Wk 9(8) Cell Bio

winniesmith's version from 2017-01-25 22:02

Section 1

Question Answer
what are the golgi apparatus and endoplasmic reticulum both examples ofmembrane-enclosed organelles
what do membrane-enclosed organelles enableefficient functioning of cells through sorting and targeting of proteins- vesicular transport.
What is the ERa network of membrane-enclosed tubules and sacs (cisternae)
What are 3 domains of ERRough ER, Transitional ER, Smooth ER
Rough ERcovered by ribosomes, functions in protein processing
Transitional ER where vesicles exit to Golgi apparatus
Smooth ERhad no ribosomes and is involved in lipid metabolism
How was the role of the ER in protein processing and sorting first demonstrated by Palade&colleagues in 1960s. Studied pancreatic acinar cells that secrete digestive enzymes into the small intestine. Newly synthesized proteins were labeled with radioisotopes. Role of ER determined by tracking the secretion of radiolabelled proteins by pancreatic cells
Define the secretory pathwayRough ER--> Golgi--> secretory vesicles--> cell exterior

Section 2

Question Answer
what is the cellular structure of ribosomescomposed of RNA and proteins responsible for protein synthesis
what is co-translational translocationProteins move into theER during their synthesis on membrane-bound ribosomes.(main pathway in mammalian cells)
what is post-translational translocationProteins move into the ER after translation has been completed on free ribosomes (more common in yeast)
How are ribosomes targeted to the ERby a signal sequence at the amimo terminus (which is removed when the growing polypeptide chain enters the ER)
What is a signal sequence made up of a stretch of approx.20 hydrophobic amino acids usually located at the amino terminus of the polypeptide chain
What is step 1 of the ER co-translational pathwayAs the polypeptide chain emerges from the ribosome, signal sequence is bound by a signal recognition particle (SRP).
what do SRPsconsist of six polypeptides and a small cytoplasmic RNA (SRP RNA
what is step 2 of the ER co-translational pathwaySRP binds the ribosome as well as the signal sequence, inhibiting further translation and targeting the entire complex (the SRP, ribosome, and growing polypeptide chain) to the rough ER by binding to the SRP receptor on the ER membrane Step
what is step 3 of the ER co-translational pathwayBinding to the receptor releases the SRP from both the ribosome and the signal sequence of the growing polypeptide chain. The ribosome then binds to a protein translocation channel (translocon) in the ER membrane, and the signal sequence is inserted into a membrane channel
what is step 4 of the ER co-translational pathwayTransfer of the ribosome from the SRP to the translocation complex allows translation to resume, and the growing polypeptide chain is transferred directly into the translocon and across the ER membrane as translation proceeds.
what is step 5 of the ER co-translational pathwayAs translocation proceeds, the signal sequence is cleaved by signal peptidase and the polypeptide is released into the lumen of the ER

Section 3

Question Answer
What is step 1 of the post-translation pathwayPolypeptides synthesized on free ribosomes have signal sequences recognized by receptor proteins on the translocon. Hsp70 and Hsp40 chaperones keep the polypeptide chains unfolded so they can enter the translocon.
what is step 2 of the post-translational pathwayAnother Hsp70 chaperone in the ER (BiP) acts as a ratchet to pull the polypeptide chain through the channel and into the ER.
How are proteins destined to be incorporated into membranes transportedInitially inserted into the ER membranes.They are transported along the secretory pathway as membrane components rather than as soluble proteins.
What is step 1 of insertion of proteins into the ER membrane (1)The signal sequence is cleaved by signal peptidase during translocation, leaving the amino terminus in the ER lumen.Tranlocation halts at a stop-transfer sequence
what is step 2 of insertion of proteins into the ER membrane (1)The protein exits laterally and becomes anchored in the ER membrane.

Section 4

Question Answer
How else can proteins be anchored in the ER membraneby internal signal sequences
How does anchoring by internal signal sequences workThe protein exits laterally and becomes anchored in the ER membrane.
How are proteins that spam the membrane multiple times insertedby an alternating series of internal signal sequences and transmembrane stoptransfer sequences
when can protein folding and processing occureither during translocation across the ER membrane or in the ER lumen.
What is the ER the site of protein folding, assembly of multisubunit proteins, disulfide bond formation, the initial stages of glycosylation, and the addition of glycolipid to some plasma membrane proteins
What is the primary role of lumen proteinsassist folding and assembly of newly translocated polypeptides
What mediates the folding and assembly of multisubunit proteinsThe Hsp70 chaperone BiP. Which bound to an unfolded polypeptide chain as it crossed the membrane.
What are bonds are important in protein foldingdisulfide bonds
What promotes disulfide bond formationAn oxidizing environment (like in the ER), bond formation facilitated by protein disulfide isomerase.
what environment is the cytosolreducing environment, most cysteine residues are in the reduced (-SH) state

Section 5

Question Answer
What happens to proteins as they are translocated into the ERproteins are glycosylated (N-linked glycosylation).
Where is a oligosaccharide synthesizedon a lipid (dolichol) carrier
What does glycosylation help preventprotein aggregation in the ER and provides signals for subsequent sorting
What are GPI anchorsanchors which attach some proteins to the plasma membrane by glycolipids
Where are GPI anchors assemblsin the ER membrane and added to the carboxy terminus of some polypeptides.
How are GPI-anchored proteins transportedas membrane components via the secretory pathway. Their orientation within the ER dictates that they will be exposed on the outside of the cell

Section 6

Question Answer
How are misfolded proteins removedby ER-associated degradation (ERAD)
What is the overview of misfolded protein degradationmisfolded proteins are indentified, returned to the cytosol, and degraded by the ubiquitin-proteasome system
What acts as sensors of misfolded proteinsChaperones and protein processing enzymes in the ER lumen (for example; one pathway involves the chaperon calreticulin, which assists glycoproteins to fold correctly).
What happens to severely misfolded glycoproteins once sensedthey return to the cytosol through the translocon. In the cytosol it is marked by ubiquitination and degraded in the proteasome
What happens when an excess of unfolded proteins accumulatesa signaling pathway called the unfolded protein response is activated
What does the unfolded protein response lead toto the expansion of the ER and production of more chaperones. If protein folding can't be adjusted to a normal level, the cell undergoes programmed cell death.

Section 7

Question Answer
Where are most lipids synthesizedin the smooth ER
Where are membrane lipids synthesized and whysynthesized in association with already exisiting membranes rather than the aqueous cytosol - because they are hydrophobic
Name the 3 lipids eukaryotic membranes are made ofphospholipids, glycolipids and cholesterol
Where are phospholipids synthesizedon the cytosol side of the ER membrane from water soluble precursors (glycerol)
How are phospholipids added to the bilayerBecause they are synthesized on the cytocolic side of the ER membrane, they are added only to the cytosolic half of the bilyar. they are then translocated across the membrane by phospholipid flippases, resulting in even growth of both halves of the phospholipid bilyar

Section 8

Question Answer
where are cholesterol and ceramide synthesized in the ER. (Ceramide is converted to glycolipids or sphingomyelin in the Golgi apparatus)
What type of ER is abundant in cells with active lipid metabolismSmooth ER
Why is smooth ER abundant in cells with active lipid metabolismAs Steroid hormones are synthesized from cholesterol in the ER; abundant smooth ER is found in cells of the testis and ovary.
How are proteins and phospholipids exported exported from the ER in vesicles that bud from the transitional ER. they then move through the ER-Golgi intermediate compartment (ERGIC), then move to the golgi apparatus.
Some proteins must stay in the ER- how?They have a target sequence (KDEL or KKXX) at the carboxy terminus that signals retrieval from the ERGIC or Golgi complex via a recycling pathway.