Question

Relate the following complexes to cell-cell interactions (adherens junctions, desmosomes, tight junctions, & gap junctions) or cell-ECM interactions

oCadherins

oIntegrins

oClaudins

oActin

oCatenins

oIntermediate filaments (keratin)

oConnexins

oElastin/collagen

oFibronectin/laminin

oProteoglycans

Answer 1

Cadherins:

Cadherins are the transmembrane proteins that help in cell to cell adhesion in animals.

It plays a vital role in tissue morphogenesis and homeostasis.

• During morphogenesis tissue can change in size and shape and form a distinct cell layers.
• Cadherins functions in tissue morphogenesis by controlling cell-cell adhesion and cell signalling.

Cadherins and cell adhesion:

When cell contact cadherins forms a transbonds across at the site of contact between the two cells.

Once the transbonds are formed cadherins regulate the cell – cell contact in 3 ways

• Reduces local interfacial tension directly through cell adhesion tension.
• Reduces local interfacial tension indirectly by signalling to actomyosin cytoskeleton.
• Establishes the mechanical coupling of contacting cells.

Integrins:

Integrins are the transmembrane cell-matrix adhesion receptors. Integrins serve as cell-cell adhesion molecules, which help cells to bind to other cells and to the extracellular matrix.

Integrins are transmembrane heterodimers, consist of two non convalently associated transmembrane glycoprotein subunits ? and ?.

Claudins:

Claudins are tight junction membrane proteins.

It is the integral membrane proteins, which is found in tight junctions of epithelia and endothelia cells.

Claudins act as paracellular barriers and pores which determines the permeability properties of epithelial and endothelial cells.

Actin:

Actin is the abundant protein in eukaryotic cells. There transition between monomeric G-actin and filamentous F-actin plays a important role in cellular functions such as cell motility, maintenance of cell shape and polarity to the regulation of transcription etc.

The interaction of filamentous actin with myosin forms the basis of muscle contraction.

Catenins:

Catenins are cell adhesion proteins which is found along with cadherin. There are 2 catenins identified ?-catenin and ?-catenin.

?-catenin binds to ?-catenin and they can bind to actin.

Catenins play a important role in cellular organisations and polarity.

Intermediate filaments – keratin:

Intermediate filaments have the diameter of 10nm, which is intermediate between actin – 7nm and microtubules 25nm and thus it has got its name as intermediate filaments.

• They help in providing mechanical strength to cells and tissues.
• Type I and type II consist of two groups of keratins which include 15 different proteins, expressed in epithelial cells.
• Each epithelial cells secrete at least one acidic and one type II neural/basic keratin.
• Which together polymerise to form filaments.
• Some type I and type II together forms hard keratins such as hairs, nails and horns.
• Other type I and type II together forms soft keratins that are present in the cytoplasm of epithelial cells.
• Vimentin – type III intermediate filament present in smooth muscle, fibroblasts.
• Desmin – type III intermediate filament present in muscle cells.
• Neurofilament proteins(NF proteins) – type IV present in mature neurons.

Connexins:

Connexins are proteins present in the gap junctions, which helps in direct intra cellular communication.

Connexins contains four transmembrane segments and two extra-cellular loops.

Connexins assemble in six to form hemichannels or connexons. Two hemichannels joins to form gap junctions.

Collagen/Elastin:

collagen is the structural protein present in the connective tissue. It is the elongated fibrils present in tendons, ligaments and skin.

Elastin:

Elastin is also the structural protein present in the connective tissue. It is the elastic protein which helps in resuming the shape after stretching or contracting. Elastin is present in lungs, skin, bladder.

Fibronectin/laminin:

Laminin:

Laminin is the protein present in the extra-cellular matrix.

Laminin is important in forming basal lamina. It is important in cell differentiation, migration and adhesion.

• Laminin is the heterotrimeric protein which contains ?? and ? chains.
• It forms cross-like structures with other cell membranes and extracellular matrix.

Fibronectin:

Fibronectin is the protein of extracellular matrix which binds to receptor proteins and integrins.

• Fibronectin is a dimer, two monomer united by a disulphide bond.
• Fibronectin plays a important role in cell adhesion, growth, differentiation and migration.
• Fibronectin is also plays important role in wound healing process.

Proteoglycans:

Proteoglycans are the proteins present in the extracellular matrix.

These are the proteins that fill the space between the cells.

• Proteoglycans are the proteins that are highly glycosylated.
• Proteoglycans along with collagen forms cartilage.
• They play a important role in binding cations and water. It also functions in regulating the movement of molecules through the matrix.