Subsequently, RNA-bound streptavidin beads were incubated with cell extracts for 2 h at 4°C. Proteins bound to the 5′UTR assembly were eluted by D-biotin (Invitrogen), examined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS–PAGE) and identified and quantified by liquid chromatography–tandem mass spectrometry (LC–MS/MS).