Translation is the stage of gene expression in which a ribosome reads messenger RNA (mRNA) and builds a chain of amino acids. But a newly made chain is not necessarily a finished, functional protein. After translation, it may need to fold into a specific three-dimensional shape, undergo chemical modifications, reach the correct part of the cell, assemble with other molecules, and pass quality-control checks. Eventually, like other cellular components, it may be broken down and its amino acids recycled.
What happens next depends on the protein. Some proteins become functional almost immediately after synthesis, while others undergo several processing and transport steps before they can do their jobs.
The new protein begins as a chain of amino acids
During translation, the ribosome links amino acids together in the order specified by the mRNA. The result is a polypeptide, a linear chain with a defined amino acid sequence.
That sequence contains much of the information needed to determine how the protein will function. As the chain emerges from the ribosome, chemical interactions among its amino acids begin to drive it toward a particular three-dimensional structure. In many cases, folding starts before translation is even complete.
A protein’s final shape matters because its structure determines how it interacts with other molecules. An enzyme, for example, must form an appropriate active site to catalyze a chemical reaction. A membrane protein must have the right arrangement of hydrophobic and hydrophilic regions to reside properly in a membrane.
Folding turns the chain into a functional structure
Protein folding is the process by which a newly synthesized polypeptide adopts its functional three-dimensional conformation. The process is influenced by the amino acid sequence, the cellular environment, and interactions with other molecules.
Some proteins can fold on their own. Others require assistance from proteins called molecular chaperones. Chaperones do not usually determine the final shape of a protein; instead, they help newly synthesized or damaged proteins avoid inappropriate interactions and reach or regain suitable conformations.
Folding is not always successful. Proteins can become misfolded or form abnormal aggregates. Because misfolded proteins can interfere with normal cellular processes, cells have quality-control systems that recognize and manage them.
Some proteins are chemically modified after translation
Cells can alter proteins after they are synthesized through a process called post-translational modification. These modifications can change a protein’s activity, stability, location, interactions, or lifetime.
One common modification is phosphorylation, in which a phosphate group is added to particular amino acids. Phosphorylation can activate or inhibit proteins and is an important way cells regulate signaling pathways.
Other proteins are modified by the addition of carbohydrates, lipids, methyl groups, acetyl groups, or other chemical groups. Some proteins are also cleaved into smaller pieces to become functional. For example, certain proteins are produced initially as inactive precursors and are activated only after specific peptide bonds are cut.
Post-translational modifications are not simply finishing touches. In many cases, they are essential parts of how a protein is regulated and directed to the right place.
The cell directs proteins to their proper locations
A protein cannot perform its normal function if it ends up in the wrong part of the cell. Newly synthesized proteins therefore contain information that can help determine where they should go.
Some proteins remain in the cytosol, the fluid portion of the cell. Others are directed to organelles such as the nucleus, mitochondria, or peroxisomes. Proteins destined for secretion or for many cellular membranes enter the secretory pathway, beginning with the endoplasmic reticulum.
The endoplasmic reticulum is especially important for proteins that will be secreted, inserted into certain membranes, or delivered to parts of the endomembrane system. These proteins can undergo folding and chemical modification as they move through the endoplasmic reticulum and later through the Golgi apparatus.
Targeting signals can therefore be thought of as molecular instructions that help the cell distinguish where a protein belongs. The signal may be a short sequence within the protein or a structural feature recognized by cellular machinery.
Proteins may assemble with other proteins
Not every functional protein operates as a single polypeptide chain. Some proteins work as complexes made of two or more subunits.
After synthesis, individual protein chains may associate with one another to form these larger structures. Proper assembly can be necessary for activity, stability, or regulation.
A protein may also interact with nonprotein components. Some enzymes require metal ions or small organic molecules called cofactors. These components can help create the chemical environment needed for the protein to function.
Quality control determines whether a protein is usable
Cells continually monitor newly synthesized proteins. A protein that folds and assembles correctly can enter the cellular pool of functional molecules. A protein that fails to meet quality-control standards may be refolded, retained in a particular compartment, or targeted for destruction.
This quality control is especially important in the endoplasmic reticulum, where proteins entering the secretory pathway are monitored before they proceed further through the cell.
Protein degradation is not merely a response to mistakes. Cells deliberately destroy many normal proteins when they are no longer needed, when their activity must be stopped, or when their levels need to be tightly regulated.
Proteins eventually have to be broken down
Proteins do not remain intact indefinitely. Cellular systems continuously remove proteins through regulated degradation pathways.
One major system is the ubiquitin-proteasome pathway. In this process, certain proteins are tagged with ubiquitin, a small protein that can serve as a signal for degradation. The tagged protein can then be recognized and broken down by the proteasome, a large molecular complex that dismantles proteins into smaller peptides.
Cells also use lysosomes and related pathways to degrade proteins and other cellular material, particularly material delivered through specific forms of cellular trafficking or recycling.
The resulting amino acids can be reused to synthesize new proteins or contribute to other cellular processes.
The fate of a protein depends on its identity
There is no single sequence of events that every protein follows after translation. A typical soluble protein might fold, acquire a modification, interact with other proteins, perform its function, and eventually be degraded. A secreted protein may instead enter the endoplasmic reticulum, undergo additional processing, move through the Golgi apparatus, and be released from the cell. A membrane protein may be inserted into a membrane and transported to a particular cellular compartment.
The key point is that translation produces a polypeptide, not necessarily a finished protein. After the amino acid chain is built, the cell must ensure that it acquires the right structure, modifications, location, interactions, and lifetime. These post-translational processes allow the same basic act of protein synthesis to produce the enormous variety of functional proteins required by living cells.
