Concept 10.5: Proteins Are Modified after Translation

The site of a polypeptide’s function in the cell may be far away from its point of synthesis at the ribosome. This is especially true for eukaryotes, where a polypeptide may be moved into an organelle. Furthermore, polypeptides are often modified by the addition of new chemical groups that contribute to the function of the mature protein. In this section we examine these posttranslational aspects of protein synthesis.

Signal sequences in proteins direct them to their cellular destinations

Protein synthesis always begins on free ribosomes floating in the cytoplasm, and the “default” location for a protein is the cytosol. As the polypeptide chain emerges from the ribosome it may simply fold into its three-dimensional shape and perform its cellular role in the cytosol. However, a newly formed polypeptide may contain a signal sequence (or signal peptide)—a short stretch of amino acids that indicates where in the cell the polypeptide belongs. Proteins destined for different locations have different signals.

In the absence of a signal sequence, the protein will remain in the same cellular compartment where it was synthesized. Some proteins, however, contain signal sequences that “target” them to the nucleus, mitochondria, plastids, or peroxisomes (FIGURE 10.19, LEFT). A signal sequence binds to a specific receptor protein at the surface of the organelle. Once it has bound, a channel forms in the organelle membrane, allowing the targeted protein to move into the organelle. For example, here is a nuclear localization signal (NLS):

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

The function of the NLS was established using experiments like the one illustrated in FIGURE 10.20. Proteins with or without this peptide were introduced into cells and then located by labeling the proteins with fluorescent dyes. Only proteins with the nuclear localization signal were found in the nucleus.

Figure 10.19: 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 directed. Once the protein has bound to it, the protein enters the organelle through a channel in the membrane.

Investigation

HYPOTHESIS

A nuclear localization signal is necessary for importing a protein into the cell nucleus.

Figure 10.20: Testing the Signal A series of experiments were used to test whether a nuclear localization signal (NLS) sequence is all that is needed to direct a protein to the nucleus.a

CONCLUSION

An NLS is essential for nuclear protein import and will direct a normally cytoplasmic protein to the nucleus.

Go to LaunchPad for discussion and relevant links for all INVESTIGATION figures.

a C. Dingwall et al. 1988. Journal of Cell Biology 107: 841-849.

If a polypeptide carries a particular signal sequence of five to ten hydrophobic amino acids at its N terminus, it will be directed to the rough endoplasmic reticulum (RER) for further processing (FIGURE 10.19, RIGHT AND BOTTOM). 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) or 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 for destinations within the endomembrane system, they are usually secreted from the cell via vesicles that fuse with the cell membrane.

LINK

The endomembrane system and its functions are described in Concept 4.3

Many proteins are modified after translation

Most mature proteins are not identical to the polypeptide chains that are translated from mRNA on the ribosomes. Instead, most polypeptides are modified in any of a number of ways after translation (FIGURE 10.21). These modifications are essential to the final functioning of the protein.

Figure 10.21: Posttranslational Modifications of Proteins Most polypeptides must be modified after translation in order to become functional proteins.

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CHECKpoint CONCEPT 10.5

  • Describe how signal sequences determine where a protein will go after it is made.
  • What are some ways in which posttranslational modifications alter protein structure and function?
  • Describe an experiment you would perform to test a proposed chloroplast-targeting signal sequence. Be specific about the type of cell and the proteins you would use. Describe the results you would expect if the sequence is indeed a chloroplast-targeting signal.

Question 10.2

How do antibiotics target bacterial protein synthesis?

ANSWER Tetracyclines are antibiotics that are effective against some strains of MRSA and many other bacterial infections. They derive their name from the four hydrocarbon rings that are common to this family of molecules. Tetracyclines kill bacteria by interrupting translation (Concept 10.1). They do this by binding noncovalently to the small subunit of bacterial ribosomes (Concept 10.4), where binding changes ribosome structure such that charged tRNAs can no longer bind to the A site on the ribosome (FIGURE 10.22). The specificity of antibiotics for bacterial ribosomes comes from the fact that bacterial and eukaryotic ribosomes have different proteins and RNAs. The target protein for tetracyclines is not present in eukaryotic ribosomes, so these antibiotics disrupt translation in prokaryotes but not in eukaryotes, including humans.

Figure 10.22: An Antibiotic at the Ribosome The antibiotic tetracycline binds to the small subunit of bacterial ribosomes. This causes a change in the structure of the A site, preventing tRNAs from binding, and protein synthesis stops.

Strains of MRSA with resistance to tetracyclines are emerging. The genes that confer resistance to this group of antibiotics are carried on mobile genetic elements such as plasmids, which can move at high frequencies between bacteria, by bacterial conjugation (see Concept 8.4). Some of the resistance genes encode proteins that transfer the tetracyclines out of the cell, whereas others encode proteins that prevent the antibiotics from binding to the ribosomes. These resistance genes present a major challenge, because MRSA can be lethal. To overcome resistance, new antibiotics are being developed and tried. The evolutionary race between genetically caused drug resistance and new therapies continues. In the meantime, health-care providers and the general public are being advised to take precautions to prevent the spread of MRSA.

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