cell bio exam 2 - L-51: Posttranlational Modification And...

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(659-669) 1.Describe the differences between a signal sequence and a signal patch. Both signal sequences and signal patches direct proteins to the correct cellular address Signal sequence: short continuous sequence of amino acid that determines the eventual location of a protein in the cell. (An example is the N-terminal sequence of 20 or so amino acids that directs nascent secretory and transmembrane proteins to the endoplasmic reticulum.) are used to direct proteins from the cytosol into the ER, mitochondria, choloroplasts, and microsomes…also used to transport proteins from the nucleus to the cytosol and from the Golgi apparatus to the ER Each signal sequence specifies a particular destination in the cell Are removed from the finished protein by specialized signal peptidases Signal Patch: protein sorting signal that consists of a specific 3D arrangement of atoms on the folded protein’s surface. Far more difficult to analyze than signal sequences, so less is known about their structure Often result from a complex 3D protein folding pattern, and they cannot be easily transferred experimentally from one protein to another *Both sorting sinals are recognized by complementary sorting receptors that guide proteins to their appropriate destination, where the receptors unload their cargo. 2.You discover a protein you think is normally found in the nucleus. How can you determine A.)If it has a nuclear targeting sequence If it is experimentally extracted from the nucleus and reintroduced into the cytosol, even large proteins will accumulate efficiently into the nucleus. B.) Examining the amino acid sequence of your protein, how would you identify potential nuclear targeting sequences? If you were to compare the amino acid sequences of proteins that are found in the nucleus and find an amino acid sequence that they have in common…signal sequence of proteins having the same destination are functionally interchangeable, physical properties such as hydrophobicity often seem to be more important in signal recognition process than the exact amino acid sequence.
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C.) Design an experiment that will test to see if your candidate signal sequence is necessary for nuclear import. If you were to remove the particular amino acid sequence that you though was the signal sequence and the protein was no longer transported to the nucleus, you would know that this was the signaling sequence. D.) Design an experiment that will test to see if your candidate sequence is sufficient. Transfer of targeting signal into another protein, it should go into the nucleus…also mutation and removal of a the targeting sequence, the protein should no longer go to the nucleus. To figure out whether the candidate sequence was sufficient you could take that particular sequence and insert it into a protein that does not have a nuclear targeting signal, and if it ended up being moved to the nucleus you would know that the candidate sequence is indeed sufficient. E.) What would you conclude about the nature of the targeting signal if you found
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This note was uploaded on 04/06/2008 for the course MCB 410 taught by Professor Ward during the Spring '08 term at University of Arizona- Tucson.

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cell bio exam 2 - L-51: Posttranlational Modification And...

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