Evidence for low-level translation in human erythrocytes

It is generally believed that human mature erythrocytes do not possess functional ribosomes and therefore cannot synthesize proteins. However, the absence of translation is not consistent with the long lifespan of mature erythrocytes. They stay viable and functional for about 115 d in the circulatory system. Here, using a highly pure preparation of human mature erythrocytes, we demonstrate the presence of translation by polysome profiling, [S-35] methionine labeling, and RiboPuromycylation. [S-35]methionine labeling revealed that the translation in mature erythrocytes is about 10% of that observed in reticulocytes. We could observe polysomes by transmission electron microscopy in these cells. RNA-seq and quantitative real-time PCR performed on polysome fractions of these cells revealed that HBA (alpha-globin) and HBB (beta-globin) transcripts are translated. Using a luciferase-based reporter assay and mutational studies, we show that the sequence of the 5' untranslated region is crucial for the translation of these transcripts. Furthermore, mature erythrocytes showed reduced expression of globin proteins (alpha- and beta-) when treated with translation inhibitors. Overall, we provide multiple lines of evidence for translation of globin mRNAs in human mature erythrocytes.