ABSTRACT
Integrins are heterodimeric transmembrane glycoproteins, consisting of noncovalently bound α and β subunits. Eighteen α and eight β subunits are described in humans, assembling into 24 diff erent integrins (Arnaout et al. 2007). Th e α and β chains share no homology, being distinct polypeptides with specifi c domain structures. Th e extracellular domains from both subunits contribute to the ligandbinding site of the heterodimer (Takada et al. 2007). Th e extracellular domain of the α subunit contains seven repeats of approximately 60 residues, which fold into a seven-bladed propeller structure with β-sheets arranged around a central axis. A subset of integrin α chains have an insertion domain (αA) containing a cation binding site, located between repeats two and three of the propeller. C-terminal to the propeller is an Ig-like Th igh domain, followed by two β-sandwich modules, Calf-1 and Calf-2, and a small transmembrane domain (Fig. 3A) (Humphries et al. 2003, Arnaout et al. 2007). Th e intracellular domains of the α subunits show little homology, except for a conserved motif proximal to the transmembrane region that associates with the cytoplasmic tail of the β chain (Arnaout et al. 2007).
