12. Protein processing and the endomembrane system Flashcards Preview

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after translation, all proteins must be sorted to their destinations:
some proteins:

1. remain in the cytoplasm
2. move to cellular compartments by the cytoplasm
3. enter the endomembrane system, where they are modified and transported through a series of membrane-bound organelles


transport of proteins in the endomembrane system

1. enter the endoplasmic reticulum
2. move by transport vesicles (budding and fusing "carriers" b/w other compartments)
3. to golgi, then to the :
4. trans-golgi network, then to:
a) lysosomes or b) storage vesicles or c) secretory vesicles and plasma membrane


in addition to the lumen or ER (interior space), the proteins may _______, _______ and _______ within the membranes of each these compartments.

reside, transfer and function


general pathways of new proteins

starts in the cytoplasm: 1. nucleus
2. mitochondrion
3. R.E.R
* golgi: lysosome or secreted


ER structure

network of membranous tubules in the cytoplasm of a cell
two types: rough and smooth


ER is involved in..

the production of phospholipids, proteins, and other functions


Rough ER:

studded with ribosomes, continuous with nuclear envelope
-in most cells:
*synthesis of all proteins to be compartmentalized in lysosomes secretory vesicles
*synthesis of integral (membrane bound) proteins
*membrane biosynthesis (can assume some SER functions)
*glycosylation of many proteins


Smooth ER:

no ribosomes; site of synthesis of specialized enzymes, functions
-concentrated in specialized cells:
skeletal muscle, kidney tubules, some endocrine glands, secretory cells
synthesis of steroids in gonad and adrenal cortex
synthesis of detoxification enzymes in liver
enzymes involved in glucose release from liver
release Ca2+ ions used in muscles contraction
lipid biosynthesis (membrane biosynthesis)



production of chemical compound by living organism


chemical modifications of proteins in the ER and golgi

1.disulfid bond formation (cysteines) and folding
2.addition of extra chemical groups (methyl,acetyl,formyl,sulfate,hydroxyl.)
3.addition of lipids: lipoproteins are molecules of the cell membranes
4.formation of multimeric proteins (quaternary structure)
5.proeolytic cleavage
6. glycosylation


disulfide bond formation and folding:

-protein folding: proteins have a correct or native state; the tertiary structure of a protein needs to be correct for it to work properly. if can be mis folded or denatured
-molecular chaperones are proteins that bind to and alter the folding of newly forming protein
*quality control: proteins that misfold cannot bind to chaperones properly and are destroyed
*with several cysteines, the order in which the disulfide bonds form is important-a protein disulfide isomerase in the lumen of ER can help



most proteins destined for secretion, the extracellular matrix, the endomembrane system and the plasma membrane are glycoproteins


RER is the site of synthesis of proteins destined for :

-integral membrane proteins and proteins within lumen of the endomembrane system
-endoplasmic reticulum, golgi, lysosomes, vesicles, plant vacuoles


proteins for the following locations are made within the cytoplasm, not the RER :

-cytosolic proteins
-peripheral proteins of the cell membrane
-proteins sent to the nucleus
-proteins destined for chloroplasts, mitochondria, peroxisomes


what is different about the protein that is destined for the rough endoplasmic reticulum?

the main difference is that is has a hydrophobic signal sequce


hydrophobic signal sequence

-after the signal sequence is synthesized, protein synthesis temporarily stops until the signal sequence is detected by a protein complex on the RER
-the signal peptide enters the ER through a channel and it guides the protein into the lumen of the RER. protein synthesis resumes and the rest of the protein is inserted in the lumen
-a signal peptidase near the inner surface of the membrane works to cleave the signal sequence from the growing peptide


all proteins are made on ________ in the cytoplasm

(some ribosomes are free in cytoplasm and some become attached to endoplasmic reticulum)


polyribosomes can be found on the RER

-the fluidity of the membrane allows the ribosome to be docked at its receptor site and also move along the mRNA
-translocons can move around on the ER membrane, because the membrane has fluidity
-polyribosomes can form, to speed up translation rate


integral proteins

proteins embedded in the membrane
-some proteins get incorporated into the membrane- they do not end up in the ER Lumen
-some proteins have a single transmembrane domain-a stretch of at least 15 hydrophobic amino acids that is embedded in the lipid bilayer
-the translocon can open to allow lateral movement of the hydrophobic segment into the lipid bilayer
translocon opens a side door to let hydrophobic sequence move into a hydrophobic environment


orientation of the integral protein is dependent on amino acids next to hydrophobic sequence

-presence of positively charged amino acids flanking the hydrophobic sequence will determine the orientation of the protein, such that the positive amino acids are pointing toward the cytosolic side (as opposed to the lumen side)
-N terminal could be in the lumen or the cytoplasm, orientation of protein is adjust so that the flanking positively-charged amino acids are aligned with negatively charged translocon amino acids


some integral proteins have multiple transmembrane domains

page 15
-here is the insertion of a double pass transmembrane protein within the rough endoplasmic reticulum. the red coloured signal sequence starts the membrane insertion process. this continues until a second hydrophobic signal (orange) is reached. that anchors the second membrane passage
-these alternating hydrophobic regions are called start-transfer and stop transfer sequences


multipass transmembrane proteins

membrane proteins that pass through the membrane multiple times have multiple start and stop signals. a hydrophobicity plot can predict the number of transmembrane domains a protein can have


roles of transmembrane proteins

-communication b/w cells
-communication b/w organelles and cytoplasm
-ion transport
-nutrient transport
-connections for cytoskeleton
-25% of cell's proteins