BIOLOGY 2B03 Chapter Notes - Chapter 5: Golgi Apparatus, Secretion, Cisterna

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BIOLOGY 2B03 - Module 5 Lecture II

Protein Trafficking Through Endomembranes

Cisternae: series of elongated , fact sacs that compromise Golgi complex

● Proteins are transported to the Golgi trans-cisternae via vesicles

Exocytosis: A process when vesicles fuse with the cell membrane to release proteins from the cell

● The constitutive secretory pathway is used by proteins that are released immediately after

protein synthesis and transport

○ The secretory vesicles move straight from the trans-Golgi to the cell membrane

● Regulated Secretory pathway: used by proteins that are kept in the cell until a signal

triggers release

● Secretory Granules: secretory vesicle held in the cell

● Other proteins are carried away from the Golgi trans-cisternae in vesicle to form the lysosome

○ These vesicles will fuse with endosomes

○ Endosomes: vesicles formed at the cell membrane; captures and transport

macromolecules from outside of the cell

Golgi Complex

Distinguish the membrane of the Golgi complex from the membrane of other organelles?

● Fluorescently-labeled wheat germ agglutinin is used to recognize the Golgi complex

● Wheat germ agglutinin: a lectin that recognizes N-linked polysaccharides found in the Golgi

cisternae

Golgi Cisternae

Golgi complex: contains a series of elongated vesicles sac; cisternae

● Cis-Golgi network: a collection of coalescing vesicles emanating from ER to form the cis-

cisternae

● Medial cisternae: in the middle of the complex

● Trans-cisternae; furthest from the ER and these break out into vesicles to form the trans-

Golgi network

● Entire complex is very dynamic with vesicles fusing to form new cisternae

○ Cisternae change shape and move up through the complex

● Cisternae themselves contain resident proteins that are necessary for further PTM of

transported proteins

Anterograde Transport Through the Golgi complex

Anterograde Transport: movement of proteins from the Er to the cell membrane

● Means moving in normal or forward direction

● Proteins must go through Golgi complex before they reach their destination

Models of Anterograde Transport

● Model A: vesicles carrying proteins cargo move from cis- to medial-cisternae and from

medial- to trans-cisternae

● Model B: proteins stay in cisternae but that the cisternae themselves are moving forward

through the Golgi complex

○ Still requires the use of vesicles that move backwards

Proteins in the Golgi Complex

Cell membrane protein: is found only in the cisternal sacs, but not in any associated vesicles

● Suggestion: vesicles are not required for anterograde movement

● Alternative: proteins stay in a cisterna as it moves forward through the complex

Resident Medial-Golgi Protein: should only be found in the medial-Golgi

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● If the cisternae are moving through the Golgi-complex, a medial-Golgi cisterna would

eventually move to the position of a trans-Golgi cisterna

● While cargo has moved in the anterograde direction, medial-Golgi resident proteins are

mislocalized

○ Retrograde direction in vesicles → re-sorting of proteins

We can see the antibodies to the medial-Golgi protein in the medial Golgi cisternae, also identify

vesicles containing the protein → consistent with Model B

● Model B is aka Cisternal Progression Model

Animation of Cisternal Maturation

Implications of this model: new cis-cisternae will be formed by coalescing vesicles from ER

● That the trans-Golgi network will dissipate into secretory vesicles

● And that Golgi-resident proteins will be mislocalized

● Vesicles are used to resort Golgi proteins in the retrograde direction as the bulk of the proteins

are cargo being transported in the anterograde direction

Cisternal Maturation of Cisternal Progression

● Illustrates the maturation of the Golgi cisternae carrying protein cargo en masse in the

anterograde direction

○ At trans-Golgi network, vesicles form that carry cargo to their next destination

Protein Trafficking Requires Vesicles

The movie reveals the movement of vesicles

● The fluorescence is following a secreted protein in the ER, Golgi and then in secretory

vesicles that leave the Golgi

Transport Vesicle Formation and Function

Four steps to consider about vesicular trafficking:

1) Budding: buds arise from the membrane of the donor compartment

2) Loading: cargo proteins are loaded into buds via cargo signal sequences and receptors

3) Vesicle formation and release

4) Vesicle docking and fusion to membranes of the ‘recipient’ compartment

Three Types of Coated Vesicles

There are clathrin-coated vesicles and the COPI and COPII coated vesicles

● All of these are small GTP binding proteins with GTPase activity

● G-protein bind to nucleotides GTP and GDP

○ The protein is active when it is bound to GTP: G-proteins have an internal GTPase

activity that can hydrolyze the GTP to form GDP

○ Inactive when it’s bound to GDP

● Activation/inactivation require GTPase mediated hydrolysis of GTP and GDP

○ Assisted by GAP (GTPase accelerating protein)

● Inactive → active : requires displacement of GDP and its replacement by GTP

○ Assisted by a guanine exchange factor (GEF)

Functions of Each Type of Vesicle

The three types of coated vesicles are required at different point in the vesicular transport process

● Clathrin-coated vesicles: are required for transport way from the trans-Golgi network to the

endosome and to the cell membrane

○ Also used in the process of endocytosis: uses the cell membrane to transport

macromolecules into the cell

● COP I Coated Vesicles: used specifically for retrograde transport; Golgi back → the ER

● COP II Coated Vesicles: required for transport from the RER to the cis-Golgi network

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Document Summary

Cisternae: series of elongated , fact sacs that compromise golgi complex. Proteins are transported to the golgi trans-cisternae via vesicles. Exocytosis: a process when vesicles fuse with the cell membrane to release proteins from the cell. The constitutive secretory pathway is used by proteins that are released immediately after protein synthesis and transport. The secretory vesicles move straight from the trans-golgi to the cell membrane. Regulated secretory pathway: used by proteins that are kept in the cell until a signal triggers release. Secretory granules: secretory vesicle held in the cell. Other proteins are carried away from the golgi trans-cisternae in vesicle to form the lysosome. Endosomes: vesicles formed at the cell membrane; captures and transport macromolecules from outside of the cell. Fluorescently-labeled wheat germ agglutinin is used to recognize the golgi complex. Wheat germ agglutinin: a lectin that recognizes n-linked polysaccharides found in the golgi cisternae. Golgi complex: contains a series of elongated vesicles sac; cisternae.

Related Questions

Which compartments are involved in vesicular transport? How are transport vesicles formed and targeted appropriately? What proteins are involved and what are their roles? What kinds of modifications are added to proteins in the ER? What determines whether a protein stays in the ER or moves on? Be sure to understand how vesicles fuse with their target membranes, and the consequences of fusion for the vesicleâs cargo as well as the proteins and lipids that are in the membrane. Know the structure of the Golgi apparatus and its role in intracellular trafficking. Understand the secretory process, endocytosis, exocytosis, function of the endosomes, and function of the lysosomes. Understand why endocytosis happens and how it can be regulated to take up specific molecules.

1. How do viral proteins get localized to the nucleus?

2. How do viral proteins get localized to the plasma membrane?

3. How does a nascent protein get targeted to the ER?

4. What amino acid residue on a target protein is attached to the sugar?

5.Where in the cell are N-linked oligosaccharides FIRST added to a glycoprotein?

6.Where in the cell does the initial sugar get processed into its mature form?

7. That is, which exact compartment(s) of the secretory pathway is mannose removed? Galactose added? Sialic acid added? How does the position of glycosylation (N-linked vs internal) affect which chaperone is used to properly fold proteins in the ER?

8. What compartments comprise the secretory pathway in mammalian cells?

9.How do proteins get transported from the ER to the Golgi?

10.How do the transport vesicles return from the Golgi to the ER?

11.What happens to their components when they get there? How does poliovirus inhibit the secretory pathway?

SHORT answers please

1. The histones that form the core of the nucleosome get into the nucleus by which of the following?

a) Passive diffusion through nuclear pores

b) Selective and facilitated active transport through nuclear pores

c) Ribosomes located in the nucleus synthetize histones in the nucleus

d) Histones are made at the ER membrane and then diffuse to the nucleus

2. Membrane proteins that belong to the golgi, lysosome, or plasma membrane first pass through which organelle before reaching their final subcellular destination?

a) The golgi apparatus

b) The smooth ER

c) The nucleus

d) The ER membrane

e) The ER lumen

3. Where in the cell would a ribosomal protein be located if it has had its nuclear export signal inactivated (non-functional or non-existent).

a) The protein would be found in the ER lumen

b) The protein would remain in the nucleus

c) The protein would be secreted

d) The protein would remain in the cytoplasm

e) The protein would be in the ER membrane

4. Where would be the final destination of a protein if it contained a NLS anywhere in the protein sequence (and which is exposed to the cytoplasm after protein folding) but also contained an ER signal sequence at the carboxy terminus?

a) The ER lumen

b) The nucleus

c) The cytoplasm

d) The inner nuclear membrane

e) The ER membrane

5. The sequence K-D-E-L serves as a retention/retrieval signal for ER proteins.

a) True

b) False

6. The following are true for both cell membrane proteins and secretory proteins EXCEPT?

a) Precursors of the mature proteins contain signal sequence.

b) Both types of proteins contain stop-transfer sequences

c) Both types of proteins enter the ER while being synthetized

d) Both types of proteins pas through the golgi apparatus

e) Both are carried by clatherin-coated vesicles

7. If you isolate a regulated secreted protein from a secretory/storage vesicle and inject it into the cytosol, is will?

a) Be secreted through channels in the plasma membrane

b) Be taken up into secretory vesicle and secreted

c) Remain in the cytosol until it is degraded

d) Be taken up into the rough ER and follow the secretory pathway

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BIOLOGY 2B03 Chapter Notes - Chapter 5: Golgi Apparatus, Secretion, Cisterna

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