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continuous network of membranous tubules and sacs that run throughout the cell
gives rise to golgi, lysosomes, and new cell membrane
rough ER
transitional ER
smooth ER


rough ER

has ribosomes on cytosolic surface
important in protein processing


transitional ER

involved with budding that sends vesicles to the golgi
important in protein processing


smooth ER

no ribosomes attached
involved in lipid metabolism



process of sending a protein into the ER
some proteins targeted for lumen of ER or to be embedded in its membrane


where does all protein synthesis begin

free ribosomes in the cytosol (unattached to ER)
proteins destined to remain in cytosol complete synthesis on free ribosomes


destination of proteins that complete synthesis on free ribosomes

remain in cytosol


destination of proteins that complete synthesis on membrane bound ribosomes

plasma membrane
secretory vesicles
endosomes --> lysosomes


cotranslational translocation

some proteins headed for lumen of ER enter as they are being made (during translation)


cotranslational translocation process

synthesis begins on a free ribosome in the cytosol
proteins have a unique signal sequence near the N-terminus that is about 20 AA long and contains a stretch of hydrophobic AA
signal recognition particle (SRP) that is a protein-RNA complex recognizes signal sequence and binds to the SS and ribosome which halts translation
mRNA-ribosome-polypeptide-SRP complex binds to protein on ER called SRP receptor (SRP receptor binds SRP and SS on polypeptide binds to protein complex next to SRP receptor called translocon
translocon forms channel into ER lumen
binding of SRP receptor to SRP causes SRP to be released from SS and ribosome allowing translation to resume
growing polypeptide inserted into channel in translocon but the SS is retained within the translocon bound to the wall of the channel
signal peptidase associated with translocon on lumen side and cleaves SS releasing polypeptide into lumen when translation is complete


posttranslational translocation

some ER lumen proteins made on free ribosome then translocated into ER


ER membrane proteins with an N-terminus SS and an internal stop transfer sequence

single pass membrane proteins
translocation proceeds as described in cotranslational translocation but midway through synthesis there is a stop transfer sequence that stops translocation (by altering translocon) so the remainder of polypeptide remains on cytosolic side
stop transfer sequence passes through wall of translocon into phospholipid bilayer
when finished - N-terminus on lumen side and C-terminus on cytosolic side with stop transfer sequence embedded in membrane


ER membrane proteins with internal signal sequence(s) and/or internal stop transfer sequence(s)

single or multiple pass membrane proteins
orientation of single pass membrane proteins may be in either orientation (N-terminus or C-terminus on outside)
some proteins have multiple internal signal sequences and stop transfer sequences which results in multiple pass membrane proteins


protein folding and processing in the ER

disulfide bridge formation
glycosylation/other modifications


chaperones and folding

polypeptides must be in correct folding patter to function properly
correct folding mediated by chaperones (which are also proteins)
complete polypeptide will assume correct folding spontaneously but before translation is complete it can assume an incorrect pattern or aggregate with other partially complete polypeptides
chaperones in ER and cytosol bind to nascent polypeptide to keep it from interacting with anything else until synthesis is complete



improperly folded protein
can be disease causing unit
can't be destroyed by heat
can interact with other properly folded proteins and turn them into prions
ex: mad cow disease



many polypeptides have AAs removed after translation
removal of inital methionine
extensive cleavage as with preproinsulin


cleavage process to form insulin

preproinsulin has N-terminal SS which is removed inside Er to make proinsulin
proinsulin becomes insulin when internal AA sequence is removed in ER lumen and 2 polypeptide fragments are joined by disulfide bridges


disulfide bridge formation

protein disulfide isomerase responsible for making and breaking disulfide bridges until most stable configuration is formed
disulfide bridge is a covalent bond between 2 cysteine residues
disulfide bridges only found in proteins that are to be secreted or are exterior membrane proteins because the cytosol contains reducing agents that would break the bonds


glycosylation and other modifications

other chemical modifications occur in the ER lumen
glycosylation: addition of oligosaccharides (carbs)
external membrane proteins glycosylated this way


lipids synthesized in the smooth ER

most membrane lipids including phospholipids, glycolipids, and cholesterol


phsopholipid synthesis

occurs in outer layer of ER membrane bilayer
enzyme flippase moves phospholipids to inner membrane layer after synthesized on outer membrane layer


export from ER

vesicles bud off the ER from transitional ER and carry ER lumen content and ER membrane components to the golgi
vesicles first fuse to ER-golgi intermediate compartment which gradually becomes cis-face cisternae of golgi which gradually becomes trans-face cisternae
from trans-face cisternae vesicles bud off to fuse with cell membrane (secretion and cell membrane formation) or fuse with endosomes/lysosomes


where is golgi most abundant

secreting cells (exocytosis)


clathrin coated vesicles

as vesicles bud off the ER, the region to form a vesicle first becomes coated in protein (ex: clathrin) through complex series of reactions
protein forms in network pattern
binding stimulates budding because it distorts membrane
after vesicle buds off, clathrin is removed


membrane fusion

a vesicle fuses with either cell membrane or another membrane (ex: endosome)
process involves binding of v-SNARES (on vesicle) to t-SNARES (on target membrane)



small membranous sacs containing lysosomal acid hydrolases (powerful hydrolytic enzyme)


lysosomal hydrolases

about 50 kinds
can break down all cellular organic compounds
only work in acidic environment (pH ~5) of lysosome (cell would be okay if one burst because wouldn't be functional in non-acidic cytosol)



extracellular material brought into a vesicle by this



small scale endocytosis
involves clathrin-coated endocytic vesicles