ding site or thei!
             Each member of the integrin family consists of a non-covalently linked heterodimer of an 'a' chain and a 'b' chain, which are transmembrane proteins with short cytoplasmic tails. By binding to the matrix protein outside the cell and to the actin cytoskeleton (via the attachment proteins talin and alpha-actinin) inside the cell, the protein serves as a transmembrane linker. The 'a' and 'b' chains are both glycosylated and are held together by noncovalent bonds.4 Until now 16 alpha and 8 beta subunits have been identified. From these subunits some 22 integrins are formed in nature, which implicates that not all possible combinations exist. The beta-4 subunit for instance can only form a heterodimer with the alpha-6 subunit. On the other hand the beta-1 subunit can form heterodimers with ten different alpha subunits. Because not all the beta-1 alpha heterodimers have the same ligand specificities, it is believed that the alpha chain is at least partly involved in the ligand spe!
             cificity. The diversity of the integrins is increased by the alternative splicing of some integrin messenger RNA's
             Integrins differ from other cell-surface receptors in that they bind their ligands with a low affinity (10^6-10^9 liters/mole) and that they are usually present at 10-100 fold higher concentration on the cell surface. The integrins however can only bind their ligands when they exceed a certain minimal number of integrins at certain places, called focal contacts and hemidesmosomes. So when the integrins are diffusely distributed over the cell surface, no adhesion will be present, but when after a certain stimuli these integrins cluster for example in focal contacts their combined weak affinities give rise to a spot on the cell surface which has enough adhesive (sticking) capacity to adhere to the extracellular matrix. This is a very usefull situation, because in this way cells can bi...