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Lecture 6: Endoplasmic Recticulum
I want you to look at every single step as a biochemical equation
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Then if the two molecules that
interact with each other because of their chemical characteristics
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Now the first example we gave is that protein synthesis in the rough
ER and we are talking about a soluble proteins
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Where does it start
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The signal sequence is there, the mRNA and the tRNA
are there
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Signal
recognition particle is a protein that is circulating in the cytosol
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So they bind and the binding
takes place between the signal sequence and the Signal recognition Particle
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The translation pause domain will interact with the ribosome complex
with the effect of stopping the synthesis of the nascent peptide
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It itself has undergone some
conformation changes and this protein has a site available to bind to the SRP receptor
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But these SRP can only bind to the SRP
receptor after it has complexed with the ribosomal complex
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When it is bound you have the signal sequence
bound to the signal sequence binding domain
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You have a site that is open that will allow this complex to bind to the SRP receptor protein
that is present in the ER membrane
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This net binding brings the ribosome close to the
translocator protein
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When the SRP complex is brought close to the ER it is in the right position and I mentioned that the
signal sequence will trigger the opening of a channel in the translocator protein
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There is some details
about the channel
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The binding of the signal sequence also causes the change of the ribosome SRP complex
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When the SRP leaves the ribosome it also changes the
configuration of the ribosome
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This is the reason why the ribosome is staying in place and these several points will anchor the protein in
place
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So after the signal sequence interacts with the translocator protein a channel will be opened that will
allow the nascent protein to work its way through
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I think it is the interaction of the signal sequence with the
translocator protein that will cause it to imbed with the N-terminus facing the lumen
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Signal peptidase will exert its function
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The signal peptidase will cut the nascent protein
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What is
the mechanism that will help us to move the protein into the lumen
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Question: you said that the nascent peptide goes in with the N-terminus? Yes, always N-terminus
because I want it to be loud and clear
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So the example I had used was that
of a soluble protein, the protein that is made in lumen of the ER
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So you have your membrane can oriented in a way where you have
the amino group on the side and the carboxylic end on the outside, this is how it usually is because the
amino group comes through first
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Membrane protein
The bottom line is that the formation of a protein with a single helix with the cooh outside and the Nterminus inside, so how this formed? With the new peptide being synthesized imbedded in this peptide
sequence is this stop transfer sequence
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” There is no more peptide will be pushed through any more
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As it pushes through it continues to grow
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So what happens when it stops? Signal peptides will clip this off so that on
the lumen side you have a sequence with N-terminus, but translation will continue in the ribosome and
finish the rest of the protein with C-terminus in the ctyosol
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Lets look at another situation where you have an internal signal sequence, which means that
the signal sequence is not at the beginning of the peptide but it is imbedded
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If the order of the signal sequence is as such that the start side enters and it is closer
to the NH2 side…
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The key is which part of the signal sequence starts getting imbedded in the ER
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The key thing is the direction of the signal sequence, which amino acid is the first one that is
imbedded in the membrane
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How do you make a protein like this? In the nascent
peptide there are two important sequence
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Then following it has a stop
transfer sequence
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So it is a combination of a stop transfer sequence and a stop
transfer sequence that these proteins are imbedded
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You have a transmembrane protein that transverses the membrane many times
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If you stretch out the amino acid sequence, you have the start and the stop signal sequence
that is responsible for the first group and then it stops and starts and stop and start again
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In the cytosol you have a lot of polysomes, which are kind of like molecular beads
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The ribosomes on these polysomes can process a single mRNA and it can translate
multiple peptide
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This is a membrane protein, in this membrane protein you have usually a
sequence of asparagines and it is followed by serine or therione, both of these amino acids have a
hydroxyl group, And you have a complex of carbohydrates attached at the asparagines side chain but
these signals of asparagines, serine or therione
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This is a very important carbohydrate complex? But
how does this carbohydrate complex get on a membrane protein? Well in the ER membrane this
carbohydrate complex is attached to a lipid in the ER called Dolichol
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So when you have a new
peptide being translated, the asparagines and this enzyme called oligiosaccharide transferase will
transfer this whole polysaccharide complex and link it to asparagines
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