Mus musculus (house mouse)
Unicellular organisms have an easy life. They have the cell interior surrounded by a lipid membrane and maybe some means like flagella to help then move from one place to another. For multicellular organisms like for instance human beings the situation is more complicated. Were their cells just a collection of individual cells, the result might be more something of a liquid or gel and not a human body. While the cells themselves already have quite complex structures, there must be ways to keep cells near each other and assist in the formation of organs.
A class of compounds helping cells stick together are proteins like the one seen here, E-cadherin. This protein helps individual cells stay together. It is made of three basic types of building blocks: a part that extends into the cytoplasm at the inside of the cell, a membrane bound part that anchors it to the surface of the cell and an extra- cellular part that points to the outside of the cell. The structure here shows only one fragment of the extracellular domain but it can help explain how it acts as a glue for cells. Whether this extracellular part is sticky or not is controlled by the binding of calcium. To act as a glue it needs to have calcium bound and to give up its adhesive properties the calcium has to be removed. Cells with defective cadherins where this mechanism does not work properly may be responsible for the metastasis of tumor cells making them more able to move through the body.
Protein Data Bank (PDB)
Nagar, B. Overduin, M. Ikura, M. Rini, J.M.; "Structural basis of calcium-induced E-cadherin rigidification and dimerization."; Nature; (1996) 380:360-364 PubMed:8598933.
author: Arno Paehler