Signaalilipeidejä

Signaalilipeidejä, often translated as "signal peptides" or "signal sequences," are short amino acid sequences, typically found at the N-terminus of a newly synthesized protein. Their primary function is to direct the protein to a specific cellular location or pathway, most commonly for secretion out of the cell or insertion into a membrane. These peptides act as molecular address tags, recognized by cellular machinery that facilitates protein transport.

The process usually begins with the ribosome synthesizing the protein. As the signal peptide emerges, it is recognized by a signal recognition particle (SRP). The SRP then binds to an SRP receptor on the endoplasmic reticulum (ER) membrane. This interaction docks the ribosome-membrane complex to a protein translocation channel called a translocon. The nascent polypeptide chain is threaded through the translocon into the ER lumen or embedded into the ER membrane. Once the protein is properly targeted, the signal peptide is typically cleaved off by a signal peptidase enzyme. This cleavage is crucial as the signal peptide itself is usually not part of the mature, functional protein. Proteins destined for secretion, lysosomes, or the plasma membrane often utilize this ER-mediated pathway initiated by a signal peptide. Variations of signal peptides also exist for targeting proteins to other organelles like mitochondria or the nucleus, though these often involve different recognition mechanisms and transport systems.