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MGU - ANAT 212 - Class Notes - Week 6

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MGU - ANAT 212 - Class Notes - Week 6

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background image Lecture 7: Membranes 3
Membrane Biogenesis 
• Cellular membranes can only be made by expanding pre- existing membranes, not de novo   • You   need   to   base   it   off   an   organelle,   you   can   trace every   organelle   formation   from   cell   division   back   to
evolution 
• With   very   few   exceptions,   all   proteins   are   encoded   by nuclear genes and translated in the cytosol • All   proteins   are   encoded   in   the   nucleus   and   all   the ribosomes are in the cytosol • Proteins must be sorted during or after translation to their correct compartment or membrane • Sorting information is carried inside the proteins themselves • Information for sorting proteins is within the proteins themselves 
• All proteins  are   being made  in   the  cytosol,   and  they  need  targetting
pathways to move around  Targeting Signals  • The sequence of a protein can encode a signal or signals that specify its organelle localization • Targeting signals: – are often independent from the structure or biochemical function of proteins – recognized by their pattern, but usually not an exact sequence
– may   be   removed   by   proteolysis   after   targeting   is   complete,   or
form part of the native structure  – Sometimes separate from the rest of the protein
– Will be an extra part that will signal localization, independent from
protein sometimes or it is part of final structure – Signals are not exact sequences sometimes, a lot of time there are   patterns   of   amino   acid   properties   instead   of   just   exact
sequences 
– If  the targeting sequence is  independent  and  once  targeting  is done, it can be removed  • Targeting steps:  • recognize a signal on a protein
• connect protein to the membrane
• translocate protein into or across the membrane
1
background image Signal Hypothesis  • Secretory proteins were known to enter ER during their translation
• Observation: a newly translated secretory protein is larger than its final
form • Hypothesis: extra sequence is a targeting signal peptide whose main function is to direct insertion into ER – signal   peptide   must   start   mechanism   to   connect   ribosome   to translocation pore – signal peptide is cleaved off after targeting is finished – If you look at protein in secretory pathway in its newly translated form, the original form is larger than the new form, so there is a
piece   of   the   proteins   that   are   missing   (which   is   the   targeting
sequence)
– He predicted there was a pore in the membrane too
– There needs to be different part of pathways à he came up with
this  Secretory Signal Sequences  • Secretory signal sequences have a typical pattern of amino acids
• Hydrophobic central region 8 or more residues long, with short polar
regions on each side • In many cases, signal sequences are at the N-terminus, makes sense because you can target the protein as its being made by the ribosomes • They   all   need   to   start   in   the   ER   and   they   all   need   the   secretory sequences • Signal sequences only have a purpose of targetting proteins  – shorter hydrophobic regions (8-16 residues) often cleaved off after translocation  into the membrane  • In   other   cases,   signal   sequences   can   be   in   different   places   in   the protein and become TM helices (not cleaved off) – longer hydrophobic region (18-24 residues)
– termed signal anchors
Signal Sequence Targeting  • Targeting steps:  1. recognize a signal on a protein • ribosome begins translating polypeptide with a signal 2
background image • Signal Recognition Particle (SRP) is a soluble protein that binds signal and ribosome during translation 2. connect protein to the membrane • SRP Receptor (SRP-R) is a membrane protein that binds the ribosome-SRP complex,  INTEGRAL MEMBRANE PROTEIN 
• SRP-R links ribosome to Sec61 translocon pore in ER
3. translocate protein into or across the membrane • energy   of   translation   on   ribosome   drives   polypeptide through the translocon • Ribosome will keep translating the protein through the pore (aqueous pore)  Ribosome Exist Tunnel  • Nascent   polypeptides   exit   the   ribosome through a tunnel in the large (60s) subunit Tunnel is neutral, polar, too small for tertiary
folding, 
possibility of alpha helix  • Surface   around   exit   site   provides   binding sites for ER targeting mechanisms • 30   to   40   amino   acids   of   nascent   polypeptide   between   peptidyl- transferase site and the exit • You have small and large subunit
• Nascent polypeptide will go through large subunit
SRP • Ribonucleoprotein:   6   protein   subunits   and   1 RNA • Signal   sequence   recognition   subunit   with GTPase activity • Translation regulatory domain at the opposite end • RNA strand forms flexible linker • End, you have domain which recognizes signal sequence 3

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School: McGill University
Department: OTHER
Course: MOLECULAR MECHANISM OF CELLULAR FUNCTION
Professor: Jason Young
Term: Spring 2018
Tags:
Name: anat 212
Description: anat 212 notes
Uploaded: 03/20/2018
9 Pages 27 Views 21 Unlocks
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