BIOL 2020: Lecture 8 (golgi apparatus)

Secretory pathway 

the route through which proteins and lipids are synthesized, processed, and transported to their final destinations, either within the cell or for secretion outside the cell.

1. translates from mRNA in the ribosomes in the cytoplasm 

2. enters the ER lumen 

3. goes from the ER to the golgi in a vesicle 

4. transits the golgi 

5. leaves the golgi in a vesicle 

6. the vesicle fuses the cell membrane 

7. it is outside 

Vesicular transport

• transport vesicles bud off from one compartment and fuse with another

  • as they do, they carry material as cargo

  • these buds are called coated vesicles, which have a distinctive cage of proteins

Endocytic pathway

• the series of steps a cell uses to internalize substances from its surface

• leads inwards from the plasma membrane, whereas secretory pathway leads outwards from the ER towards the golgi, and cell surface 

Coated vesicles 

• vesicles which bud off from one compartment to fuse with another, all while carrying material as cargo 

• have a distinctive cage of proteins 

• 4 well-characterized types, each used for different transport steps:

  • clathrin-coated

  • COPI-coated

  • COPII-coated

    • able to accomodate large cargos by assembling tubes instead of vesicles 

  • retromer-coater

Coats

• geometrical structures that assemble into vesicle cages 

  • coated vessicles 

Rab proteins

• guide transport vesicles to their target membrane

  • all transport vesicles display surface markers that identify them and target membranes display complementary receptors

SNARE proteins 

• mediate membrane fusion by putting membranes in close proximity

ERES

• ER exit site

• where proteins leave the ER via COPII-coated transport vesicles 

Cargo receptors

• inside vesicles there are cargo receptors which ensure that the vesicles are loaded 

Golgi apparatus 

• consists of a collection of flattened, membrane-enclosed compartments

  • cisternae

  • in some species, cisternae are not stacked

• glycosylation and phosphorylation at the golgi are destination codes

• transport through golgi occurs by 2 mechanisms:

  • vesicle transport mechanism 

  • cisternal maturation mechanism 

  • evidence of this through both fixed samples, and fluorescent live imaging

Vesicle transport mechanism

• vesicle transport of molecules between cisternae 

Cisternal maturation mechanism

• cisternal mature from cis to trans together with cargo molecules 

COPI vesicles

• form of transport-vesicles 

• bring ER proteins back from the golgi 

KDEL sequence/receptor

• ER proteins have the KDEL sequence

• KDEL receptor initiates vesicle formation 

Non-selective constitutive secretory pathway

• mechanism which transports most other proteins directly to the cell surface 

• no specific signal yields the secretion of protein

  • default pathway 

• specific signals are needed to direct secretory proteins into secretory vesicles and lysosomal proteins into different specialized transport vesicles. 

Parts of the golgi 

• cisternae

  • flattened, sac-like membrane compartments that make up the endoplasmic reticulum (ER) and the Golgi apparatus.

  • cisternae are organized in stacks and are responsible for modifying, packaging, and transporting proteins and lipids.

• CisGN

  • "forming," face of the Golgi apparatus, which receives proteins and lipids from the endoplasmic reticulum (ER).

  • It is located on the convex side of the Golgi stack, facing the ER. Its function is to receive these materials and prepare them for further modification as they move through the medial and trans compartments of the Golgi before they exit

• TransGN

  • final sorting and dispatch station of the Golgi apparatus, where proteins and lipids are packaged into vesicles for delivery to their final destinations, such as lysosomes, the cell membrane, or secretion outside the cell.

  • located on concave side of the Golgi stack