11.3Adhesion of Cells snd Extracellular Matrix
Many types of molecule are known to be involved in the adhesion between cells and extracellular matrix, of which integrins - proteins located in the plasma membrane - play the most important role.
Integrins are transmembrane proteins that penetrate the plasma membrane and bind to extracellular matrix using their extracellular domain. They convey extracellular signals (e.g., a signal indicating binding with extracellular matrix material) into the cell using the intracellular domain. Like cadherins, the intracellular domain is bound with actin filaments (Fig. 11-6).
Integrins function as dimers for the two distinct subunits of α and β. Since there are many types of α and β, many combinations of subunits forming a dimer are possible. The large number of combinations reflects the diversity of the tissues where the dimers are located, the target molecules to be adhered, and the integrin functions.
Extracellular matrix components, to which integrins attach, have a common structure for adhering to integrins that consists of the three-amino-acid sequence (arginine-glycine-aspartic acid). Integrins recognize this sequence and selectively adhere to it.
The adhesion between integrins and extracellular matrix serves to connect cells to extracellular matrix. The adhesion of epithelial cells and basal laminae is an example of this (see the Column in 11.2). This adhesion plays the further role of conveying extracellular signals into the cell, a process in which particular extracellular matrix components serve as signaling molecules. The signals from extracellular matrix have a great impact on cell division, migration, differentiation and survival.
Fig. 11-6. A model of adhesion between integrins and extracellular matrix
Integrins not only serve as adhesive molecules that bind to extracellular matrix, but are also involved in conveying extracellular signals into the cell and regulating cytoskeleton assembly. The intracellular domain of an integrin is therefore bound with focal adhesion kinase (FAK) - a signaling protein - and actin filaments of the cytoskeleton.
Extracellular Matrix in Plants
The main extracellular matrix components of plants is the cell wall that surrounds cells. Besides wrapping cells, cell walls provide plant tissues with structural support by solidly binding cells together. The main constituents of cell walls include cellulose, hemicellulose, pectin and lignin. Cellulose, hemicellulose and pectin are polysaccharides consisting of linear chains of sugars. Lignin is a molecule in which propyl benzene derivatives (the aromatic rings with hydroxyl and methoxyl groups) are intricately combined, and reinforces cell walls. The cell walls are permeable, and molecules with a molecular mass of less than 20,000 can penetrate by diffusion.