how do proteins that fold in ER move to Golgi?
packaging proteins from the ER into vesicles
vesicles bud, are targeted to right organelle, and fuse w/ membrane of Golgi
what is the structure of the Golgi apparatus?
stack of flattened membranous disks that have distinct curved appearance
curvature creates polarity w/ a "cis" convex face and "trans" concave face
what happens to proteins as they move through the golgi?
which part of golgi does what?
they move and they're modified via post-translational modification; golgi does transit and modification
cis stack: phosphorylation
medial stack: modifications to carbohydrates, i.e. cutting manose units of carbs that were added as single unit in ER
trans stack: sulphation of tyrosines
what is function of PTMs that occur to proteins in the golgi?
1) increase complexity of proteins traveling through the Golgi
2) specify subsequent localization to other membrane systems
how do proteins get transported from the ER to the Golgi?
in lipid vesicles that bud from teh ER membrane, are targeted to teh Golgi, and fuse at Cis side of Golgi
what is the ER retrieval pathway for?
how does it work?
when proteins go from ER to Golgi, some of the chaperones & folding enzymes that worked on protein in ER go w/ vesicles to the Golgi, and have to be returned to ER via this pathway
operates via a receptor that recognizes peptide seuqnce of KDEL at c-terminus of resident ER proteins that've escaped to the Golgi
in cis face of Golgi, KDEL receptor interacts w/ these KDEL sequences, sequester, and return chaperones to the ER in vesicles
where in the cell does the Golgi sit?
what does glycosylation of proteins in the ER do?
gives proteins added level of complexity, easier to recognize
what are the models that explain how proteins that go through stacks of the Golgi maintain their identity, if diff biochemical pathways are occurring in each stack of Golgi?
1) vesicular transport model: vesicles budd and fuse from stacks; proteins bud and fuse in those vesicles as they move from 1 stack to another
but this is discounted model, b/c some of large protein assemblies in ER are too big to go this way
2) cisternal maturation model: stacks themselves are consistently moving as they reach the transgolgi network, enzymes that do these rxns are packaged into vesicles, move backwards to proper destination in cis golgi
what is the lysosome?
single-membrane organelle that does recycling
comprises hundreds of different enzymes that chew things up
lysosomal enzymes destined for lysosome start in the ER
what is unique about enzymes destined for the lysosome?
synthesized in the ER
sorted in the Golgi
these enzymes = 2 units of glucose, 8 of manose; a phosphotransferase adds a single phosphate to a manose unit while in the cis golgi
marks lysosomal enzymes in the golgi
how to lysosomal enzymes get from the cis golgi -> lysosome?
phosphatase adds a phosphate onto a manonse subunit of lysosomal enzymes while they are in the cis golgi
once in the trans golgi, enzyme is connected to a manose-6-phosphate receptor that connnects to the phosphate added onto the lysosomal enzyme
M6P receptor gets bound in a vesicle containing the enzyme; goes to endosome, fuses w/ it
how does lysosomal enzyme get delivered from the trans golgi network to the lysosome? describe steps, pH changes
1) M6P receptor binds the phosphate attached to the lysosomal enzyme while it is in trans golgi network
2) M6P gets bound in a vesicle w/ lysosomal enzyme
this goes to endosome, fuses w/ it
3) pH of trans golgi network = 6; pH of endosome = 5.5
this is 5x difference
so when vesicles fuse w/ endosome, pH decrease interferes w/ binding of the lysosomal protein to M6P receptor
4) 2 kinds of vesicles are created:
M6P receptor is removed, bound to vesicle, and transported usually back to the trans-golgi network, but several vesicles w/ M6P receptors go to the plasma membrane
protein destined for lysosome are put into vesicles and sent to lysosome, where pH=5 so lysosomal enzymes get dumped once get there
what are the pH changes in the lysosomal enzyme delivery system?
pH trans golgi network = 6, M6P binds to lysosomal enzyme's phosphate
pH endosome = 5.5, allows dissociation of lysosomal enzymes into vesicles and M6P receptors to other vesicles
pH lysosome = 5, lysosomal proteins get delivered there
what are lysosomal storage diseases caused by?
missing an enzyme that breaks down glycolipids
so glycolipid is stored, cannot be broken down
i.e. Gaucher's Disease, Fabry's Disease
what causes Gaucher's disease?
what is gene therapy for it?
mutation in glucocerebroside which breaks down glycolipids
causes glycolipids' accumulation
gene therapy method: purify glucocerebroside from a mammalian cell in culture, add back to the blood via infusion of purified protein that has M6P and carbs
glucocerebroside gets into cells b/c some errant M6P receptors go to plasma membrane, capture the purified glucocerebroside, take it back into cell via endocytosis
what causes fabry's disease?
what is gene threapy for it?
caused by alpha galactosidase (alpha gal-a) mutation due to mistolding of proteins in the ER
proteins misfolded in the ER will be destroyed by proteasome in cytosol, have no Alpha Gal A
so work w/ DGJ, a competitive inhibotor that binds strongly to alpha Gal-A at pH=7, in the ER; carries alpha gal-A from ER -> Golgi -> lysosome via M6P receptor pathway, and once at lysosome of pH=5, compound falls off enzyme so enzyme now has free active site
this takes advantage of protein folding & competitive inhibitors
what happens in the final processing step for proteins that undergo regulated secretion?
describe case of insulin as example of peptide hormone that undergoes regulated secretion
peptide hormones (i.e. insulin) are synthesized as larger precursors, have to be trimmed down before proteins are biologically active
this occurs in vesicles for these peptide hormones
precursor of insulin has a PRO region
when insulin protein precursor is in ER, signal peptide cleavage occurs so protein travels through ER and Golgi complex, then to trans golgi network, packaged into secretory vesicle
secretory vesicle contains both the PRO insulin and 2 enzymes that cleave the PRO region out
insulin thus matures in the secretory vesicle; disulfide bonds form in the vesicle right before insulin's secreted
insulin thus exits vesicle in disulfide-bonded form that's highly concentrated (b/c also have enzymes in vesicle that take out water from insulin)
what is the PRO region of a peptide precursor
region that is cleaved out while in vesicle, does not become part of active protein
what are examples of acid hydrolases? - lysosomal enzymes?
nucleases, proteases, glycosidases, lipases, phosphatases, sulfatases, phospholipases
What are the functions of N-linked glycosylation of proteins entering the ER?