The Golgi apparatus Flashcards
what is the structure of the Golgi apparatus?
it has many cisterna connected by tubules. the cis Golgi network is near the rough ER and accepts cargo from the ER. the trans Golgi network is where cargo is finally modified and secreted in vesicles
how does the Golgi differ between plant and animal cells?
Animal: located near the cell nucleus and centrosome
Plant: may contain hundreds of golgi more evenly dispersed throughout the cell
what cells have the most prominent golgi?
secretory cells such as intestinal goblet cell (secretes mucus). these cells contain unusually large secretory vesicles on trans-side of golgi
what is the process of vesicle transport from the ER to the golgi
Cargo leaves the ER in COPII coated transport vesicles.
vesicles bud from smooth ER regions called exit sites which are already curved so energy required for budding is reduced
cargo receptor proteins contain built in exit signals that are recognized and recruited by coat proteins. only correctly folded proteins bind cargo receptors (chaperones involved)
transmembrane proteins with exit signals become concentrated within vesicles and can function as a cargo receptor to concentrate proteins within vesicles
Sar1-GTP functions like adaptin and helps with coat shedding.
What do ER resident proteins have instead of exit signals
retention or retrieval signals that return them to the ER if the leak out by retrieval pathways
What does BiP do in the ER
BiP is a chaperone protein that binds to partially assembled antibodies and retains them in the ER. BiP no longer binds after antibody is fully assembled, allowing it to enter vesicle for transport out of ER
what are vesicular tubular clusters?
after vesicles bud from the ER they coalesce into larger structures called vesicular tubular clusters. These mediate transport between ER and Golgi and are carried along microtubules by motor proteins. They are short lived and dynamic structures formed by homotypic fusion
how does homotypic fusion occur?
the t-SNAREs and v-SNAREs of vesicles are complementary. NSF unraveled the t from the v to allow them to interact with other vesicles t and v and fuse vesicles
what is the advantage of tubular clusters?
it saves energy by moving a large cluster all at one time instead of many smaller vesicles
How are ER resident proteins returned to the ER from the Golgi?
ER retrieval signals (such as KDEL and KKXX) are built into the C-terminus of the proteins. KDEL is a signal for soluble proteins and binds the KDEL receptor. KKXX is a signal for membrane proteins and binds to COPI coat protein
how does the KDEL signal work?
the KDEL receptor has a pH dependent affinity for the KDEL sequence. receptor has high affinity for KDEL in low pH (golgi) and low affinity for KDEL in neutral pH (ER).
Vesicular transport model
Golgi cisternae are static in pace. enzymes remain within the appropriate subcompartment. vesicles transport forwards and backwards
cisternal maturation model
cisternae actually migrate cis to trans through the golgi. they are mature when they reach trans. enzymes only transported backwards by vesicles
two main classes of N-linked oligosaccharides attached to glycoproteins
- complex oligosaccharides: has sialic acid NANA and other residues
- High mannose oligosaccharides: has many mannose residues
how are complex oligosaccharides constructed?
as glycoproteins transit through the ER and Golgi the oligosaccharide attachments are modified. it is step-wise, so each product is the exclusive substrate for the next enzyme in the process.
oligosaccharides which are inaccessible to appropriate enzyme will remain as high mannose oligosaccharides
