How are proteins directed to their cellular destinations?

As a polypeptide chain emerges from the ribosome it may simply fold into its three-dimensional shape and perform its cellular role locally in the cytosol. However, if a newly formed polypeptide is meant to do its work elsewhere, a signal sequence (or signal peptide)—a short stretch of amino acids attached to the polypeptide—will tell the polypeptide where in the cell it belongs. Proteins destined for different locations have different signals.

Protein synthesis always begins on free ribosomes, and the “default” location for a protein is the cytosol. In the absence of a signal sequence, the protein will remain in the same cellular compartment in which it was synthesized. Some proteins contain signal sequences that “target” them to the nucleus, mitochondria, plastids, or peroxisomes (Figure 14.17). A signal sequence binds to a specific receptor protein at the surface of the organelle. Once it has bound, the targeted protein moves into the organelle. For example, here is a nuclear localization signal (NLS):

-Pro-Pro-Lys-Lys-Lys-Arg-Lys-Val-

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Figure 14.17 Destinations for Newly Translated Polypeptides in a Eukaryotic Cell Signal sequences on newly synthesized polypeptides bind to specific receptor proteins on the outer membranes of the organelles to which they are “addressed.” Once the protein has bound to it, the receptor forms a channel in the membrane, and the protein enters the organelle.

Question

Q: What happens to a protein that has no amino acid sequence “address”?

A protein with no “address” stays in the cytoplasm.

How do we know this signal sequence directs the protein to the nucleus? The function of this NLS peptide was established using experiments like the one illustrated in Figure 14.18. Proteins were made in the laboratory with or without the peptide, and then tested by injecting them into cells. Only proteins with the NLS were found in the nucleus.

experiment

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Figure 14.18 Testing the Signal

Original Paper: Dingwall, C. et al. 1988. The nucleoplasmin nuclear location sequence is larger and more complex than that of SV-40 large T antigen. Journal of Cell Biology 107: 841–849.

A. Richardson and his colleagues performed a series of experiments to test whether the nuclear localization signal (NLS) is all that is needed to direct a protein to the nucleus.

If a polypeptide carries a signal of about 20 hydrophobic amino acids at its N terminus, it will be directed to the rough endoplasmic reticulum (RER) for further processing (see Figure 14.17). Note that this is not a specific sequence of amino acids, just a generally hydrophobic sequence at the N terminus that is first translated. Translation will pause, and the ribosome will bind to a receptor at the RER membrane. Once the polypeptide–ribosome complex is bound, translation will resume, and as elongation continues, the protein will traverse the RER membrane. Such proteins may be retained in the lumen (the inside of the RER) or in membrane of the RER, or they may move elsewhere within the endomembrane system (Golgi apparatus, lysosomes, and cell membrane). If the proteins lack specific signals or modifications (see below) that specify destinations within the endomembrane system, they are usually secreted from the cell via vesicles that fuse with the cell membrane.

The importance of signals is shown by Inclusion-cell (I-cell) disease, an inherited disease that causes death in early childhood. People with this disease have a mutation in the gene encoding a Golgi enzyme that adds specific sugars to proteins destined for the lysosomes. These sugars act like signal sequences; without them, enzymes that are essential for the hydrolysis of various macromolecules cannot reach the lysosomes, where the enzymes are normally active. Without these enzymes, the macromolecules accumulate in the lysosomes, and this lack of cellular recycling has drastic effects, resulting in early death.