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.