Cell-Cell junctions and the ECM

Cell Junctions

physically connect one cell to the next and anchor cells to the ECM

Cadherins

transmembrane proteins that connect cells to other cells

Why are there different types of cadherins on different types of cells?

To ensure that only the correct cell types are able to attach to each other

Process of cadherin binding:

The extracellular domain of a cadherin molecule binds to the extracellular domain to a cadherin of the same type of on an adjacent cell.

The cytoplasmic portion of the cadherin is linked to the internal cytoskeleton.

What is the structural importance of cadherin arrangement?

Provides structural continuity from the cytoskeleton of one cell to the cytoskeleton of another, increasing the strength of tissues and organs

Integrins

transmembrane protein that enables cells to adhere to the ECM

Function of integrins

provides structural reinforcement to tissues under physical stress

Integrin binding process

Thy cytoplasmic domain (intracellular) binds to a microfilament and the extracellular domain binds to an intermediate filament in the ECM

3 types of cell junctions

Anchoring, Barrier, and Communicating

3 types of Anchoring cell junctions

Adherens junctions, desmosomes, and hemidesmosomes

Adherens Junctions

Belt-like junctional complex of cadherins that goes around the circumference of the cell

Uses cadherins

Adherens Junctions binding process

The belt of cadherins attaches to a band of actin filaments in the cytoplasm. The cadherins bind to other cadherins in adherens junctions in the extracellular space

Desmosomes

button-like points of adhesion that hold the plasma membrane of adjacent cells together

uses cadherins

Desmosomes binding process

Cadherins in the desmosomes of one cell bind to the cadherins in the desmosomes of adjacent cells. Connect intermediate filaments of one cell to the intermediate filaments of another cell.

Hemidesmosomes

version of the desmosome

anchors epithelial cells to the basal lamina

Hemidesmosomes binding process

Integrins connect intermediate filaments inside the cell to the ECM

What do adherens junctions and desmosomes NOT prevent?

the free passage of materials through the spaces between cells they connect

A tight junction is a _______ cell junction.

Barrier

Tight junction

junctional complex that establishes a seal between cells so that the only way a substance can travel from one side of a sheet of epithelial cells to the other is by moving through the cells by means of cellular transport

Function of tight junctions

prevent passage of materials between cells

Claudins and Occludins

proteins that bind to each other to create a waterproof/movement proof seal

Like stitching cells together

What are the two sides of the cells created by tight junctions?

Apical (top) and Basal (bottom)

2 types of communication cell junctions

Gap Junctions and Plasmodesmata

Gap Junctions and Plasmodesmata

Connections between the plasma membrane of adjacent cells that permit materials to pass directly from the cytoplasm of one cell to the cytoplasm of another

Gap junctions

Form when a set of integral proteins arranged in a ring connects to a similar ring of proteins in the membrane of another cell

What passes through gap junctions?

ions and signaling molecules

Extracellular Matrix

insoluble meshwork composed of proteins and polysaccharides

Contributes to structual support and provides informational cues that determine the activity of the cells that are in contact with it

Secreted by cells

Composition of ECM

large fibrous proteins including collagen, elastin, and laminin

Function of ECM

• Cell Shape

• Structural support

• Signaling between cell

Collagen

provides strength to ECM

3 polypeptide wound around one another in a triple helix

Elastin

provides flexibility to ECM

Has a lot of stretches of unstructured polypeptide chains that are cross-linked together

Unstructured regions tend to contain very few hydrophobic amino acids that usually make it a polypeptide core.