Protein Targeting

Protein targeting refers to the methods cells use to get proteins to the proper location after synthesis. Proteins play a major role in most cellular processes but must be located properly to serve their functions. Knowing how newly synthesized proteins target within cells is essential for understanding pro­tein function.

Proteins are synthesized either in the cytosol or on the endoplasmic reticulum. When synthesized in the cytosol on free ribosomes, most pro­teins diffuse freely until they are bound to a particular substrate or assem­ble into a larger complex. Protein diffusion in the cytosol is usually rapid, so an unbound protein is capable of diffusing across the cell in only a few seconds.

One way cytosolic proteins are targeted within cells is by forming large macromolecular assemblies. Many proteins can exist either as monomers, which freely diffuse through the cytoplasm, or as polymers, which form large-scale structures that dynamically distribute to distinct locations in the cell. The cytoskeletal proteins actin and tubulin, for example, have a pair of complementary self-binding sites on their surfaces that allow them to poly­merize into long helical filaments that stretch across the cell. These fila­ments form the cytoskeleton of the cell, which reorganizes continuously as the cell changes shape, divides, and responds to its environment.

Conformational changes in a protein often lead to changes in the pro­tein’s affinity toward a particular substrate. This process can play a crucial role in regulating the intracellular localization of a protein. An example of this type of regulation is protein phosphorylation, or addition of a phosphate group. This can dramatically change a protein’s affinity for a substrate and can thereby lead to rapid changes in the protein’s location. This type of reg­ulation of protein localization is crucial for enabling cells to coordinate their activities under different growth conditions and during cell division.

Some cytoplasmic proteins are targeted to a particular site in the cell because they contain a specific amino acid sequence that causes them to bind to receptors located at that site. An example of such a targeting se­quence is the so-called nuclear localization signal (NLS), which consists of two short stretches of basic amino acids divided by a 10-11 amino acid spacer region. This sequence of amino acids allows a protein possessing it to bind to nuclear localization receptors found in the nucleus. Once a pro­tein containing an NLS signal binds to a nuclear receptor it is no longer able to freely diffuse and becomes “localized” to the nucleus.

Many proteins are embedded within or associated with membranes. In eukaryotic cells, membranes form the boundaries of a variety of distinct compartments, including the nucleus, mitochondria, endoplasmic reticu­lum (ER), and Golgi complex. Some proteins are synthesized in the cytosol and are then modified with a lipid “anchor,” and association with mem­branes is simply a matter of embedding in the membrane’s outer lipid layer. Signaling molecules, such as the GTP-binding protein Ras, localize to mem­branes in this manner.

All other proteins that target to intracellular membranes require sort­ing signals that direct their transport from the cytosol. For example, pro­teins targeted to mitochondria contain a specific peptide sequence of 20-80 amino acids that mediates their import. This sequence is found at the amino terminus of the protein and after import is rapidly removed by a protease.

Proteins localized within those membranes involved in endocytosis and exocytoses are first targeted to the ER, and then use membrane transport pathways to reach other compartments. Targeting of proteins to the ER be­gins before the polypeptide chain is completely synthesized. This is in con­trast to import of proteins to mitochondria, chloroplasts, and peroxisomes, which occurs after synthesis is completed. An ER signal peptide, localized at the amino terminus of these proteins, directs the ribosome to attach to the ER membrane before the protein has been completely translated. The ER signal peptide is guided to the ER membrane by a signal-recognition particle (SRP), which binds to the signal peptide, and an SRP receptor in ER membranes.

From the ER proteins use a variety of mechanisms to reach different fi­nal destinations in the cell. Proteins destined for the nuclear envelope sim­ply diffuse there and stick, since the nuclear envelope is in direct continuity with the ER. To reach the Golgi complex, plasma membrane, endosomes, and lysosomes, however, proteins must enter the secretory pathway and use membrane trafficking pathways. For membrane proteins, entry into the secretory pathway is thought to require their concentration and sorting at ER exit sites. In contrast, proteins that are soluble in the ER lumen (inner space) move out by a bulk flow process. After leaving the ER, most soluble proteins are eventually secreted by the cell. Many membrane proteins are directed to specific organelles within the secretory and endocytic pathways because they contain specific sorting signals in their cytoplasmic tails, which function much like zip codes. Alternatively, sorting may be due to proper­ties of a protein’s transmembrane domain, a region of the protein that gives its affinity for different lipid environments characteristic of different or­ganelles.

References

Alberts, Bruce, et al. Molecular Biology of the Cell, 4th ed. New York: Garland Pub­lishing, 2000.