Anchoring junctions occur in two functionally different forms:Adherens junctions and desmosomes hold cells together and are formed bytransmembrane adhesion proteins that belong to the cadherin family.Focal adhesions and hemidesmosomes bind cells to the extracellular matrixand are formed by transmembrane adhesion proteins of the integrin family.On the intracellular side of the membrane, adherens junctions and focaladhesions serve as connection sites for actin filaments, while desmosomes andhemidesmosomes serve as connection sites for intermediate filaments. Adherens Junctions Connect Bundles of Actin Filaments from Cell to Cell. Adherens junctions occur in various forms. In many nonepithel ...view middle of the document...
The assembly of tight junctions between epithelial cells seems to require theprior formation of adherens junctions. Anti-cadherin antibodies that block theformation of adherens junctions, for example, also block the formation of tightjunctions.Desmosomes Connect Intermediate Filaments from Cell toCellDesmosomes are buttonlike points of intercellular contact that rivet cellsTogether. Inside the cell, they serve as anchoring sites forropelike intermediate filaments, which form a structural framework of greattensile strength. Through desmosomes, the intermediatefilaments of adjacent cells are linked into a net that extends throughout themany cells of a tissue. The particular type of intermediate filaments attached tothe desmosomes depends on the cell type: they are keratin filaments in mostepithelial cells, for example, and desmin filaments in heart muscle cells.The junction hasa dense cytoplasmic plaque composed of a complex of intracellular anchorproteins (plakoglobin and desmoplakin) that are responsible for connecting thecytoskeleton to the transmembrane adhesion proteins. These adhesion proteins(desmoglein and desmocollin), like those at an adherens junction, belong to thecadherin family. They interact through their extracellular domains to hold theadjacent plasma membranes together.The importance of desmosome junctions is demonstrated by some forms of thepotentially fatal skin disease pemphigus. Affected individuals make antibodiesagainst one of their own desmosomal cadherin proteins. These antibodies bindto and disrupt the desmosomes that hold their skin epithelial cells(keratinocytes) together. This results in a severe blistering of the skin, withleakage of body fluids into the loosened epithelium.Anchoring Junctions Formed by Integrins Bind Cells to theExtracellular Matrix: Focal Adhesions andHemidesmosomesSome anchoring junctions bind cells to the extracellular matrix rather than toother cells. The transmembrane adhesion proteins in these cell-matrixjunctions are integrins a large family of proteins distinct from the cadherins.Focal adhesions enable cells to get a hold on the extracellular matrix through integrins that link intracellularly to actin filaments. In this way, muscle cells,for example, attach to their tendons at the myotendinous junction. Likewise,when cultured fibroblasts migrate on an artificial substratum coated withextracellular matrix molecules, they also grip the substratum at focaladhesions, where bundles of actin filaments terminate. At all such adhesions,the extracellular domains of transmembrane integrin proteins bind to a proteincomponent of the extracellular matrix, while their intracellular domains bindindirectly to bundles of actin filaments via the intracellular anchor proteinstalin, a-actinin, filamin, and vinculin.Hemidesmosomes, or half-desmosomes, resemble desmosomesmorphologically and in connecting to intermediate filaments, and, likedesmosomes, they act as rivets to distribute tensile or shearing for...
