Analysis of the intracellular localization and assembly of Ro ribonucleoprotein particles by microinjection into Xenopus laevis oocytes.

Xenopus laevis oocytes have been used to determine the intracellular localization of components of Ro ribonucleoprotein particles (Ro RNPs) and to study the assembly of these RNA-protein complexes. Microinjection of the protein components of human Ro RNPs, i.e., La, Ro60, and Ro52, in X. laevis oocytes showed that all three proteins are able to enter the nucleus, albeit with different efficiencies. In contrast, the RNA components of human Ro RNPs (the Y RNAs) accumulate in the X. laevis cytoplasm upon injection. Localization studies performed at low temperatures indicated that both nuclear import of Ro RNP proteins and nuclear export of Y RNAs are mediated by active transport mechanisms. Immunoprecipitation experiments using monospecific anti-La and anti-Ro60 antibodies showed that the X. laevis La and Ro60 homologues were cross-reactive with the respective antibodies and that both X. laevis proteins were able to interact with human Y1 RNA. Further analyses indicated that: (a) association of X. laevis La and Ro60 with Y RNAs most likely takes place in the nucleus; (b) once formed, Ro RNPs are rapidly exported out of the nucleus; and (c) the association with La is lost during or shortly after nuclear export.

Intracellular localization and nucleocytoplasmic transport of Ro RNP components.

The Ro and La autoantigens, which were originally identified by sera from autoimmune patients, consist of RNA-protein complexes (RNPs), in which the protein components carry most of the autoantigenic determinants. In human cells, Ro RNPs consist of one of four small RNA molecules, termed hY1, hY3, hY4 and hY5 [35], associated with several proteins, Ro60, Ro52 and La [8, 35, 106] and possibly other, yet unidentified polypeptides. Controversial data have been published on the association of the protein calreticulin with Ro RNPs [53, 58, 75, 78], but recently in vitro evidence has been obtained that non-phosphorylated calreticulin is able to interact with hY RNAs directly [17]. The physiological significance of this interaction remains to be established. In addition to its association with hY RNAs, La is (in most cases transiently) associated with all other RNA polymerase III transcripts. The hY RNAs are the only known cellular RNAs stably bound by La. While the Ro RNPs were found to reside mainly in the cytoplasm, the other La RNPs are mainly nuclear. In this review recent progress made on the intracellular localization and transport of Ro RNP components and of assembled Ro RNPs will be discussed.

The interactions with Ro60 and La differentially affect nuclear export of hY1 RNA.

Ro RNPs are evolutionarily conserved ribonucleoprotein particles that consist of a small RNA, known as Y RNA, associated with several proteins, such as La, Ro60, and Ro52. The Y RNAs (Y1-Y5), which are transcribed by RNA polymerase III, have been shown to reside almost exclusively in the cytoplasm as Ro RNPs. To obtain more insight into the nuclear export pathway of Y RNAs, hY1 RNA export was studied in Xenopus laevis oocytes. Injection of various hY1 RNA mutants showed that an intact Ro60 binding site is a prerequisite for nuclear export, whereas the presence of an intact La binding site resulted in strong nuclear retention of hY1 RNA. Competition studies with various classes of RNAs indicated that, in addition to Ro60, another titratable factor was necessary for nuclear export of hY1 RNA. This factor appears also to be involved in nuclear export of tRNA. Because export of hY1 RNA could not be blocked by a synthetic peptide containing the recently identified nuclear export signal of the HIV-1 Rev protein, nuclear export of hY1 RNA does not seem to be dependent on a Rev-like nuclear export signal.

Characterization of cis-acting signals for nuclear import and retention of the La (SS-B) autoantigen.

The La (SS-B) autoantigen is a 47-kDa protein which binds to the 3' termini of nascent RNA polymerase III transcripts and to a number of viral leader RNAs. The La protein plays a direct role in the termination of RNA polymerase III transcription and recent findings have suggested an additional role in several aspects of translation of (viral) mRNAs. In this study we have addressed the intracellular trafficking of the La protein and characterized cis-acting elements involved in nuclear import and retention in Xenopus laevis oocytes by microinjection of in vitro translated La protein. The steady-state distribution of recombinant human La protein was, like the endogenous Xenopus La protein, mainly nuclear. Nuclear import of La appeared to be energy-dependent and is governed by a nuclear localization signal (NLS) located in the extreme C-terminal part of the protein, resembling the consensus bipartite NLS. Another sequence element in La, which completely corresponds to the bipartite NLS consensus, appeared to be nonfunctional in nuclear import of the La protein. Nuclear accumulation of La was found to be mediated by retention in the nuclear compartment. The N-terminal RNA binding domain of La is not involved in this retention, but sequence elements in the central region of the polypeptide (amino acids 165 to 337) appear to be required. Amino acids 266-269 as well as 313-337 were found to be of major importance for retention in the nucleus.

Evolution of Volatile Sulfur Compounds during Laboratory-Scale Incubations and Indoor Preparation of Compost Used as a Substrate in Mushroom Cultivation.

Volatile sulfur compounds are known to be produced during the preparation of compost used as a substrate in mushroom cultivation. Because they cause odor problems, attempts have been made to reduce the production of these compounds. The influences of temperature and various additions on the production of volatile sulfur compounds from composting material were tested on laboratory-scale preparations. The production of H(2)S, COS, CH(3)SH, and (CH(3))(2)S was proven to be a biological process with an optimal temperature that coincides with the optimal temperature for biological activity. The formation of CS(2) and (CH(3))(2)S(2) was shown to be a nonbiological process. The emission of volatile sulfur compounds during the indoor preparation of mushroom compost appeared to be remarkably reduced (about 90%) as compared with the emission during the conventional outdoor process. Introduction of this indoor composting process would result in a significant reduction in environmental pollution.