Kinetic estimation of the base sequence complexity of poly(A)-containing mRNA in Ehrlich-ascites-tumor cells and its abundance in nuclear RNA

Translational regulation in rat brain hemispheres

Present studies on the sensitivity of the transcription process in rat cerebral hemispheres showed that the relative abundance and translation of free and bound poly (A)+ mRNAs in a reticulocyte mRNA-dependent system were reduced following 1 h and 4 h of ethanol or pentobarbital administration with free being affected to a greater extent than the bound poly (A)+ mRNAs. In addition, the energy-dependent nucleocytoplasmic transport of in vivo [3H]labeled nuclear RNA to surrogate cytoplasm was modified in response to ethanol exposure. The translocation of the labeled nuclear RNA fraction occurred only to the microsomal/ribosomal fractions of the cytoplasm, was stimulated by cyclic cAMP and abolished when the cytoplasm was depleted of its protein factors following streptomycin treatment, thereby establishing the translocated RNA as messenger RNA. It is concluded that the neural cell, in response to ethanol exposure, modifies the efficiency of nuclear processing and transport of mRNA. This nuclear restriction probably occurs at multi-levels during the post-transcriptional modification of mRNAs.

Total poly(A+)-messenger RNA from bovine lens cofractionates with sucrose purified fiber cell plasma membrane

Poly(A+)-messenger RNA was isolated from bovine lens as well as poly(A+)-mRNA from crude and sucrose-purified plasma membrane. Bovine lens MP26 antisera was also propagated in rabbits, from which anti-MP26 IgG was partially purified and used to assay for the specific synthesis of MP26 during in vitro translation of the isolated poly(A+)-mRNAs. We found that membrane-associated poly(A+)-mRNA supported the synthesis of proteins which were identical to those translated from total lens fiber cell poly(A+)-mRNA. These proteins included MP26, as assayed by immunoprecipitation of [35S]-labeled MP26.

Characterization of a messenger RNA transport protein

A cytoplasmic protein which facilitates the energy-dependent transport of mRNA from isolated nuclei to a specified medium has been further characterized, since it could have relevance to the mechanism of mRNA nucleo-cytoplasmic transport in vivo. This protein is now shown, by cDNA hybridization analysis using appropriate recombinant probes, to be obligatory for the transport of alpha 2u-globulin and albumin mRNA from male rat liver nuclei. It is concentrated in the cytoplasm. When isolated under conditions where they retain nuclear proteins, the nuclei contain less than 2% of the total mRNA transport activity. Approx. 20% is recovered in the cytosol, while the rest (80%) copurifies with the messenger ribonucleoproteins in the polyribosome fraction. The protein is eluted from the poly A-messenger ribonucleoproteins between 0.25 and 0.50 M NaCl. The activities of the cytosolic- and messenger ribonucleoprotein-derived transport proteins were mutually additive below saturation of the transport system. Further, the activities of both fractions were increased when they were fortified with the catalytic subunit of the cAMP-dependent protein kinase in the presence of ATP. On the other hand, protein kinase-induced thiophosphorylation of the protein with ATP[S] decreased transport activity. The molecular weight of the transport protein from either cell compartment as judged by molecular sieving is approx. 35,000. It has now been purified 2000-fold and requires manganese ions and serum albumin for stabilization of activity. The highly purified transport factor from the cytosol is tentatively assigned a molecular weight of 32,000 by SDS-polyacrylamide gel electrophoresis.

Regulated transport of messenger ribonucleic acid from isolated liver nuclei by nucleic acid binding proteins

Rat liver nucleocytosolic messenger ribonucleic acid (mRNA) transport is shown to be regulated by proteins with a high affinity for nucleic acids. In the cell-free system described, the energy-dependent transport of all RNA classes [transfer RNA (tRNA), mRNA, and ribosomal RNA (rRNA)] exhibited a dependence upon the availability of discrete minor sets of cytosol proteins. In addition to having a different level of saturation, only the mRNA "transport protein" activities are increased by adenosine cyclic 3',5'-phosphate (cAMP), an effect most likely mediated by a cAMP-dependent protein kinase. The mRNA transport proteins were isolated from cytosol by precipitation with streptomycin sulfate followed by deoxyribonucleic acid (DNA)-cellulose affinity chromatography, or from oligo-(thymidylate)-cellulose bound cytoplasmic messenger ribonucleoprotein (mRNP) particles by high-salt extraction. Either method yielded a protein fraction which exhibited a 1000-fold increase in mRNA transport activity as compared to cytosol. Over one-half of the mRNA transport activity is associated with the mRNP of the cell. A partial homology between the cytosol and mRNP-derived proteins was demonstrated by polyacrylamide gel electrophoresis. One major (20 000 daltons) and several minor proteins (23 000, 52 000, 54 000, and 72 000 daltons) were in common. Nuclear 4-5S exited from in vitro incubated nuclei in three phases, according to their differential in vivo rates of labeling and intranuclear pool sizes. The amount of nuclear RNA transported in vitro as mRNA (about 1.0%) agrees wtih the in vivo estimates. Additional evidence for in vivo equivalence was provided by the physicochemical characterization and bioassay of the RNA. The transported mRNA sedimented in urea-sucrose gradients as an 8-18S heterodisperse product. This RNA initiated cell-free translation with the synthesis of precursor peptides as diverse in size as those for albumin and alpha 2U-globulin. The relative abundancies of various transported mRNAs were different than the corresponding abundancies of liver cytoplasmic mRNAs.