In the absence of ribosomal RNA synthesis, the ribosomal proteins of HeLa Cells are synthesized normally and degraded rapidly

Abstract

It is possible to solubilize essentially the total protein complement from a lysate of HeLa cells. When such an extract is displayed on a two-dimensional polyacrylamide gel (pH 5·0 × sodium dodecyl sulfate) it is possible to isolate and identify 45 ribosomal proteins, five histones, and numerous other proteins. Using this method we have measured directly the synthesis of ribosomal proteins in the presence of actinomycin D. The results show that the synthesis of ribosomal proteins does not depend on concurrent transcription of ribosomal precursor RNA. Furthermore, if ribosomal precursor RNA synthesis is blocked for 25 hours by 10 ng of actinomycin/ml, ribosomal protein synthesis as well as histone synthesis continues normally. This suggests either that messenger RNA for ribosomal proteins is exceedingly long-lived or that the synthesis of mRNA for ribosomal proteins can be uncoupled from the synthesis of ribosomal precursor RNA. Ribosomal proteins synthesized in the presence of either high or low concentrations of actinomycin are unstable. They disappear, presumably through degradation, with half-lives of 30 to 100 minutes, depending on the individual proteins. Under these conditions, histones and several other cell proteins are stable.