcriptional events take place near the junction of the fibrillar centers and the dense fibrillar component; nascent transcripts occupy the dense fibrillar component, and the later stages of pre-ribosome formation take place in the granular component. We review current knowledge of non-ribosomal nucleolar proteins. Nucleoli contain a group of proteins that bind RNA and are likely to act as chaperones to facilitate the molecular structural events in the folding and interaction of the many components of ribosomes. Some of these nucleolar proteins are also engaged in a shuttling cycle between the nucleus and the cytoplasm and may serve to transport other proteins.

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