Week 11(L20-21): Transmembrane proteins, Golgi, COP, endocytosis, autophagy, cytoskeleton

How are transmembrane proteins integrated into a membrane?

single-pass transmembrane

* TMD gets stuck in translocon
* TMD released into ER membrane

What are the 2 options for a protein targeted to the ER lumen after fully synthesized and folded?

1. retained in ER lumen
2. transported ER→golgi complex for further modification + delivered to distal parts of biosynthetic pathway

The Golgi complex receives transport vesicles at the __ and sends them off from the ____

CGN

TGN

What is critical for cell function?

getting proteins to where they need to be

What does the **Golgi** **complex** do?

* receives proteins + lipids from ER
* sorts to other organelles, PM, or cell exterior

In what direction do materials move from ER→Golgi→other compartments?

proximal → distal

Describe the structure of Golgi complex

* smooth, flattened, disk-like cisterna
* diameter = 0.5-1 micron
*

Golgi complex shows polarity

cis-medial-trans cisternae

Cisternae are …

biochemically unique; different cisternae contain different enzymes to modify proteins

The golgi complex membrane is supported by what?

protein skeleton - actin, spectrin

Scaffold linked to ________ that direct vesicle movement in/out of Golgi

motor proteins

What does the CGN act as?

sorting station

What does the CGN do?

sort whether protein continues to next Golgi station OR shipped back to ER

What does TGN do?

sorts proteins into different vesicles

Where do the vesicles go?

* other **PM**
* other **intracellular** destinations (lysosomes)

Golgi is the ________ of the cell

processing plant

In addition to sorting, what else is the Golgi involved in?

* polysaccharide synthesis
* protein + lipid modification (glycosylation + proteolytic)

proteins are modified ______ as traverse the Golgi

step-wise

Where are proteins fully processed?

at the TGN

Fully processed proteins are sorted to the _________, then sorted/delivered to final destinations

TGN

Mucin secretion is an example of what?

**constitutive** secretory pathway

Insulin and neurotransmitter release is an example of what?

**regulated** secretory pathway

What does the goblet cell in the GI tract produce?

protective mucus containing glycoprotein mucin

Describe **cystic** **fibrosis**

CFTR mutated and degraded in ER

* fails to reach surface/other sites

In CF, what does an F508 mutation cause?

* deletion of 3 nucleotides
* loss of phenylalanine (F) at 508th position of protein

How is the transfer of vesicles ER→Golgi + b/t Golgi sub-compartments achieved?

coat proteins

Coat proteins have 2 functions:

1. form vesicle
2. select cargo for vesicle

What are **COPI** + **COPII**?

* **coat protein** **complexes**
* assemble on the cytosolic surface of donor compartment membranes where budding takes place

What direction do COPI-coated vesicles move?

retrograde

What direction do COPII-coated vesicles move?

anterograde

What direction does retrograde run in?

distal-proximal

What direction does anterograde run in?

proximal→distal

Lysosomes are ______ organelles

digestive

What is the size of lysosomes?

25 nm → 1 μm

What is the internal pH of lysosomes?

4\.6

What do lysosomes contain?

hydrolytic enzymes - acid hydrolases

The lysosomal membrane is composed of what?

glycosylated proteins

What do glycosylated proteins act as?

protective lining next to acidic lumen

What directs vesicles to other parts of the cell in late biosynthetic secretory pathway?

* trans golgi → endosomes
* trans golgi → lysosomes
* PM → endosomes

What does a two-layered coat consist of?

* clathrin
* AP complex
* AP/clathrin-coated vesicles

What is clathrin?

* coat protein - OUTER layer
* vesicle formation + structure

AP complex

* clathrin adaptor protein complex - INNER layer
* 3 diff AP complexes
* select cargo
* different complexes associated with different trafficking routes

AP/Clathrin coated vesicles move ______-

from TGN →other compartments

Autophagy

normal disassembly of unnecessary/dysfunctional cellular components -organelle turnover

What are the phases of autophagic pathway?

autophagasome formation → lysosome recruitment → autolysosome → digestion & release

Describe steps of autophagy

1. isolation membrane engulfs target organelles → forms autophagosome/autophagic vesicle
2. lysosome fuses w/ER-derived AV to form an autolysosome
3. autolysosome content enzymatically digested and released (EXOCYTOSIS)

What is the isolation membrane derived from?

ER

Degradation of internalized material

* recycling plasma membrane components (receptors/extracellular material)
* destroy pathogens - only in phagocytic cells

Describe **phagocytosis**

* pathogen internalized by phagocytic cell
* pathogen degraded by lysosomes associated w/pathogen-containing vesicle
* hydrolytic enzymes degrade + kill pathogen
* debris released from cell

Describe the plant cell wall

rigid barrier composed of polysaccharides

Plant vacuoles take up ____% of the cell’s volume

90%

Vacuoles are _____-filled__ and ___-bound

fluid

membrane

Chloroplasts enable plant cells to do what?

harness sunlight energy to synthesize sugars

What do plasmodesmata connect?

neighbouring plant cells

What are vacuoles?

organelles contributing to structural rigidity of plants by maintaining turgor pressure against cell walls

Vacuoles are involved in what?

* cytoplasmic pH regulation
* cell turgor (rigidity) regulation
* toxic ion sequestration
* AA, sugar, CO2 storage in form of malate

Tonoplast

* vacuolar membrane
* contains active transport systems
* allow ion + molecule transport

What are the 3 functions of plant vacuoles?

1. intracellular digestion
2. mechanical support
3. storage

intracellular digestion

* similar to lysosomes
* pH = 5
* acid hydrolases

Vacuole mechanical support

* turgor pressure
* gives plant rigidity
* supports soft tissues
* stretches cell wall during growth

\

Vacuole storage

* solutes
* macromolecules
* chemical storage → anthocyanin (pigment)

Cytoskeleton

* dynamic network of interconnected filaments and tubes
* extends through cytosol of eukaryotes

What are the functions of the cytoskeleton?

* structural support
* spatial organization
* intracellular transport
* contractility
* motility

Microfilaments

subunit = actin

structure = 7-9 nm

Microtubules

subunit = alpha beta-Tubulin dimer

structure = 25 nm

intermediate filaments

subunit = various

structure = 10 nm