BIO 1140 Lecture Notes - Lecture 16: Signal Peptidase, Translocator Protein, Transmembrane Protein

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BIO1140 B Dr. Caroline Petit-Turcotte
Protein Sorting (Lecture) 03.13.17
Co-Translation
Proteins to be Secreted ribosomes assemble with the membrane on the ER to translate transmembrane proteins
to allow them to span the membrane and stay there, a vesicle forms and delivers the protein to where it needs to
go
soluble proteins that remain in cytosol are translated using free cytosolic ribosomes; so proteins that
need to function in the membrane will translate as part of the membrane (transmembrane proteins)
N-terminal of polypeptide extending out of the ribosome are the signal sequence (aka signal peptide; 20-
30 aas at N-terminal) i ost ases, the ill e leaed o’t e pat of fial potei; tue fo
proteins that need to be matured through ER
o Ctosoli poteis o’t e atued though ER, ad golgi to e distiuted
As translation is still going on, signal peptide is recognized by signal recognition particle (SRP; protein-
RNA complex) SRP are looking for specific combos of aas to let the SRP to know where it needs to go
SRP halts (pause) translation to dock the ribosome to SRP receptor and line up the ribosome with the
translocator protein so that when translation resumes
the polypeptide can be translated across the membrane (to be entirely soluble in the membrane of the ER
lumen), or translocation can be interrupted to finish translation to get a transmembrane protein
o when SRP docks to its receptor it hands over the signal sequence to a portion of the translocation
protein signal peptidase; it remains bound until everything is in place to resume translation
as translation continues, signal peptidase (anchored to translocation protein) forces the
peptide to grow into the rough ER
mRNA and ribosome is on exterior of ER, peptide chain enters ER
when enough of the peptide has made it across the translocation protein, the signal peptidase cleaves the
peptide bond, the signal sequence remains attached the translocator protein
when stop codon is reached, polypeptide is completely in the ER lumen; the ribosomal subunits dissociate
o translocator protein spans the membrane and creates a favorable environment for the protein to
span the membrane creates pore for peptide to get across into ER
these proteins will then be packaged in vesicles to be sent to the golgi for more processing and sent to be
secreted
Transmembrane Proteins same process but additional steps; when translation
resumes, as the polypeptide is being translated across the membrane, there is stop
transfer sequence in polypeptide that will be recognized by the translocator complex
signal peptide is recognized by SRP, ribosome is aligned with translocator
complex, peptide is handed off to signal peptidase to anchor it, translation
continues; but within the sequence of the peptide, there is a stop transfer
sequence that the translocator complex will recognize to stop transferring the
peptide across the membrane; single co-translation, transmembrane protein
spans membrane once
o translation makes it all the way to the end of the stop codon on mRNA; stop transfer sequence
doesn’t interfere with translation, interferes with translocation
the finished product is a transmembrane protein once signal peptidase cleaves signal sequence,
translation is finished, and the ribosome has disassembled due to release factor
o i.e single transmembrane protein enzyme receptors, ion channels
o transmembrane proteins that span the membrane more than once (i.e G-coupled protein
receptors) will have multiple stop transfer sequences and additional signal sequences to restart
translocation across membrane; snake across membrane
Chaperon Proteins: assist a newly formed polypeptide/protein to fold into its final three-dimensional shape; small
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