Integrin/cacodylate ion complex
Homo sapiens (human)
Similar cells group together in tissues in order to perform specific functions . For example, in animals five different kinds of tissue can being distinguished : epithelial tissue, connective tissue, blood, nervous tissue, and muscle. Often cells in the tissues are surrounded by an extracellular matrix, an amorphous substance composed of polysaccharides and proteins , which acts as "glue" that keeps them together. The attachment of cells to this matrix is accomplished by a family of membrane proteins called integrins. More than 20 of them have been identified so far in nature. Integrins are crucial for such important functions as wound healing and apoptosis (cell suicide). For example, in cases of blood vessel damage, integrins are responsible for the aggregation of cells at the site of damage, thus stopping blood leakage. At the same time, unnecessary formation of such aggregates in the blood stream leads to thrombosis, which is often the cause of strokes and heart attacks. Arthritis, sclerosis and cancer are other diseases linked to improper integrin function.
Here we can see the structure of one of them, integrin alpha-IIb-beta3 , with a cacodylate ion acting as a pseudo ligand. Like all integrins, alpha-IIb-beta3 is formed by two subunits - alpha-subunit and beta-subunit. They are connected on the extracellular side, forming the "head" of the integrin. From this "head" two "legs" span across the membrane, one from each subunit. The space between the two subunits in the "head" contains a ligand-binding site. This is also the place where the integrin binds three cations, usually Ca2+ and Mg2+. When no ligand is bound , the molecule is in a "bent" form. The head is facing towards the membrane, thus lowering the affinity for forming bonds with other molecules. When the integrin "stretches" and adopts an "extended" conformation (like the one you can see here) , the head is ready for a ligand binding. When this happens, additional conformational changes in the molecule occur. The "leg" of the beta-subunit is pushed away from the alpha-subunit "leg" and kept apart. This is believed to be the signal for subsequent processes inside the cell, although their nature is not yet fully understood. The structure of this integrin is a starting point for elucidating the structures of other complexes between integrins and different ligands. This will help for the further development of drugs targeting cell adhesion, and the diseases linked to it.
Protein Data Bank (PDB)
Xiao, T. Takagi, J. Coller, B.S. Wang, J.-H. Springer, T.A.; "Structural basis for allostery in integrins and binding to fibrinogen-mimetic therapeutics"; Nature; (2004) 432:59-67 PubMed:15378069.
Related PDBj ID
author: Rossen Apostolov