In Key Concept 5.4 you learned about the extracellular matrix of animal cells, which is composed of collagen protein arranged in fibers in a gelatinous matrix of proteoglycans. The attachment of a cell to the extracellular matrix is important in maintaining the integrity of a tissue. In addition, some cells can detach from their neighbors, move, and attach to other cells; this is often mediated by interactions with the extracellular matrix.

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A transmembrane protein called integrin often mediates the attachment of epithelial cells to the extracellular matrix (Figure 6.8A). More than 24 different integrins have been described in human cells. All of them bind to a protein in the extracellular matrix outside the cell, and to actin filaments, which are part of the cytoskeleton, inside the cell. So in addition to adhesion, integrin has a role in maintaining cell structure via its interaction with the cytoskeleton.

The binding of integrin to the extracellular matrix is noncovalent and reversible. When a cell moves its location within a tissue or organism, one side of the cell detaches from the extracellular matrix while the other side extends in the direction of movement, forming new attachments in that direction (Figure 6.8B). The integrin at the “back” of the cell (away from the direction of movement) is brought into the cytoplasm by endocytosis (see Key Concept 6.5) so that it can be recycled and used for new attachments at the “front” of the cell. These events are important for cell movement within the developing embryo and for the spread of cancer cells.

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