Oct 24th - Intracellular membrane traffic

COPII-coated vesicles travel from the ER to the Golgo

• soluable cargo proteins have exit signals that are recognized by cargo receptors

• vesicles are formed and coated in COPII at ER exit sites

• proteins need to be correctly folded and all units assembled in multi-unit proteins (quaternary structure) before departure - prevents misfolded proteins from moving forward

  • e.g. cystic fibrous

• COPII-coated vesicles fuse into the vesicular tubular clusters that are carried to the Golgi

  • COPII-coated vesicles budding from the ER fuse to form a bigger tubular structure — vesicular tubular structure

  • vesicular tubular structure travels along microtubules to fuse with the Golgi

  • as it moves, COPI-coated vesicles bud off, carrying ER cargo

    • uses retrieval pathway to transport proteins like BiP and SNAREs back to the ER

KDEL is a return signal for resident ER proteins

• cargo that are retrieved have retrieval signals

  • e.g. KDEL in BiP is recognized by KDEL receptors — bind escaped BiP

  • this is followed by COPI-coated vesicle formation

the Golgi apparatus is a protein modifying and sorting system

• it is a collection of flattened membranous compartments called cisternae

• protein modification happens progressively as they move up from cis to trans cisternae

• vesicles carrying the modified proteins bud off from trans cisternae

N-linked oligosaccharides started in the ER are modified in the Golgi

• glycosylation contributes to the multiple functions

  • protection of proteins from proteases

  • facilitating folding by shielding hydrophobic regions, preventing aggregation and recruiting chaperones that recognize a “glycol-code” of sugar patterns

  • fine-tuning protein-protein interactions

  • the glycocalyx might have evolved as an animal version of a cell wall

• a group of glycoproteins called proteoglycans contains sulfated sugars that are modified in the Golgi