Downloaded from http://cshperspectives.cshlp.org/ on September 23, 2021 - Published by Cold Spring Harbor Laboratory Press Integrin Structure, Activation, and Interactions Iain D. Campbell1 and Martin J. Humphries2 1Department of Biochemistry, University of Oxford, Oxford OX1 3QU, United Kingdom 2Wellcome Trust Centre for Cell-Matrix Research, University of Manchester, Manchester, M13 9PT, United Kingdom Correspondence: [email protected] Integrins are large, membrane-spanning, heterodimeric proteins that are essential for a met- azoan existence. All members of the integrin family adopt a shape that resembles a large “head” on two “legs,” with the head containing the sites for ligand binding and subunit association. Most of the receptor dimer is extracellular, but both subunits traverse the plasma membrane and terminate in short cytoplasmic domains. These domains initiate the assembly of large signaling complexes and thereby bridge the extracellular matrix to the intracellular cytoskeleton. To allow cells to sample and respond to a dynamic pericellular environment, integrins have evolved a highly responsive receptor activation mechanism that is regulated primarily by changes in tertiary and quaternary structure. This review summar- izes recent progress in the structural and molecular functional studies of this important class of adhesion receptor. he name “integrin” was suggested for an INTEGRIN STRUCTURE integral membrane protein complex first T Overall Structure characterized in 1986 (Tamkun et al. 1986). The name was devised because the protein Integrins are heterodimers of non-covalently identified linked the extracellular matrix to the associated a and b subunits. In vertebrates, cytoskeleton (early developments in this field there are 18 a and 8 b subunits that can assem- have been well described [Hynes 2004]). In the ble into 24 different receptors with different 25 years since that first characterization, a vast binding properties and different tissue distribu- amount of work has been performed, with con- tion (Hynes 2002; Barczyk et al. 2010). The a sequent increased understanding. The essential and b subunits are constructed from several role of integrins in tissue organization and cell domains with flexible linkers between them. development, their signal transduction mecha- Each subunit has a single membrane-spanning nisms (from outside to in and inside to out!), helix and, usually, a short unstructured cyto- and their potential as therapeutic targets is plasmic tail. The size varies but typically the now established. In this article, we provide an a- and b-subunits contain around 1000 and overview of the structure of integrins, the con- 750 amino acids, respectively. Numerous re- formational changes that determine activation views on integrin structure and function have state, and the mechanisms of ligand binding. been published (Arnaout et al. 2007; Luo et al. Editors: Richard Hynes and Kenneth Yamada Additional Perspectives on Extracellular Matrix Biology available at www.cshperspectives.org Copyright # 2011 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a004994 Cite this article as Cold Spring Harb Perspect Biol 2011;3:a004994 1 Downloaded from http://cshperspectives.cshlp.org/ on September 23, 2021 - Published by Cold Spring Harbor Laboratory Press I.D. Campbell and M.J. Humphries 2007; Askari et al. 2009; Bennett et al. 2009) Current knowledge of ectodomains has also so here we concentrate mainly on the impli- been enhanced by structures of various integrin cations of recent structural work that includes fragments including isolated a-I domains (Lee studies of intact ectodomains, membrane et al. 1995a) and b2-leg fragments (Beglova spanning regions, cytoplasmic tails and their et al. 2002; Xiao et al. 2004; Shi et al. 2005). ligands. Because of observed flexibility in studies by electron microscopy (EM) and the existence of conformationally sensitive antibody recognition The Ectodomains sites (Humphries 2004), there is a general accep- The breakthrough crystal structure of aVb3 tance that conformations other than the bent (Xiong et al. 2001) started a deluge of structural one are possible and are functionally relevant information about integrin ectodomains. (see below). The structural studies of intact ecto- Structures of aVb3, with and without ligand domains (Xiong et al. 2001; Xie et al. 2009; Zhu (Xiong et al. 2001, 2002), aIIbb3 (Zhu et al. et al. 2009) all postulated that an upright struc- 2008), and axb2 (Xie et al. 2009) are all now ture of the sort illustrated in Figure 1C could exist available. These crystal structures are all in a as well as the bent structure. There has, however, similar overall “bent” conformation that would been controversy about whether this large change place the ligand binding site near the membrane between bent and upright structures—the surface. The overall topology and structure of “switchblade” model (Luo et al. 2007)—has to integrin ectodomains is illustrated in Figure 1 take place or if more conservative changes ar- for the case of axb2 (Xie et al. 2009), which ound the bent structure can explain the data— has an inserted a-I domain. the “deadbolt” model (Arnaout et al. 2005). α-I ~4 nm 1 126 327 597 800 901 1086 1146 C 1 2 α-I 3 4 5 6 7 thigh calf1 calf2 β-I 1109 N β A 142358 101 342 460513 747680597552 -propeller Hyb Psi Hyb β-I Hyb E1 E2 E3 E4 β-T 426 702 Psi thigh E1 N E2 ~19 nm calf1 E3 ~11 nm ~1000 aa E4 B calf2 β-T ~4 nm ~8 nm C ~50 aa C Figure 1. Integrin structure. (A) Domain structure of axb2 (Xie et al. 2009); (B) structure of axb2 using same color code as A (drawn with PyMOL [DeLano Scientific] using PDB coordinates 3K6S); (C) cartoon represen- tation of bent and upright conformations showing approximate dimensions. 2 Cite this article as Cold Spring Harb Perspect Biol 2011;3:a004994 Downloaded from http://cshperspectives.cshlp.org/ on September 23, 2021 - Published by Cold Spring Harbor Laboratory Press Integrin Structure, Activation, and Interactions The Structure of a Subunit Ectodomains Extrinsic ligand The a-chain consists of four or five extracellular α7 domains: a seven-bladed b-propeller,athigh, and two calf domains. Nine of the 18 integrin a chains have an a-I domain of around 200 amino-acids, inserted between blades 2 and 3 of the b-propeller (Larson et al. 1989). The I domain, a copy of which also appears in the b-chain, has five b-sheets surrounded by seven a helices; it is similar to von Willebrand A do- mains. The last three or four blades of the α7 Extrinsic 2þ ligand b-propeller contain domains that bind Ca Intrinsic on the lower side of the blades facing away ligand 2þ from the ligand-binding surface. Ca binding α7 to these sites has been shown to influence ligand binding (Oxvig and Springer 1998; Humphries et al. 2003). The thigh and calf domains have similar, immunoglobulin-like, b-sandwich folds (Xiong et al. 2001). They have 140–170 residues with more b-strands than typical Ig-like domains (100 residues). There are two main regions of interdomain flexibility. One is the linker An illustration of the movement of a7 helix b Figure 2. between the -propeller and the thigh; the other in the I domains and the swing-out of the hybrid is the “genu” or knee at the bend between the domain (the domains are defined in Fig. 1). The thigh and the calf-1 domain. The a-subunit top pair corresponds to the closed and open confor- genu is located close to the similar bend in the mations of an integrin without an inserted a-I b subunit, thereby allowing extension by a hing- domain whereas the lower pair represents the situa- ing at the knees. The a-I domain in axb2is tion when there is an a-I domain present. The intrin- inserted in the b-propeller domain with flexible sic ligand is a glutamate (E310 in aL). linkers (Fig. 1C). Unlike the other four a-leg domains, which have relatively rigid structures, hybrid domain. The small PSI domain, with I domains show conformational changes within an a/b fold, is also split into two portions the domain that are important for regulating (Xiao et al. 2004; Xiong et al. 2004) connected, binding affinity (see below and Fig. 2). in b3, by a long-range Cys-13 to Cys-435 disul- fide bond. Unusual EGF module boundaries were first The Structure of b Subunit Ectodomains proposed in the aVb3 structure (Xiong et al. The b-leg has seven domains with flexible and 2004), but recent crystal structures suggest complex interconnections. A b-I domain is that each EGF module has an even number of inserted in a hybrid domain, which is, in turn, eight cysteines, bonded in a C1-C5, C2-C4, inserted in a plexin-semaphorin-integrin (PSI) C3-C6, and C7-C8 pattern except for EGF1, domain. These domains are followed by four which lacks the C2-C4 disulfide. The aIIbb3 cysteine-rich epidermal growth factor (EGF) structure shows that all 56 cysteines in the integ- modules and a b-tail domain. The hybrid rin b3 subunit are disulfide bonded (Zhu et al. domain in the upper b-leg has a b-sandwich 2008). construction. The b-I domain, which is homol- The b-tail domain has an a þ b fold (Xiong ogous to the a-I domain, is inserted into the et al. 2001). The weak electron density of this Cite this article as Cold Spring Harb Perspect Biol 2011;3:a004994 3 Downloaded from http://cshperspectives.cshlp.org/ on September 23, 2021 - Published by Cold Spring Harbor Laboratory Press I.D. Campbell and M.J. Humphries domain observed in the aIIbb3 crystal structure was taken to suggest a flexible connection to other regions of the b-leg by a mobile “ankle” 126 (Zhu et al.