Читать книгу Introduction to the Human Cell - Danton PhD O'Day - Страница 28

Desmosomes were the first junctional adhesion complex members to be recognized. This is because they are the most distinct component appearing as paired dark and dense masses adjacent to the cell membrane of adjacent epithelial cells. Desmosomes provide strong adhesion between cells. They are typically found in epithelial cells and other cell types that are subjected to stress or shear (e.g., cardiac muscle, epithelium of skin, cervix of the vagina). The following electron microscope picture (left) is false-colored (right) to show the different components of the desmosome more clearly (Figure 3.12).

The paired dark masses that reside on opposite sides of the intercellular space are called desmosomal adhesion plaques. Early work showed that mild digestion with dilute protein digesting enzymes (proteases), such as trypsin, caused the desmosomes to disappear and the cells to separate. Thus it became clear that these structures were primarily made up of proteins involved in cell adhesion. The adhesion plaques link to tonofilaments in the cytoplasm. It was discovered that the tonofilaments are keratin intermediate filaments, so both terms are used when desmosomes are discussed.

Figure 3.12. The ultrastructure of a desmosome.

Adhesion between cells is mediated by desmogleins and desmocollins which are desmosomal forms of cadherins (Figure 3.13). These extend from the cell membrane across the intercellular space. They differ from other cadherins in their intracellular domains which is why desmosomal cadherins associate with keratin while those in adherens junctions are linked to actin filaments. The dense plaques on the inner side of the membrane are sites where the desmoplakin and plakoglobin linker molecules link the cytoplasmic tails of the desmogleins and desmocollins to the tonofilaments (keratin intermediate filaments). Plakoglobin for example is very similar to ß-catenin.

Figure 3.13. The structure of a desmosome and its protein components.

A close-up of the proteins found in desmosomes is shown in the following figure (Figure 3.14). Homotypic binding occurs between the desmosomal cadherins desmoglein and desmocolin.

Figure 3.14. The organization of desmosomal proteins.