Cell-Cell Junctions

Question 1

multicellular organisms have a means of joining cells in long term associations to form tissues and organs, what is this means?

Answer 1

cell-cell junctions

Question 2

what are cell-cell junctions?

Answer 2

specialized structures where two cells come together

Question 3

What are the three common types of junctions in animal cells?

what do plants have?

Answer 3

1. adhesive junctions
2. tight junctions
3. gap junctions

plants have plasmodesmata

Question 4

Function of adhesive junctions?(2)
What do they contain that enables them to do this?
what are the two main types of adhesive junction?

Answer 4

1. link adjoining cells together, enabling them to function as a unit
2. adhesive junctions link the cytoskeleton to the cell surface
   - they have intracellular attachment proteins on the inside of the cell and cadherins protruding from the outer surface
   - two main types of adhesive junctions are adherens junctions, and desmosomes

Question 5

Adhesive junctions: Adherens junction

what are adherens junctions? where are they found most?

Answer 5

-cadherin-mediated adhesive junctions that interact with actin. They are especially prominent in epithelial cells

Question 6

Adhesive junctions: Adherens junction
How do adherens junctions work?
What is a disadvantage of these adhesion systems?

Answer 6

• Protein B-catenin binds to cytosolic tail of cadherin
• B-catenin is bound by a second protein, a-catenin, which recruits actin to the junction; sometimes plakoglbulin is present alongside of B-catenin
• p120-catenin binds to the cytoplasmic tail of cadherins near the plasma membrane, regulates stability of cadherin at cell surface as well as the activity of Rho, an actin regulator
• many pathogens infect the body by using these same adhesion systems to gain entry into the cells

Question 7

Adhesive junctions: Desmosomes
What are demosomes and what do they do?
where are they abundant in?

Answer 7

• button-like points of strong adhesion between adjacent cells in a tissue.
• desmosomes give tissue structural integrity
• abundant in skin, heart muscle, and the neck of the uterus

Question 8

Adhesive junctions: Desmosomes

What is the extracellular space between the two connected cells called? what does this consist of?

Answer 8

• called the desmosome core

- desmosome core consists of desmosomal cadherins called desmocollin, and desmoglein, desmoglein bind to cytoskeleton

Question 9

Adhesive junctions: Desmosomes

How do desmosome work?

Answer 9

• B-catenin family protein plakoglobulin binds to desmocollin, and a protein called desmoplakin
• desmoplakin attaches to tonofilaments, which are composed of intermediate filaments such as vimentin, desmin, and keratin
• a thick plaque with these linker proteins and tonofilaments is located beneath the membrane of the two adjoining cells

Question 10

Adhesive junctions: Tight Junctions
what do they do?
where are they found?

Answer 10

• tight junctions seal epithelial layers, leaving no space between the plasma membranes of adjacent cells, thus preventing movement of molecules across cell layers
• especially prominent in intestinal epilthelial cells

Question 11

Adhesive junctions: Tight Junctions

if there is no space inbetween a tight junction, then how do molecules cross?

Answer 11

-the tight junctions between adjacent cells form a continuous belt around the apical ends of lateral surfaces of each cell, these belts together form a formidable barrier, so that molecules must typically cross the cell layer by passing through the cells themselves.

Question 12

Adhesive junctions: Tight Junctions
Structure of tight junctions?
What do the scaffolding proteins do?

Answer 12

• tight junctions appear as a series of ridges forming an interconnected network extending across the junction
• scaffolding proteins at tight junctions recruit proteins such as F-actin to the junctions

Question 13

Adhesive junctions: Tight Junctions

Role of tight junctions in blocking lateral movement of membrane proteins and lipids?

Answer 13

• tight junctions also block lateral movement of lipids and proteins in the plasma membrane
• movement of lipids is blocked only on the outer monolayer
• movement of integral membrane proteins is completely blocked

Question 14

Adhesive junctions: Tight Junctions

Tight junctions contain?

Answer 14

Claudins

Question 15

Adhesive junctions: Tight Junctions 
what are claudins?
what are they thought to do?
relationship to ion passage 
different tissues express..?

Answer 15

• proteins with four membrane spanning domains
• in adjacent cells, claudins are thought to interlock to form a tight seal; the large extracellular loop forms ion-selective pores to allow passage of specific ions
• b/c ions move between cells, it is called paracellular transport, which is probably regulated by claudins
• different tissues express different claudins

Question 16

Adhesive junctions: Gap Junctions

what are they? what do they do?

Answer 16

• region where the plasma membrane of two cells are aligned and brought into intimate contact, with a very small gap between
• gap junctions allow direct electrical and chemical communication between cells

Question 17

Adhesive junctions: Gap Junctions

structure and how its related to function?

Answer 17

-the gap is spanned by small molecular “pipelines” , thus gap junctions provide a point of cytoplasmic contact between two adjacent cells through which ions and small molecules can pass directly from one cell to another

Question 18

Adhesive junctions: Gap Junctions

How are adjacent cells joined at gap junctions?

Answer 18

• joined by hollow cylinders called connexons

Question 19

Adhesive junctions: Gap Junctions

structure of connexons?

Answer 19

• each connexon is an assembly of six subunits of connexon proteins, which form a circle
• each connexon has a channel about 3nm wide, to small to allow proteins, nucleic acids, or organelles through
• different tissues have different types of connexons but they all function the same way