Lecture 21

Question 1

Biosynthetic/
Secretory Pathway
Protein Sorting

Answer 1

A protein targeted to the ER lumen,
after it is fully synthesized and properly folded, has 1 of 2 options:
Option 1) It is retained in the ER
lumen, if that is where it functions.
Option 2) It is transported from the ER to the Golgi complex for further modification and delivered to distal parts of the biosynthetic/secretory pathway (e.g.Golgi, lysosomes, plasma membrane). In some cases, the final destination could be outside the cell.

Question 2

ER to Golgi Complex “route”

Answer 2

Material moves from ER to Golgi and then to other compartments and the plasma membrane in a proximal to distal direction.

Question 3

Structure of Golgi Complex

Answer 3

Smooth, flattened, disk-like cisterna
(~0.5-1 micron in diameter).
* ~8 (or fewer) cisternae/stack—range
from a few to several 1000 stacks per
cell.
* Curved like a shallow bowl.
* Shows polarity: cis–medial–trans
cisternae.
* Cisternae are biochemically unique.
* Membrane supported by protein
skeleton (actin, spectrin).
* Scaffold linked to motor proteins that direct movement of vesicles
into and out of the Golgi.

Question 4

Golgi Complex - what are the structural differences?

Answer 4

CGN acts as a sorting station (i.e.,
sorts whether proteins should continue
on to the next Golgi station or be
shipped back to the ER).
‣ TGN sorts protein into different types of vesicles—vesicles go to plasma
membrane or other intracellular
destinations (e.g. lysosomes).

Question 5

Biochemical Diversity of Golgi Complex

Answer 5

• Golgi is processing plant of the cell.
• In addition to sorting, the Golgi is also involved in synthesis of polysaccharides and specific modification of proteins and lipids (glycosylation and proteolytic modification).
  -Different cisternae of the Golgi contain different enzymes that modify proteins.
• Proteins are modified step-wise as they traverse the Golgi; fully processed proteins
  at the TGN
• The differential staining of the Golgi cisternae reflects their biochemical differences

Question 6

Biosynthetic/Secretory
Pathway Protein
Sorting steps

Answer 6

Fully processed proteins are sorted to the trans-Golgi network (TGN) and then sorted and delivered to their final destinations.

The transfer of vesicles from ER to
Golgi, and between the Golgi sub-
compartments, is achieved by coat
proteins.
Coat proteins have 2 functions:
1. Helps form the vesicle.
2. Helps select ‘cargo’ (i.e. material
inside or on vesicle).

Question 7

Goblet cell in GI tract

Answer 7

produces protective
mucus that contains glycoprotein mucin

Question 8

Abnormal Protein
Trafficking and
Mislocalization in
Diseases

Answer 8

Cystic fibrosis (CF) is caused by a mutation in the
cystic fibrosis transmembrane conductance regulator
(CFTR) gene. The most common mutation, ΔF508, is
a deletion of three nucleotides that results in a loss of
the amino acid phenylalanine (F) at the 508th
position of the protein. This mutation accounts for
two-thirds (66–70%) of CF cases worldwide

Question 9

Mucus secretion is an example of:

Answer 9

Constitutive secretory pathway

Question 10

The release of insulin and neurotransmitters is an example of:

Answer 10

Regulated secretory pathway.

Question 11

Coatomer: COPI and COPII:
What are they?
What do they do?

Answer 11

• COPI and COPII are cytosolic protein complexes that assemble on the cytosolic surface of donor compartment membranes at sites where budding takes place.
• Move from the ER to the Golgi and back to the ER
• they help select cargo and direct transport vesicles in opposite directions

Question 12

COP stands for

Answer 12

COat Protein complex

Question 13

COP-I

Answer 13

COPI-coated vesicles move in
retrograde direction.

Question 14

COP-II

Answer 14

COPII-coated vesicles move in
anterograde direction

Question 15

Lysosomes: Key Features

Answer 15

• Digestive organelles
• Size: 25 nm to 1 μm
• Internal pH of 4.6 (proton
  pump or H+-ATPase)
• Contains hydrolytic enzymes:
  acid hydrolases
• Lysosomal membrane is
  composed of glycosylated
  proteins that act as a
  protective lining next to acidic
  lumen

Question 16

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

Answer 16

• Trans Golgi to endosomes
• Trans Golgi to lysosomes
• PM to endosomes

Question 17

Lysosomes: Function 1

Answer 17

1. Autophagy: Normal disassembly of unnecessary or dysfunctional
   cellular components—organelle turnover.
   i. Isolation membrane derived from ER engulf target organelles to form an
   autophagosome (also known as autophagic vesicle).
   ii. Lysosome fuses with ER-derived autophagic vesicle to form an autolysosome.
   iii. Content of autolysosome is enzymatically digested and released

Autophagosome formation → Lysosome recruitment → Autolysosome → Digestion and release

Question 18

Lysosomes: Function 2

Answer 18

2. Degradation of internalized material.
   i. Recycling of plasma membrane components like receptors and extracellular
   material.
   ii. Destroy pathogens like bacteria and viruses—only in phagocytic cells

Question 19

Plant Vacuoles

Answer 19

• vacuoles are fluid filled membrane bound
  -They can take up ~90% of the
  cell’s volume.
• Vacuoles are involved in the regulation of cytoplasmic pH, sequestration of toxic ions, regulation of cell turgor (rigidity), storage of amino acids, sugars, and CO2 in the form of malate

Question 20

Tonoplast

Answer 20

Vacuolar membrane that
contains active transport systems that allows ion and molecule transport.

Question 21

Function of Plant Vacuoles

Answer 21

1. Intracellular digestion
   - comparable to lysosomes—slightly low pH (5.0), acid hydrolases.
2. Mechanical support, turgor pressure
   * Gives rigidity to plant—supports soft tissues
   * Stretches cell wall during growth
   Function of Plant Vacuoles
3. Storage
   * Solutes and macromolecules
   * Chemical storage (toxic
   compounds as well as
   pigments like anthocyanin).

Question 22

Cytoskeleton

Answer 22

Dynamic network of interconnected filaments and tubes that extends
throughout the cytosol (and some organelles) of eukaryotes.
Functions of the cytoskeleton:
1) structural support,
2) spatial organization within
cell,
3) intracellular transport, and
4) contractility and motility.

Question 23

Microtubules (MT)

Answer 23

Largest cytoskeletal element (25 nm diameter).
* Polymer of two different proteins (monomers):
α-tubulin and β-tubulin.
* 2 major types:
Axonemal MT
* Highly organized, stable
* Part of structures (axoneme) involved in cell
movement (e.g., cilia, flagella)
Cytoplasmic MT
* Loosely organized, very dynamic
* Located in cytosol

Question 24

Microtubule-Organizing Center

Answer 24

Microtubule-Organizing Center (MTOC) is the central site of MT assembly.
* Only found in eukaryotic cells.
* Best studied MTOCs are 1) the basal bodies associated with cilia and flagella
and 2) the centrosome associated with spindle formation.

Question 25

MT-Associated Non-Motor Proteins

Answer 25

Control MT organization in cytosol.
* Many other MAPs; different MAPs expressed in different cell types;
contribute to the growth disassembly of Mts
Stabilize MTs or stimulate assembly.

Question 26

MT-Associated Motor Proteins

Answer 26

Two main types—kinesin and dynein.
* Use ATP to generate force.
* Can move material along MT track.
* Can generate sliding force between MT’s
Kinesin: plus end directed
Dynein: minus end directed