Nucleocytoplasmic RNA transport

Abstract

A number of closely related post-transcriptional facets of RNA metabolism show nuclear compartmentation, including capping, methylation, splicing reactions, and packaging in ribonucleoprotein particles (RNP). These nuclear 'processing' events are followed by the translocation of the finished product across the nuclear envelope. Due to the inherent complexity of these interrelated events, in vitro systems have been designed to examine the processes separately, particularly so with regard to translocation. A few studies have utilized nuclear transplantation/microinjection techniques and specialized systems to show that RNA transport occurs as a regulated phenomenon. While isolated nuclei swell in aqueous media and dramatic loss of nuclear protein is associated with this swelling, loss of RNA is not substantial, and most studies on RNA translocation have employed isolated nuclei. The quantity of RNA transported from isolated nuclei is related to hydrolysis of high-energy phosphate bonds in nucleotide additives. The RNA is released predominantly in RNP: messenger-like RNA is released in RNP which have buoyant density and polypeptide composition similar to cytoplasmic messenger RNP, but which have distinctly different composition from those in heterogeneous nuclear RNP. Mature 18 and 28S ribosomal RNA is released in 40 and 60S RNP which represent mature ribosomal subunits. RNA transport proceeds with characteristics of an energy-requiring process, and proceeds independently of the presence or state of fluidity of nuclear membranes. The energy for transport appears to be utilized by a nucleoside triphosphatase (NTPase) which is distributed mainly within heterochromatin at the peripheral lamina. Photoaffinity labeling has identified the pertinent NTPase as a 46 kD polypeptide which is associated with nuclear envelope and matrix preparations. The NTPase does not appear to be modulated via direct phosphorylation or to reflect kinase-phosphatase activities. A large number of additives (including RNA and insulin) produce parallel effects upon RNA transport and nuclear envelope NTPase, strengthening the correlative relationship between these activities. Of particular interest has been the finding that carcinogens induce specific, long-lasting increases in nuclear envelope (and matrix) NTPase; this derangement may underlie the alterations in RNA transport associated with cancer and carcinogenesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Cytoplasmic transport of ribosomal subunits microinjected into the Xenopus laevis oocyte nucleus: a generalized, facilitated process.

“…By using isolated nuclei many groups have shown that release of endogenous RNP complexes is temperature and energy dependent, suggesting active transport (2,15). The in vitro rate of RNP efflux has been correlated with the level of activity of a particular nuclear envelope NTPase; the NTPase is stimulated by exogenous RNA but its exact function is unknown (2,7,15).…”

“…The in vitro rate of RNP efflux has been correlated with the level of activity of a particular nuclear envelope NTPase; the NTPase is stimulated by exogenous RNA but its exact function is unknown (2,7,15). Further evidence for a facilitated transport mechanism was obtained by Zasloff who showed by microinjection of tRNAs into Xenopus oocyte nuclei that the export rate was saturable and sensitive to temperature (60).…”

“…This procedure is free of many drawbacks such as swelling, extensive loss of protein, and contamination with cytoplasmic RNP that accompany the use of isolated nuclei (2,15). Furthermore, since transport is initiated by microinjection, the assay is not limited to quantitation of release of preformed endogenous complexes without control over their composition or interaction with possible transport facilitating components.…”

Control of gene expression by steroid hormones

“…It is thus important to consider tant splicing must occur within highly organized ribonucleoprotein multicomponents, somewhat analogous to ribosomes (Brody and Abelson, 1985 ;Grabowski et al, 1985 ;Frendewey and Keller, 1985 (Nevins, 1983) and poly(A) addition (Huez et al, 1981) have been suggested as protecting RNA against nucleolytic degradation. Mature RNA is then transported into cytoplasm through the nuclear pore complex (for a recent review see Clawson et al, 1985). The precise mechanism of transport remains unknown, but it does exhibit selectivity towards correctly processed mRNA (Webb et al, 1981) or rRNA (Wunderlich, 1981).…”

“…Accurate splicing of pre-RNA to mature RNA is apparently a prerequisite for nucleocytoplasmic transport. RNA transport is also an energy-dependent process and involves a nucleoside triphosphatase associated with nuclear envelope and matrix (Clawson et al, 1985).…”