Cell-Cell Junctions Flashcards
multicellular organisms have a means of joining cells in long term associations to form tissues and organs, what is this means?
cell-cell junctions
what are cell-cell junctions?
specialized structures where two cells come together
What are the three common types of junctions in animal cells?
what do plants have?
- adhesive junctions
- tight junctions
- gap junctions
plants have plasmodesmata
Function of adhesive junctions?(2)
What do they contain that enables them to do this?
what are the two main types of adhesive junction?
- link adjoining cells together, enabling them to function as a unit
- 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
Adhesive junctions: Adherens junction
what are adherens junctions? where are they found most?
-cadherin-mediated adhesive junctions that interact with actin. They are especially prominent in epithelial cells
Adhesive junctions: Adherens junction
How do adherens junctions work?
What is a disadvantage of these adhesion systems?
- 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
Adhesive junctions: Desmosomes
What are demosomes and what do they do?
where are they abundant in?
- 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
Adhesive junctions: Desmosomes
What is the extracellular space between the two connected cells called? what does this consist of?
- called the desmosome core
- desmosome core consists of desmosomal cadherins called desmocollin, and desmoglein, desmoglein bind to cytoskeleton
Adhesive junctions: Desmosomes
How do desmosome work?
- 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
Adhesive junctions: Tight Junctions
what do they do?
where are they found?
- 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
Adhesive junctions: Tight Junctions
if there is no space inbetween a tight junction, then how do molecules cross?
-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.
Adhesive junctions: Tight Junctions
Structure of tight junctions?
What do the scaffolding proteins do?
- 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
Adhesive junctions: Tight Junctions
Role of tight junctions in blocking lateral movement of membrane proteins and lipids?
- 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
Adhesive junctions: Tight Junctions
Tight junctions contain?
Claudins
Adhesive junctions: Tight Junctions what are claudins? what are they thought to do? relationship to ion passage different tissues express..?
- 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
Adhesive junctions: Gap Junctions
what are they? what do they do?
- 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
Adhesive junctions: Gap Junctions
structure and how its related to function?
-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
Adhesive junctions: Gap Junctions
How are adjacent cells joined at gap junctions?
- joined by hollow cylinders called connexons
Adhesive junctions: Gap Junctions
structure of connexons?
- 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
