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Nov 11, 2019 · The translational science spectrum represents each stage of research along the path from the biological basis of health and disease to interventions that improve the health of individuals and the public. The spectrum is not linear or unidirectional; each stage builds upon and informs the others.
- Kazuwa Nakao
- 10.2183/pjab.95.037
- 2019
- 2019/11/11
- Overview
- Introduction
- Overview of cellular shipping routes
- The endomembrane system and secretory pathway
- Signal peptides
- Transport through the endomembrane system
- Targeting to non-endomembrane organelles
How molecular labels are used to direct proteins to different parts of the cell (and to the cell exterior).
•Proteins are fed into the ER during translation if they have an amino sequence called a signal peptide. In general, proteins bound for organelles in the endomembrane system (such as the ER, Golgi apparatus, and lysosome) or for the exterior of the cell must enter the ER at this stage.
Different proteins need to be sent to different parts of a eukaryotic cell, or, in some cases, exported out of the cell and into the extracellular space. How do the right proteins get to the right places?
Cells have various shipping systems, kind of like molecular versions of the postal service, to make sure that proteins arrive at their correct destinations. In these systems, molecular labels (often, amino acid sequences) are used to "address" proteins for delivery to specific locations. Let’s take a look at how these shipping systems work.
Translation of all proteins in a eukaryotic cell begins in the cytosol (except for a few proteins made in mitochondria and chloroplasts). As a protein is made, it passes step by step through a shipping "decision tree." At each stage, the protein is checked for molecular tags to see if it needs to be re-routed to a different pathway or destination.
The first major branch point comes shortly after translation starts. At this point, the protein will either remain in the cytosol for the rest of translation, or be fed into the endoplasmic reticulum (ER) as it is translated2 .
•Proteins are fed into the ER during translation if they have an amino sequence called a signal peptide. In general, proteins bound for organelles in the endomembrane system (such as the ER, Golgi apparatus, and lysosome) or for the exterior of the cell must enter the ER at this stage.
•Proteins that do not have a signal peptide stay in the cytosol for the rest of translation. If they lack other "address labels," they'll stay in the cytosol permanently. However, if they have the right labels, they can be sent to the mitochondria, chloroplasts, peroxisomes, or nucleus after translation.
Proteins destined for any part of the endomembrane system (or the outside of the cell) are brought to the ER during translation and fed in as they're made.
The signal peptide that sends a protein into the endoplasmic reticulum during translation is a series of hydrophobic (“water-fearing”) amino acids, usually found near the beginning (N-terminus) of the protein. When this sequence sticks out of the ribosome, it’s recognized by a protein complex called the signal-recognition particle (SRP), which takes the ribosome to the ER. There, the ribosome feeds its amino acid chain into the ER lumen (interior) as it's made.
In some cases, the signal peptide is snipped off during translation and the finished protein is released into the interior of the ER (as shown above). In other cases, the signal peptide or another stretch of hydrophobic amino acids gets embedded in the ER membrane. This creates a transmembrane (membrane-crossing) segment that anchors the protein to the membrane.
In the ER, proteins fold into their correct shapes, and may also get sugar groups attached to them. Most proteins are then transported to the Golgi apparatus in membrane vesicles. Some proteins, however, need to stay in the ER and do their jobs there. These proteins have amino acid tags that ensure they are shipped back to the ER if they "escape" into the Golgi.
In the Golgi apparatus, proteins may undergo more modifications (such as addition of sugar groups) and before going on to their final destinations. These destinations include lysosomes, the plasma membrane, and the cell exterior. Some proteins need to do their jobs in the Golgi (are "Golgi-resident), and a variety of molecular signals, including amino acid tags and structural features, are used to keep them there or bring them back3 .
If they don't have any specific tags, proteins are sent from the Golgi to the cell surface, where they’re secreted to the cell exterior (if they’re free-floating) or delivered to the plasma membrane (if they’re membrane-embedded). This default pathway is shown in the diagram above for a membrane protein, colored in green, that bears sugar groups, colored in purple.
Proteins are shipped to other destinations if they contain the right molecular labels. For example, proteins destined for the lysosome have a molecular tag consisting of a sugar with a phosphate group attached. In the Golgi apparatus, proteins with this tag are sorted into vesicles bound for the lysosome.
Proteins that are made in the cytosol (don't enter ER during translation) may stay permanently in the cytosol. However, they may also be shipped to other, non-endomembrane destinations in the cell. For instance, proteins bound for the mitochondria, chloroplasts, peroxisomes, and nucleus are usually made in the cytosol and delivered after translation is complete.
[Don't mitochondria and chloroplasts have their own ribosomes?]
To be delivered to one of these organelles after translation, a protein must contain a specific amino acid "address label." The label is recognized by other proteins in the cell, which help transport the protein to the right destination.
As an example, let's consider delivery to the peroxisome, an organelle involved in detoxification. Proteins needed in the peroxisome have a specific sequence of amino acids called a peroxisomal targeting signal. The classic signal consists of just three amino acids, serine-lysine-leucine, found at the very end (C-terminus) of a protein. This pattern of amino acids is recognized by a helper protein in the cytosol, which brings the protein to the peroxisome4 .
Mitochondrial, chloroplast, and nuclear targeting are generally similar to peroxisomal targeting. That is, a certain amino acid sequence sends the protein to its target organelle (or a compartment inside that organelle). However, the nature of the "address labels" is different in each case.
[References]
The process of translation in biology is the decoding an mRNA message into a polypeptide product. Put another way, a message written in the chemical language of nucleotides is "translated" into the chemical language of amino acids.
Jul 30, 2022 · The process of translation is similar in prokaryotes and eukaryotes. Here we’ll explore how translation occurs in E. coli, a representative prokaryote, and specify any differences between prokaryotic and eukaryotic translation.
A release factor (RF) refers to a type of translation factor that triggers translation termination. Release factors fall into two classes; Class I release factors that bind the ribosome in response to the presence of a stop codon within the ribosomal A-site. Thus it binds with A site (not P site).
- No. The ribosome is the workbench and the mRNA is being moved through the ribosome shifting downwards every codon.
- IF1, Binds to the 30S subunit to the A site, prevents tRNA binding IF2, Binds initiator tRNA (f-Met) and controls its entry to the ribosome at the...
- They don't get translated! The 5' UTR is everything 5' of the start codon. The interesting question is how does the ribosome know which start codon...
- It costs 4n high-energy bonds to make a peptide chain. n= the number of amino acids in the chain. For example: How many high-energy phosphate bonds...
- Great question! The tRNA is released into the cell and can again be joined with an amino acid. (Details on the joining are in the previous section.)
- They don't. Polypeptides frequently start with a methionine for reasons that are discussed in this article ... However, the beginnings of many poly...
- Yes, it is. It cannot be considered start codon if it is not AUG at the beginning of first exon. What might confuse is that it does not have to be...
- There are two things that could happen to it. It could go back to the nucleus to be reused, or, if it is detected to have outlived its usefulness,...
People also ask
What is the process of translation in biology?
Which molecule contributes to the process of translation?
How does translation end?
What is translational science?
Step 1: transcription! Here, the DNA sequence of a gene is "rewritten" in the form of RNA. In eukaryotes like you and me, the RNA is processed (and often has a few bits snipped out of it) to make the final product, called a messenger RNA or mRNA. Step 2: translation!
May 26, 2014 · Translation complex profile sequencing (TCP-seq) is a major technological step forward for analysis of mRNA translation. Unlike ribosome profiling, TCP-seq can be used to study all stages of ...
