Properties of rat brain microsomal ribonucleic acid containing polyadenylate

The Xenopus oocyte as a surrogate secretory system. The specificity of protein export

Combining messenger RNA from one kind of secretory cell with the cytoplasm of another such cell can reveal the nature and specificity of protein export mechanisms. We show that messenger RNAs from secretory cells of chickens, rats, mice, frogs, guinea-pigs, locusts and barley plants, when injected into Xenopus oocytes, direct the synthesis and export of proteins. Chicken ovalbumin, Xenopus albumin, mouse thyroid-stimulating hormone, locust vitellin and guinea-pig milk proteins were identified using specific antibodies, whilst chicken lysozyme and ovomucoid, rat albumin, Xenopus vitellogenin and rat seminal vesicle basic proteins were identified provisionally from their molecular weights. Certain endogenous proteins are sequestered and secreted although most oocyte proteins are not exported. Similarly the major polyoma viral protein and the simian virus 40 and polyoma tumour antigens are retained within the oocyte. Radioactive proteins exported by oocytes programmed with chicken oviduct or Xenopus liver RNA are not re-exported in detectable amounts when injected into fresh oocytes, nor is there secretion of chicken oviduct or guinea-pig mammary gland primary translation products prepared using wheat germ extracts. Thus the export of secretory proteins from oocytes cannot be explained by leakage and may require a cotranslational event. The secretory system of the oocyte is neither cell-type nor species-specific yet is highly selective. We suggest that the oocyte can be used as a general surrogate system for the study of gene expression, from transcription through translation to the final subcellular or extracellular destination of the processed protein.

Poly(A)- and nonpoly(A)-RNA associated with rat brain microsomal fractions: in vivo labelling studies

The time course of incorporation of radiolabelled precursor into RNA associated with rat brain free polyribosomes, rough membranes, and smooth membranes was measured following a single intracranial injection of [3H] orotic acid. Polyadenylated RNAs were separated from nonpolyadenylated RNAs by affinity chromatography on oligo (dT)-cellulose columns. Poly(A)-RNA associated with each of the microsomal fractions became more rapidly labelled than did the nonpoly(A)-RNA of the same fractions. While the labelling profiles of the nonpoly(A)-RNA isolated from the polyribosomes and rough membranes are similar from one fraction to another, the specific radioactivity of the poly(A)-RNA isolated from free polyribosomes increased much more drastically than that of the poly(A)-RNA associated with rough membranes. The labelling profiles of RNA species isolated from smooth membranes were very different in this respect from the two ribosomal fractions. There was a lag of more than four hours before significant label appeared in the RNA associated with the smooth membrane fraction. These studies demonstrate that the different populations of brain microsomal RNA are labelled at different rates, perhaps reflecting differences in the turnover of these RNAs and differences in their function.

Nucleo-cytoplasmic relationships of high-molecular-weight ribonucleic acid, including polyadenylated species, in the developing rat brain

The metabolism of high-molecular-weight RNA in the nuclear and cytoplasmic fractions of newborn and adult rat brain was investigated after the intracranial administration of [32P]Pi. In young brain, a considerable proportion of the newly synthesized radioactive RNA is transferred to the cytoplasm, in contrast with the adult brain, where there appears to be a high intranuclear turnover. Electrophoretic analysis of the newly synthesized RNA showed that processing of the rRNA precursor to yield the 28S and 18S rRNA may be more rapid in the adult than in the young, although most of the adult rRNA in the nucleus is not transferred to the cytoplasm. In young brain, processing is probably tightly coupled to transport of rRNA into the cytoplasm, so that 28S and 18S rRNA are not subjected to possible degradation within the nucleus. Polyadenylated RNA turns over in concert with high-molecular-weight RNA in the nuclei of the adult rat brain. In the cytoplasm the polyadenylated RNA has a higher turnover rate relative to rRNA. In the young brain the polyadenylated RNA is transferred to the cytoplasm along with rRNA, although polyadenylated RNA is transported into the cytoplasm at a faster rate. The nuclear and cytoplasmic polyadenylated RNA species of young brain are larger than their corresponding adult counterparts. These results suggest that there are considerable changes in the regulation of the nucleo-cytoplasmic relationship of rRNA and polyadenylated RNA during the transition of the brain from a developing replicative phase to an adult differentiated and non-dividing state.