Integrins are the principal cell-surface receptors used by animal cells to bind to the extracellular matrix: they function as transmembrane linkers between the extracellular matrix and the cytoskeleton. Most integrins connect to actin filaments, while those at hemidesmosomes bind to intermediate filaments. Integrin molecules are heterodimers, and the binding of extracellular matrix ligands or intracellular activator proteins such as talin results in a dramatic conformational switch from an inactive to an active state. This creates an allosteric coupling between binding to matrix outside the cell and binding to the cytoskeleton inside it, allowing the integrin to convey signals in both directions across the plasma membrane. Complex assemblies of proteins become organized around the intracellular tails of activated integrins, producing intracellular signals that can influence almost any aspect of cell behavior, from proliferation and survival, as in the phenomenon of anchorage dependence, to cell polarity and guidance of migration. Integrin-based cell–matrix junctions are also capable of mechanotransduction: they can sense and respond to mechanical forces acting across the junction.
Each cell in a plant deposits, and is in turn completely enclosed by, an elaborate extracellular matrix called the plant cell wall. It was the thick cell walls of cork, visible in a primitive microscope, that in 1663 enabled Robert Hooke to distinguish and name cells for the first time. The walls of neighboring plant cells, cemented
Figure 19–60 Talin is a tension sensor at cell–matrix junctions. tension across cell–matrix junctions stimulates the local recruitment of vinculin and other actin-regulatory proteins, thereby strengthening the junction’s attachment to the cytoskeleton. the experiments presented here tested the hypothesis that tension is sensed by the talin adaptor protein that links integrins to actin filaments (see Figure 19–55). (a) the long, flexible, C-terminal region of talin is divided into a series of folded domains, some of which contain vinculin-binding sites (dark green lines) that are thought to be hidden and therefore inaccessible. One domain near the N-terminus, for example, comprises a folded bundle of 12 α helices containing five vinculin-binding sites. (B) this experiment tested the hypothesis that tension stretches the 12-helix domain, thereby exposing vinculin-binding sites.  a fragment of talin containing this domain was attached to an apparatus in which the domain could be stretched, as shown here. the fragment was labeled at its N-terminus with a tag that sticks to the surface of a glass slide on a microscope stage. the C-terminal end of the fragment was bound to a tiny magnetic bead, so the talin fragment could be stretched using a small magnetic electrode. the solution around the protein contained fluorescently tagged vinculin proteins. after the talin protein was stretched, excess vinculin solution was washed away, and the microscope was used to determine if any fluorescent vinculin proteins were bound to the talin protein. In the absence of stretching (top), most talin molecules did not bind vinculin. When the protein was stretched (bottom), two or three vinculin molecules were bound (only one is shown here for clarity). (adapted from a. del rio et al., Science 323:638– 641, 2009.) together to form the intact plant (Figure 19–61), are generally thicker, stronger, and, most important of all, more rigid than the extracellular matrix produced by animal cells. In evolving relatively rigid walls, which can be up to many micrometers thick, early plant cells forfeited the ability to crawl about and adopted a sedentary lifestyle that has persisted in all present-day plants.
the Composition of the Cell Wall Depends on the Cell type