Animal and protozoan cells do not have cell walls to help maintain shape and provide structural stability. Instead, these eukaryotic cells secrete a sticky mass of carbohydrates and proteins into the spaces between adjacent cells. This network of proteins and molecules is called an extracellular matrix or ECM.
Some ECM proteins assemble into a basement membrane to which the remaining components adhere. Proteoglycans typically form the bulk of the ECM while fibrous proteins, like collagen, provide strength. Both proteoglycans and collagen are attached to fibronectin proteins, which, in turn, are attached to integral membrane proteins called integrins.
Mammalian integrins are heterodimeric transmembrane adhesion receptors comprising α and β glycoprotein subunits. Eighteen different α-chain genes and eight different β-chain genes dimerize in various combinations resulting in 24 types of integrins specific to vertebrates. They span the membrane and interact with proteins in the cytosol and the ECM. The α subunit contains a seven-bladed β-propeller head domain while the β subunit has an I-like head domain; together, they bind ligand. The ligand binding head is connected to two leg-like structures, one from each subunit. The β subunit interacts with the cytoskeletal protein and helps transmit signals in and out of the cell.
Adapted from section 5.1 Components and structure, 4.2 Epithelial tissue, and 3.4 Unique characteristics of eukaryotic cells, Openstax biology 2e