signal sequence
/ SIG-nul SEE-kwents /
A busy company puts a shipping address on every package so the mailroom knows where it goes. A cell faces the same problem on a vast scale: it makes thousands of different proteins, and each one needs to end up in the right place — some stay in the cytosol, others must enter a particular organelle, and many must be exported out of the cell entirely. A signal sequence is the molecular 'address label' that tells the cell where a protein belongs.
A signal sequence is a short stretch of amino acids, often near the front of a newly made protein, that acts as a destination tag. The best-understood example is the one that routes proteins into the endoplasmic reticulum: as such a protein starts to emerge from the ribosome, its signal sequence is recognized by a particle that pauses translation and docks the whole ribosome onto the ER membrane. Synthesis then resumes with the growing chain threaded directly across (or into) the membrane. Once it has done its job, the signal sequence is often clipped off. Other signal sequences direct proteins to the nucleus, mitochondria, or other compartments, each read by its own recognition machinery.
Signal sequences matter because without correct addressing the cell would be chaos — a digestive enzyme made in the wrong place could damage the cell, and a hormone never exported would be useless. This is the front end of the secretory pathway, the route by which cells make and ship out proteins. A useful clarification: not all proteins carry a signal sequence. Those without one are made by free ribosomes in the cytosol and simply stay there or are sorted by other cues; the signal sequence is specifically the ticket for entering the ER-based shipping system.
Insulin is made with a signal sequence at its front. That tag steers its ribosome to the ER membrane so the hormone enters the secretory pathway; the tag is then snipped off, and the matured insulin is eventually exported from the cell into the blood.
A signal sequence is the 'ship to the ER' label on a protein bound for export.
A signal sequence is not the same as the finished protein's function — it is a temporary address tag, frequently cut off once the protein has reached the right compartment.