Lecture 22

Question 1

What are the proteins that make up the junctions?

Answer 1

Transmembrane proteins
Cytoplasmic protens
Cytoskeletal elements (actin)

Question 2

What are adherens junctions?

Answer 2

Fist junctions to form upon cell-cell contact

Question 3

What is the function of adherens junctions?

Answer 3

Function: to adhere cells together

Question 4

What are adherens junctions originally referred to as?

Answer 4

Originally referred to as Zonula Adherens/adhesion belt

Question 5

What are adherens junctions characterized by?

Answer 5

Adherens junctions are characterized based on the transmembrane proteins that they use

Question 6

What are the 2 different general types of transmembrane proteins?

Answer 6

1) Cadherins
2) Nectin (1-4)

Question 7

What are the different types of cadherins?

Answer 7

E- Cadherin (Epithelia)
N- Cadherin (Neurons)
P- Catherine (Placenta)
T- Cadherin (Early embryos and neurons)
R- Cadherin (Retina)
M- Cadherin (Myotube)

Question 8

What are the nectins (1-4)?

Answer 8

Nectin-like (Necl 1-5)

Question 9

What was the first adherens junction transmembrane protein to be discovered?

Answer 9

E-Cadherin

Question 10

How large was the first adherens junction transmembrane protein?

Answer 10

135kD

Question 11

What are E-Cadherins?

Answer 11

A transmembrane protein
5 domains
Interact with identical proteins from the neughbouring cell
Calcium dependent interaction
E-cadherin binding is relatively weak
The strength to maintain the junctions lies in their association with actin

Question 12

What are cytoplasmic binding proteins called?

Answer 12

Catenins

Question 13

What are some cytoplasmic binding protein catenins?

Answer 13

Alpha, beta, gamma (AKA. Plakoglobin. It is also found at desmosomes), amd p120 catenin

Question 14

What do catenins do?

Answer 14

Link the cytoskeleton to the transmembrane proteins

Question 15

What else can catenin do?

Answer 15

Can act as asignaling molecules
B-catenin-wnt signaling pathway

Question 16

What was thought of B-catenin?

Answer 16

For years the nuclear localization of B-catenin couldn’t be explained.
It was thought that it was a staining artifact

Question 17

What does the crux of the Wnt signalling pathway involve?

Answer 17

The crux of the Wnt signalling pathway involves 3 things:
1) Accumulation of B-catenin in cytoplasm,
2) Translocation of B-catenin to the nucleus
3) Activation of transcription factors in the nucleus

Question 18

What are the 3 key proteins involved in signalling by junction proteins in Wnt?

Answer 18

1) Wnt
2) Frizzled
3) Dishevelled

Question 19

What is Wnt?

Answer 19

Extracellular ligand
Int: (1982) proto-oncogene. A “pre” cancer gene/gene product discovered by Harold Varmus’ Lab (Nobel Prize)
Wingless: already known protein in Drosophila known for cell polarity and embryonic development. Nobel prize given to Nusslein-Volhard and Weischaus
But Int and Wingless are the same proteins… so what do you do?
Combine them Wingless + Int= Wnt

Question 20

What is Frizzled?

Answer 20

Membrane receptor

Question 21

What is Dishevelled?

Answer 21

Cytoplasmic Protein

Question 22

What are some other crucial proteins?

Answer 22

The DESTRUCTION COMPLEX:
Axin
GSK3beta (Glycogen synthase kinase 3beta)
APC (Adenomatosis polyposus coli)
CK1alpha (Casein Kinase 1alpha)

Question 23

How was nectin discovered?

Answer 23

Was looking for novel actin associated proteins and found afadin

Question 24

What is afadin?

Answer 24

A 205 kD protein that binds to a variety of junction associated proteins; actin, ZO-1, vinculin, and others

Question 25

What are the two splice variants of afadin?

Answer 25

S- afadin L- afadin

Question 26

What is S-afadin?

Answer 26

Expressed in neural tissue

Question 27

What is L-afadin?

Answer 27

Ebiquitously expressed L-afadin contains one PDZ, three proline-rich regions, one F-actin-binding region (at the carboy-terminal)

Question 28

Where are nectins and cadherins found?

Answer 28

Nectins are found primarily at the adherens junctions, wheres cadherins are located along the lateral plasma memrbane in epithelial cells

Question 29

What do nectins bind to?

Answer 29

Nectins bind to afadin through the 4 most distal amino acids (E/AxYV) in the cytoplasm of nectins and the PDZ domains of afadin (Note: X is any amino acid)

Question 30

Why is the binding process required for nectin?

Answer 30

This is required for nectins to cluster at cell-cell junctions AND for a cadherin-catenin complex to be recruited to nectin based cell-cell adhesion sites

Question 31

What does this attach the nectins to?

Answer 31

This attaches the nectins to the actin cytoskeleton

Question 32

Do Necls bind to afadin?

Answer 32

Necls do not interact with afadin, but bind other proteins

Question 33

How do nectins interact?

Answer 33

Nectins homotypically interact in cis on the same cell and cis interactions are needed for the trans interactions for cell-cell binding.

Question 34

Are cis dimer interactions needed for Necls to trans interact?

Answer 34

It is unknown whether homotypic cis dimer interactions are needed for Necls to trans interact

Question 35

What is the strongest trans interaction for nectin binding?

Answer 35

Nectin-1 and Nectin 3

Question 36

What are the other Nectins that bind?

Answer 36

Nectin-3 and Nectin-5 Nectin-2 and Nectin-3

Question 37

Do nectins form primordial junctions?

Answer 37

Nectin forms primordial junctions in cells that don't normally make junctions

Question 38

In what order does binding occur upon cell-cell contact?

Answer 38

Nectins bind first Then E-Cadherin Then JAM (The Nectin/Cadherin/JAM adhesions intitially are interdispursed and mixed-up) Then they start to segregate into tight junctions and adherens junctions once the claudins adhere

Question 39

What is the role of the nectins in contact inhibiton?

Answer 39

When 2 cells collide, Necl-5 on the surface of one cell will interact with Nectin-3 (obviously a trans interaction) This will start the formation of a cell-cell junction Then Necl-5 is down-regulated and endocytosed from the plasma membrane

Question 40

What does Nectin-3 do then?

Answer 40

Then Nectin-3 releases from Necl-5 and is kept on the plasma membrane where it will not interact with Nectin-1 Then cadherin is recruited