Schematic view of SRP in metazoans. The protein subunits with molecular weights 9, 14, 19, 54, 68 and 72 kDa are shown as well as RNA helix numbers 3, 4, 5, 6 and 8 that are referred to in the text. The human SRP RNA sequence is shown. The Alu domain contains the 5' and 3' terminal regions of the RNA as well as the SRP9 and 14 proteins. The S domain contains the central region of the RNA and the four remaining SRP proteins.