lec 7- protein synthesis Flashcards
what was done to see the path of proteins from synthesis to discharge?
a radioactively labelled amino acid (pulse) was added to cells, and was used in protein synthesis, then it was washed out and a solution with no label was added to examine where the label is. (pulse-chase)
what system has been used to identify roles of proteins involved in membrane trafficking?
cell-free systems
what are liposomes?
vesicles with a phospholipid bilayer
when are buds and vesicles produced?
They are formed after liposomes are incubated with purified proteins normally on coats on the cytosolic surface of transport vesicles in the cell
what has a network of membranes that penetrate much of the cytoplasm?
the ER
what part of the ER has ribosomes and doesn’t have ribosomes?
rough ER has and smooth ER doesn’t
what is continuous with the outer membrane of the nuclear envelope?
The ER
what are the two locations where proteins are synthesized?
one third at the RER and the rest are synthesized at ribosomes in cytosol
where does protein synthesis begin and where do they end up?
begins on ribosomes in the cytosol and ends up in an organelle, in the plasma membrane, be secreted, remain in the cytoplasm
what is co-translational translocation and post-translational translocation?
co-translational translocation- transport of most secretory proteins into the ER lumen begins while the protein is still being synthesized on the ribosome
post-translational translocation- translocation that takes place after protein is synthesized
what do the proteins in translocation have?
have a signal sequence
what is SRP?
a signal recognition particle that binds to signal sequence on nascent proteins and large ribosomal units
what happens after ribosome binds to SRP?
it is handed to the translocon (a protein channel embedded in the ER membrane). Upon attachment, the signal sequence is recognized and the polypeptide is inserted into the translocon, the signal sequence is cleaved off by signal peptidase and degraded, translocation occurs until protein is released into lumen, proteins are being modified by folding, disulfide bond formation and glycosylation
what part of the amino acid sequence stops the translocation (movement of protein across membrane)?
the stop-transfer sequence, translocation stops but translation continues
what can the orientation of a single-spanning membrane proteins be?
it can have the N-terminus facing towards the lumen of the ER or the cytosol
what do proteins produced in membrane-bound ribosomes become?
glycoproteins
how do the carbohydrate groups help proteins?
act as macromolecule binding sites and aid in protein folding and stabilization
how is glycosylation of proteins done?
sugars are added to an oligosaccharide which are then catalyzed by glycotransferases, each transfers a specific monosaccharide to the growing end of the carbohydrate chain until a 14 unit “oligosaccharide precursor” is generated and attached to the amide nitrogen of an asparagine residue
how is the oligosaccharide precursor formed?
2 GlcNAc and 5 mannose residues are added to the cytosolic face, sugars are found in the cytosol. The 7 residue oligosaccharide is then flipped to the luminal side, the sugars are then transferred to luminal side by dolichol phosphate
where is the 14 residue oligosaccharide transferred to?
an asparagine residue on the nascent polypeptide in a tripetide sequence of Asn-X-Ser/Thr and further modified
X is any amino acid apart from proline
what happens to misfolded proteins in the ER?
they need to be degraded, degradation generally takes place in the cytosol
what is Dislocation or retrotranslocation?
the movement of misfolded proteins from the ER to the cytosol
what is ERAD proteins?
ER associated degredation proteins that export improperly folded proteins from the ER to the cytosol for the degredation by proteosomes
which proteins are allowed to leave the ER?
only properly folded proteins, misfolded stay in ER
what is the unfolded protein response (UPR)?
a response where if too many misfolded proteins accumulate in the ER, the cell will make more ER, this mechanism ensures a sufficient amount of properly folded proteins entering the secretory pathway but if too much ER is made apoptosis is initiated
