Isolation and characterization of a translation inhibitor from human term placenta

Angiogenin abolishes cell-free protein synthesis by specific ribonucleolytic inactivation of 40S ribosomes

The translational capacity of a rabbit reticulocyte lysate is rapidly abolished on treatment with angiogenin, an effect that is due to cleavage of rRNA [St. Clair, D. K., Rybak, S. M., Riordan, J. F., & Vallee, B. L. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 8330-8334]. The same time course of inhibition is seen when isolated ribosomes are treated with angiogenin prior to being added to a ribosome-dependent lysate system. In both cases, the onset of inhibition occurs at a rate similar to that seen on addition of puromycin, a known inhibitor of elongation, suggesting that this is the step in the protein synthesis machinery that is inactivated by angiogenin. The action of angiogenin on ribosomes is quite specific: both 28S and 18S rRNAs are cleaved whereas 5.8S and 5S rRNAs are not. Moreover, 28S and 18S rRNAs are affected differently. Prolonged incubation with angiogenin degrades 28S rRNA extensively but only causes limited cleavage of 18S rRNA. Remarkably, it is the effect of angiogenin on 18S rRNA that seems to be responsible for the inhibition of protein synthesis rather than the nucleolytic degradation of 28S rRNA. This has been demonstrated by separating the isolated ribosomes into their 40S and 60S subunits and treating them individually with angiogenin. The pattern of rRNA cleavage is the same with the separated subunits as with intact ribosomes, but translation is abolished only on treatment of the 40S, not the 60S, subunit with angiogenin. These results confirm our previous observations on the effect of angiogenin on the rabbit reticulocyte cell-free translation system and extend the understanding of its mechanism of action on the ribosome.

Posttranscriptional control of embryonic rat skeletal muscle protein synthesis. Control at the level of translation by endogenous RNA

The onset of muscle cell differentiation is associated with increased transcription of muscle-specific mRNA. Studies from this laboratory using 19-d embryonic rat skeletal muscle, suggest that additional, posttranscriptional controls regulate maturation of muscle tissue via a quantitative effect upon translation, and that the regulatory component may reside within the poly A- RNA pool (Nathanson, M.A., E.W. Bush, and C. Vanderburg. 1986. J. Biol. Chem. 261:1477-1486). To further characterize muscle cell translational control, embryonic and adult total RNA were separated into oligo(dT)cellulose-bound (poly A+) and -unbound (poly A-) pools. Unbound material was subjected to agarose gel electrophoresis to resolve constituents of varying molecular size and mechanically cut into five fractions. Material of each fraction was electroeluted and recovered by precipitation. Equivalent loads of total RNA from 19-20-d embryonic rat skeletal muscle exhibited a 40% translational inhibition in comparison to its adult counterpart. Inhibition was not due to decreased message abundance because embryonic, as well as adult muscle, contained equivalent proportions of poly A+ mRNA. An inhibition assay, based upon the translatability of adult RNA and its inhibition by embryonic poly A- RNA, confirmed that inhibition was associated with a 160-2,000-nt poly A- fraction. Studies on the chemical composition of this fraction confirmed its RNA composition, the absence of ribonucleoprotein, and that its activity was absent from similarly fractionated adult RNA. Rescue of inhibition could be accomplished by addition of extra lysate or mRNA; however, smaller proportions of lysate were required, suggesting a strong interaction of inhibitor and components of the translational apparatus. Additional studies demonstrated that the inhibitor acted at the level of initiation, in a dose-dependent fashion. The present studies confirm the existence of translational control in skeletal muscle and suggest that it operates at the embryonic to adult transition. A model of muscle cell differentiation, based upon transcriptional control at the myoblast level, followed by translational regulation at the level of the postmitotic myoblast and/or myotube, is proposed.

Human angiogenin, an organogenic protein

Angiogenin is a 14 kD protein, initially isolated as a tumour-cell secreted product but subsequently found to be a normal constituent of human plasma. It is a potent inducer of blood vessel formation on the chorioallantoic membrane of the chick embryo. Chemical characterization of the protein reveals a remarkable homology to the pancreatic ribonuclease family and has led to the identification of a unique ribonucleolytic activity for angiogenin. It is a particularly potent inhibitor of in vitro protein synthesis. Treatment with placental ribonuclease inhibitor abolishes the biological and enzymatic activities of angiogenin, an effect with important mechanistic, physiological and pharmacologic implications.

Depurination of yeast 26S ribosomal RNA by recombinant ricin A chain

The nature of the modification of yeast ribosomes by the recombinant form of the ricin A chain has been examined. Evidence is presented that the 26S rRNA molecule is depurinated at a specific site and that the activity is inhibited by antibody raised to ricin A chain. It thus appears that the recombinant form of this toxin retains the depurination activity of the native molecule. These results are consistent with the model that the site of depurination is in a highly conserved sequence forming a loop on the surface of the ribosome, a domain involved in elongation factor-dependent binding of aminoacyl-tRNA.

Translational control at the level of initiation between mRNAs for pre-existing proteins and a 22-kDa heat-shock protein of Chlamydomonas by small cytosolic RNAs

Small cytosolic RNAs (scRNAs) from human placenta inhibit translation of poly(A)-rich RNA from Chlamydomonas in the wheat germ cell-free system. The major exception is the mRNA for a nuclear-coded 22-kDa chloroplast heat-shock protein whose translation is much less affected. Evidence is presented which suggests that scRNAs do not directly interact with the mRNAs but with a factor of the wheat germ system instead. It has been found that run-off translation of polyribosomes is not impaired by scRNAs whereas the formation of initiation complexes in vitro, again with the exception of those of the mRNA for the 22-kDa heat-shock protein, is heavily affected. From this evidence we conclude that scRNAs interfere with the action of one or more of the wheat germ initiation factors and that the translation of the mRNA for the 22-kDa heat-shock protein is much less dependent upon this (these) factor(s).

The function of proteins that interact with mRNA

Specific proteins are associated with mRNA in the cytoplasm of eukaryotic cells. The complement of associated proteins depends upon whether the mRNA is an integral component of the polysomal complex being translated, or, alternatively, whether it is part of the non-translated free mRNP fraction. By subjecting cells to ultraviolet irradiation in vivo to cross-link proteins to mRNA, mRNP proteins have been shown to be associated with specific regions of the mRNA molecule. Examination of mRNP complexes containing a unique mRNA has suggested that not all mRNA contain the same family of associated RNA binding proteins. The functions of mRNA associated proteins may include a role in providing stability for mRNA, and/or in modulating translation. With the recent demonstrations that both free and polysomal mRNPs are associated with the cytoskeletal framework, specific mRNP proteins may play a role in determining the subcellular localization of specific mRNPs.