Integrin structure, allostery, and bidirectional signaling.
Integrins are one of the major families of cell adhesion receptors (Humphries, 2000; Hynes, 2002). All integrins are non-covalently linked, heterodimeric molecules containing an and a subunit. Both subunits are type I transmembrane proteins, containing large extracellular domains and mostly short cytoplasmic domains (Springer and Wang, 2004; Arnaout et al., 2005). Mammalian genomes contain 18 subunit and 8 subunit genes, and to date 24 different combinations have been identified at the protein level. Although some subunits appear only in a single heterodimer, 12 integrins contain the 1 subunit, and five contain V.