An assessment of some methodological criticisms of studies of RNA efflux from isolated nuclei

Preparation and characterization of nuclear-envelope vesicles from rat liver nuclei

We describe a procedure for the preparation of sealed nuclear-envelope vesicles from rat liver nuclei. These vesicles are strikingly similar in their polypeptide composition when compared with those of nuclear envelopes prepared conventionally using deoxyribonuclease I. Subfractionation analysis by means of extraction with high salt and urea show that the components of the nuclear envelope, e.g. the pore-complex/lamina fraction, are present. The residual DNA content is only 1.5%, and typical preparations consist of about 80% vesicles, with the vesicular character of these envelopes shown by microscopic and biochemical studies. The vesicles can be obtained in high yield, are tight and stable for at least two days and are enriched in a nucleoside triphosphatase thought to be involved in nucleocytoplasmic transport processes. Because the vesicles are largely free of components of the nuclear interior, but retain properties of intact nuclei, we believe that they are a valuable model system to study nucleocytoplasmic transport. Although in transport studies with isolated nuclei interference from intranuclear events has to be considered, the nuclear-envelope vesicles provide the possibility of studying translocation alone. Furthermore, the less complex nature of these vesicles compared with whole nuclei should facilitate investigation of the components involved in the regulation of nuclear transport processes.

Proteins from rat liver cytosol which stimulate mRNA transport. Purification and interactions with the nuclear envelope mRNA translocation system

Two polysome-associated proteins with particular affinities for poly(A) have been purified from rat liver. These proteins stimulate the efflux of mRNA from isolated nuclei in conditions under which such efflux closely stimulates mRNA transport in vivo, and they are therefore considered as mRNA-transport-stimulatory proteins. Their interaction with the mRNA-translocation system in isolated nuclear envelopes has been studied. The results are generally consistent with the most recently proposed kinetic model of mRNA translocation. One protein, P58, has not been described previously. It inhibits the protein kinase that down-regulates the NTPase, it enhances the NTPase activity in both the presence and the absence of poly(A) and it seems to increase poly(A) binding in unphosphorylated, but not in phosphorylated, envelopes. The other protein, P31, which probably corresponds to the 35,000-Mr factor described by Webb and his colleagues, enhances the binding of poly(A) to the mRNA-binding site in the envelope, thus stimulating the phosphoprotein phosphatase and, in consequence, the NTPase. The possible physiological significance of these two proteins is discussed.

Inflammation-associated events in liver nuclei during acute-phase reaction

Isolated liver nucleoli from rats undergoing turpentine-induced inflammation (acute-phase reaction) synthesize rRNA at a rate significantly higher than normal. This increase is associated with, and possibly preceded by, an enhanced methylation of RNA, which further increases when rRNA synthesis has reached a plateau level. Five hours after turpentine treatment, before clear activation of RNA synthesis and methylations, the nucleocytoplasmic transport of rRNA (largely 40S and 60S subunits) and the related ATPase activity of isolated nuclei are significantly increased. Apparently, posttranscriptional control is affected before transcription of rRNA during the onset of the acute-phase reaction: both kinds of events eventually contribute to the expansion of the ribosome population which occurs in the liver cells from rats undergoing an inflammatory process. All these processes are activated before the liver starts the synthesis of acute-phase proteins.

Release of rRNA from liver nuclei during the early stages of the acute-phase reaction

Liver nuclei isolated from rats 5 h after turpentine injection show an increased release of rRNA, of the transport-related nucleoside-triphosphatase activity and of the amount of nuclear RNA; RNA methylation is also likely to undergo some activation. These changes occur when RNA synthesis is still normal.