Cysteine protease attribute of eukaryotic ribosomal protein S4

► The eukaryotic ribosomal protein S4 is a cysteine protease. ► It cleaves synthetic substrates commonly used for assaying cathepsins and caspases. ► Proteolysis of at least two recombinant proteins, Bcl-xL and eIF2α, is demonstrated. ► The extraribsosomal activity of S4 could have roles in cell growth and proliferation.