Plasma membrane RNase activity in mammalian cells

Plasma membranes isolated from eight different tissues from either man, rat, mouse or rabbit and from tissue culture were shown to inhibit protein synthesis in a cell-free system. From all membranal extracts an RNase endonuclease activity could be isolated which split rRNΔ. In contrast, the polyribosomal structure of rabbit reticulocytes was unaffected, showing that 9S mRNA was not destroyed under these conditions. The Triton X-100 membranal extracts blocked protein synthesis in the elongation stage and all resembled very closely the previously described RNase M activity 2]. A hypothesis is put forward, suggesting that newly formed ribosomes migrate in the cytoplasm while accomplishing protein synthesis. After being engaged in a series of such rounds of protein synthesis they meet with the plasma membrane and are inactivated by the RNase endonucleolytic splitting of their ribosomal RNA (rRNA). It is suggested that this is a mechanism common to all eukaryotic cells.