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The signal recognition particle RNA, is part of the signal recognition particle ribonucleoprotein complex. SRP recognizes the signal peptide and binds to the ribosome, halting protein synthesis. SRP-receptor is a protein that is embedded in a membrane, and which contains a transmembrane pore. When the SRP-ribosome complex binds to SRP-receptor, SRP releases the ribosome and drifts away. The ribosome resumes protein synthesis, but now the protein is moving through the SRP-receptor transmembrane pore.
In this way SRP directs the movement of proteins within the cell to bind with a transmembrane pore which allows the protein to cross the membrane to where it is needed. The RNA and protein components of this complex are highly conserved but do vary between the different kingdoms of life.
The common SINE family Alu probably originated from a 7SL RNA gene after deletion of a central sequence.