Translocation, Sec61, Integration of Integral Membrane Proteins, Start & Stop Transfer Regions

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University of Massachusetts Amherst
Biochemistry & Molecular Bio.
David Gross

March 26 • Translocation o protein-conducting channels of ribosome & Sec61 channel align when complexed to facilitate direct translocation of newly synthesized polypep into ER lumen o The protein translocator channel in the ER membrane is the Sec61 channel • The Sec61 Translocon Channel Contains o multiple transmembrane helices that form aqueous channel crossing membrane o hour class shape, prevents indiscriminate transport of solutes across ER mem. o central plug to prevent leakage in absence of protein translocation o the signal peptide sequence (and when translation begins the newly synthesized polypep chain) displaces the central plug to allow translocation to proceed o Sec61 opens in 2 ways:  transversely to allow polypeptide translocation  laterally to allow contact w/ the lipid bilayer • Evolutionary relation btwn Euk & bacteria o the ER translocation in Euk & protein secretion in bacteria evolutionarily related o major difference between co- & post-translocation systems is role of molecular chaperones & targeting factors in providing driving force for protein transport o movement of a polypep. requires hydrolysis of 650-700ATP molecs. o bacteria doesn’t really use the molec. chaps. • Protein targeting to ER o ER is entry point for vast majority of luminal & integral membrane proteins of endomembrane system, plasma membrane, & for proteins secreted to cell exterior • Targeting & Integration of Integral membrane proteins o only alpha-helical transmembrane proteins found in ER membrane, beta-barrel proteins only found in outer membranes of mitochondria, chloroplast & bacteria o the signal sequence can be @ the N-terminus or internal
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