The mammalian cell nucleus contains structurally stable functional compartments. We show here that one of them, the Cajal body (CB), can be formed de novo. Immobilization on chromatin of both CB structural components, such as coilin, and functional components of the CB, such as the SMN complex, spliceosomal small nuclear ribonucleoproteins (RNPs), small nucleolar RNPs, and small Cajal body-specific RNPs, is sufficient for the formation of a morphologically normal and apparently functional CB. Biogenesis of the CB does not follow a hierarchical assembly pathway and exhibits hallmarks of a self-organizing structure.

La is an RNA-processing-associated phosphoprotein so highly conserved that the human La protein (hLa) can replace the tRNA-processing function of the fission yeast La protein (Sla1p) in vivo. La proteins contain multiple trafficking elements that support interactions with RNAs in different subcellular locations. Prior data indicate that deletion of a nuclear retention element (NRE) causes nuclear export of La and dysfunctional processing of associated pre-tRNAs that are spliced but 5 and 3 unprocessed, with an accompanying decrease in tRNA-mediated suppression, in fission yeast. To further pursue these observations, we first identified conserved residues in the NREs of hLa and Sla1p that when substituted mimic the NRE deletion phenotype. NRE-defective La proteins then deleted of other motifs indicated that RNA recognition motif 1 (RRM1) is required for nuclear export. Mutations of conserved RRM1 residues restored nuclear accumulation of NREdefective La proteins. Some RRM1 mutations restored nuclear accumulation, prevented disordered pre-tRNA processing, and restored suppression, indicating that the tRNA-related activity of RRM1 and its nuclear export activity could be functionally separated. When mapped onto an hLa structure, the export-sensitive residues comprised surfaces distinct from the RNA-binding surface of RRM1. The data indicate that the NRE has been conserved to mask or functionally override an equally conserved nuclear export activity of RRM1. The data suggest that conserved elements mediate nuclear retention, nuclear export, and RNA-binding activities of the multifunctional La protein and that their interrelationship contributes to the ability of La to engage its different classes of RNA ligands in different cellular locations.La is an abundant protein whose capacity to bind a variety of noncoding RNAs and mRNAs lends itself to numerous activities (26). La proteins from yeast to human have been implicated in the production of tRNAs, rRNAs, ribosomal proteins, and other components of the translational machinery (13,16,19,20). In human cells, most La is phosphorylated on serine-366 by protein kinase CK2, resides in the nucleoplasm, and is associated with nascent pre-tRNAs (17, 32). Nonphosphorylated La is most concentrated in the nucleolus (15, 16) and was independently found at tRNA and other RNA polymerase III-transcribed genes (4), but it also resides in the cytoplasm associated with 5ЈTOP mRNAs that encode ribosomal proteins and translation factors (16). Trafficking signals in human La include a nuclear localization signal (NLS), a nuclear retention element (NRE) first identified by microinjection of Xenopus oocytes and later confirmed in fission yeast (14,35), and a nucleolar localization signal (10, 15). Although trafficking may be important for its different activities (19), knowledge of the functional significance of alterations in La trafficking in specific RNA pathways is limited.Sequence-specific binding to 3Ј UUU-OH, the termination motif found on nascent pre-tRNAs and other transcripts syn...