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-The sequence of a protein that directs the protein protein to a particular organelle-
"adress label" indicating where the poly pep belongs. |
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2 sets of instructions in the Amino Acid sequence can lead to |
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Complete Translation: reased to an organelle ( nucleus, mitochondria, plastids, or peroxisomes.) or stay in cytosol by default
Stop Translation: go to ER and finish translation. can remain in ER lumen or be sent to Golgi ap to be distributed. if they lack specific codes they can be secreted from the cell by vesicles.
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Destinations for Newly Translated Polypeptides in a Eukaryotic Cell
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Stephen Dilworth and colleagues at the University of Cambridge
Test Signal Sequences
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The experiments involved the nuclear protein nucleoplasmin, the cytoplasmic protein pyruvate kinase. For example, the putative nuclear signal was removed from nucleoplasmin, which normally carries it, or attached to pyruvate kinase, which does not normally carry it. The result was that it did not matter where in the cell the protein normally resided. If it had the signal, it went to the nucleus and if it did not have the signal, it stayed in the cytoplasm. |
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Signal sequence experiement |
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-A receptor protein that binds (docks) a ribosome to the membrane of the endoplasmic reticulum by binding the signal sequence attached to a new protein being made at the ribosome.-
forms a channel in the membrane of an organelle, allowing the signal-bearing protein to pass through the membrane and enter the organelle. In this process, the protein is usually unfolded by a chaperonin protein so that it can pass through the channel; then it refolds into its normal conformation |
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| Signal Recognition Particle |
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composed of protein and RNA
This binding (to ribosome) blocks further protein synthesis until the ribosome becomes attached to a specific receptor protein in the membrane of the rough ER. |
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is the cutting of a polypeptide chain. Cleavage of the signal sequence from the growing polypeptide chain in the ER is an example of proteolysis; the protein might move back out of the ER through the membrane channel if the signal sequence were not cut off. Some proteins are actually made from polyproteins (long polypeptides) that are cut into final products by enzymes called proteases.
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is the addition of sugars to proteins to form glycoproteins. In both the ER and the Golgi apparatus, resident enzymes catalyze the addition of various sugars or short sugar chains to certain amino acid R groups on proteins.
Other types are important in the conformation of proteins and their recognition functions at the cell surface. Other attached sugars help to stabilize extracellular proteins, or proteins stored in vacuoles in plant seeds. |
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