Review3 - http:/www.usc.edu/si/ Supplemental Instruction...

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http://www.usc.edu/si/ Biochemistry Supplemental Instruction Nathalie Petein Stefan Shuaib SI Worksheet Midterm #3 Review 1. The endoplasmic reticulum is the site of membrane protein synthesis and folding . It is also the site of modification of proteins. The golgi apparatus is where sugars are made and linked to starches or lipids or proteins. The lysosome has an acidic pH and degrades debris and recycles cell contents. The nucleus has pores on its surface that point out of the double membrane and allows mature mRNA out and specific proteins in. The peroxisome breaks down and detoxifies various molecules. 2. The N-terminal signal sequences involved in protein targeting (translocation) to membranes fall into two classes: hydrophobic and amphipathic. a) Briefly illustrate how hydrophobic signal sequences target proteins to Rough Endoplasmic Reticulum (RER) membranes. Hydrophobic signal sequences contain 1 or more basic amino acids followed by 6-12 hydrophobic residues that are recognized by SRPs during translation. The SRPs prevent translation temporarily until they bind SRP receptor proteins on the ER. Once bound, translation resumes and the protein moves through a channel in the ER (translocon) into the lumen where the signal sequence may now be removed by proteolytic cleavage post-translationally. b) Briefly illustrate how amphipathic signal sequences target proteins to mitochondrial or chloroplast membranes. Additionally, explain what TPRs are. Amphipathic signal sequences of 10-70 residues form alpha helices with positive charges on one side and neutral charge on the other side. After translation, protein folding is prevented by association with Hsp70. Then, the signal sequences bind to mitochondrial outer membrane translocon (TOM). This occurs by association with a complex of Tom70 proteins that have TRP (tetratricopeptide repeat) domains which allow them to form a large homocomplex with a peptide binding groove. If the proteins are meant to stay in the mitochondrial membrane, they are moved to the SAM (sorting and assembly) complex. If meant for the inner membrane, the protein flows through the TOM complex and is taken up by Tim22 to the inner membrane. If it is meant for the
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This note was uploaded on 05/03/2010 for the course BISC 330L taught by Professor Petruska,tower during the Spring '07 term at USC.

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Review3 - http:/www.usc.edu/si/ Supplemental Instruction...

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