Lecture 22 Flashcards
What are the proteins that make up the junctions?
Transmembrane proteins
Cytoplasmic protens
Cytoskeletal elements (actin)
What are adherens junctions?
Fist junctions to form upon cell-cell contact
What is the function of adherens junctions?
Function: to adhere cells together
What are adherens junctions originally referred to as?
Originally referred to as Zonula Adherens/adhesion belt
What are adherens junctions characterized by?
Adherens junctions are characterized based on the transmembrane proteins that they use
What are the 2 different general types of transmembrane proteins?
1) Cadherins
2) Nectin (1-4)
What are the different types of cadherins?
E- Cadherin (Epithelia)
N- Cadherin (Neurons)
P- Catherine (Placenta)
T- Cadherin (Early embryos and neurons)
R- Cadherin (Retina)
M- Cadherin (Myotube)
What are the nectins (1-4)?
Nectin-like (Necl 1-5)
What was the first adherens junction transmembrane protein to be discovered?
E-Cadherin
How large was the first adherens junction transmembrane protein?
135kD
What are E-Cadherins?
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
What are cytoplasmic binding proteins called?
Catenins
What are some cytoplasmic binding protein catenins?
Alpha, beta, gamma (AKA. Plakoglobin. It is also found at desmosomes), amd p120 catenin
What do catenins do?
Link the cytoskeleton to the transmembrane proteins
What else can catenin do?
Can act as asignaling molecules
B-catenin-wnt signaling pathway
What was thought of B-catenin?
For years the nuclear localization of B-catenin couldn’t be explained.
It was thought that it was a staining artifact
What does the crux of the Wnt signalling pathway involve?
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
What are the 3 key proteins involved in signalling by junction proteins in Wnt?
1) Wnt
2) Frizzled
3) Dishevelled
What is Wnt?
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
What is Frizzled?
Membrane receptor
What is Dishevelled?
Cytoplasmic Protein
What are some other crucial proteins?
The DESTRUCTION COMPLEX:
Axin
GSK3beta (Glycogen synthase kinase 3beta)
APC (Adenomatosis polyposus coli)
CK1alpha (Casein Kinase 1alpha)
How was nectin discovered?
Was looking for novel actin associated proteins and found afadin
What is afadin?
A 205 kD protein that binds to a variety of junction associated proteins; actin, ZO-1, vinculin, and others
What are the two splice variants of afadin?
S- afadin
L- afadin
What is S-afadin?
Expressed in neural tissue
What is L-afadin?
Ebiquitously expressed
L-afadin contains one PDZ, three proline-rich regions, one F-actin-binding region (at the carboy-terminal)
Where are nectins and cadherins found?
Nectins are found primarily at the adherens junctions, wheres cadherins are located along the lateral plasma memrbane in epithelial cells
What do nectins bind to?
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)
Why is the binding process required for nectin?
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
What does this attach the nectins to?
This attaches the nectins to the actin cytoskeleton
Do Necls bind to afadin?
Necls do not interact with afadin, but bind other proteins
How do nectins interact?
Nectins homotypically interact in cis on the same cell and cis interactions are needed for the trans interactions for cell-cell binding.
Are cis dimer interactions needed for Necls to trans interact?
It is unknown whether homotypic cis dimer interactions are needed for Necls to trans interact
What is the strongest trans interaction for nectin binding?
Nectin-1 and Nectin 3
What are the other Nectins that bind?
Nectin-3 and Nectin-5
Nectin-2 and Nectin-3
Do nectins form primordial junctions?
Nectin forms primordial junctions in cells that don’t normally make junctions
In what order does binding occur upon cell-cell contact?
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
What is the role of the nectins in contact inhibiton?
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
What does Nectin-3 do then?
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
