Signal recognition particle domain. Computer model showing the crystal structure of the Alu domain of the mammalian signal recognition particle (SRP). SRP is an abundant, cytosolic, universally conserved ribonucleoprotein (protein-RNA complex) that recognises and targets specific proteins to the endoplasmic reticulum (ER) in eukaryotes and the plasma membrane in prokaryotes. The Alu domain of the mammalian SRP retards the ribosomal elongation of signal-peptide-containing proteins before their engagement with the translocation machinery in the endoplasmic reticulum.