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Microbial Physiology notes lec26 11-29-10

Microbial Physiology notes lec26 11-29-10 - Microbial...

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Microbial Physio Lecture 26: Protein secretion system:Structure and Evolution (contd) November 29, 2010 1. a. SecA and SecB are the 2 proteins that are found in the cytoplasm of the bacterial cell i. SecA 1. SecA and an ATpase 2. the ATP is required for secretion, allows the secA protein to insert the 1 st part of the native polypeptide chain into the SecY channel 3. How does SecA actually fxn? a. secY makes up the channel, it’s a 10TMS protein, which arose from duplication of a 5TMS to give 10TMS b. it makes sense that its 10TMS because secY has to both allow proteins that are soluble to go out into the periplasm, and also for the membrane bound proteins, SecY has to open up laterally so that the protein can diffuse into the membrane from the secY complex c. SecA fxns like a syringe, not like the syringe for the type3 secretion systems, but what happens is that a portion of SecA inserts itself into the SecY channel, and w/ it, it brings a part—the N terminal part of the polypeptide chain, and when SecA retracts, the polypeptide chain is going to be inserted/translocated into the membrane and stays there—so SecA releases the polypeptide chain once the SecA domain is present w/in the secY channel d. So obviously the SecY channel has to be flexible as well, because it doesn’t just have to accommodate 1 polypeptide chain, but it also has to accommodate
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part of the SecA protein—the part that is going to facilitate the translocation of the transported polypeptide chain across the membrane ii. SecB 1. chaperone protein that 1 st takes the protein as its being made in the cytoplasm from the ribosome 2. secB keeps this protein unfolded and passes it onto SecA b. SecY channel forms a complex w/ SecE and SecG proteins i. The system doesn’t work at full efficiency if not all 3 are functioning—i.e if 1 of the proteins is deleted ii. Every single cell has all 3 proteins, indicates that they’re really important w/o them, most of the proteins in the cytoplasm couldn’t get out of the IM in order to fxn in the periplasm or in the extracellular space iii. SecY, E, G are ubiquitous
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