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FIG 13.6  Mixing and matching integrin subunits.

Numerous combinations of - and -subunits create integrins with specific ligand-binding characteristics. The 2-integrins are restricted to blood cells, 1 are present on all cells. Platelets have the unique combinationllb 3, vital in blood clotting. Integrins bind to specific sequences in proteins of the extracellular matrix such as RGD or EILDV.

(comparing V and L). Importantly, I- and 1-domains have similar folds and can both interact with ligand.

The -propeller structure and the 1-domain are responsible for linking the - and -subunits.26 This interaction resembles the linking of - and -subunits of heterotrimeric GTP binding proteins (see page 83). The 1-domain is a hotspot for mutations, which can result in a failure of association and can give rise to leukocyte adhesion deficiency (LAD), characterized by frequent unresolved infections (see Chapter 16).

Inactive to primed

Integrins can exist in low affinity (inactive), high affinity (primed), or ligand-bound (activated) states, determined by conformational changes in the head region.

The shift from low to high affinity is directed from inside the cell by association of proteins with the cytosolic region of the integrin - and-subunits (inside-out signalling). Association of these proteins disrupts the interaction between the juxtamembrane region of the - and -subunits

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FIG 13.7  Domain architecture of integrins. (a) Integrins, involved in cell–cell and cell–extracellular matrix contact, are composed of two non-covalently bound subunits ( and ). Two examples of a subunit are shown. The stalk region of the -subunit comprises two calf domains and a thigh domain. The protein articulates at the border of calf-1 and thigh, the PSI and hybrid domain of the -subunit being pushed outward in a switchblade movement. This plays an important role in the activation process. The heads contain various subdomains of which the -propeller (on the -subunit) and 1 domain (on the -subunit) assure the association of the two subunits. (b) Molecular structure of V and 3. Depending on the type of integrin, ligand recognition either occurs through the 1 domain or through the domain (in L). Yellow spots indicate divalent metal ions; PSI, plexin 7 integrin domain; TD, -tail domain; EGFD, EGF-like domain (1jv225).

which leads to their separation. This then alters the stalk region, which unfolds from a bent to an extended conformation, like an upward switchblade movement (Figure 13.8a). A possible candidate for the juxtamembrane positioning of the integrins is talin, which binds both integrins and components of the actin cytoskeleton. A mutation in the FERM domain (F3) of talin that prevents binding to integrin has a profound effect on the affinity of integrins for extracellular matrix in Drosophila but, surprisingly, does not perturb the organization of the actin cytoskeleton around focal adhesion complexes27 (see page 398).

The switchblade movement of the stalk region and its effect on the head structure has been studied in great detail for integrin IIb 3, expressed

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on platelets.28 As the stalk region straightens, there is a movement of the hybrid domain of the -subunit which causes a shape change of the neighbouring 1 domain (see Figure 13.8a, b). This exposes the ligand

binding site and converts the integrin into a primed, high-affinity, adhesion molecule. Similar changes occur on those integrins that carry an I domain ( L 2) but here the change has to be propagated by means of an intrinsic ligand (Figure 13.8c).29,30

The integrins of circulating cells such as leukocytes are maintained in an inactive state and are stimulated by chemokines acting through G-protein- coupled 7TM receptors. In contrast, for tissue cells the integrins appear to be constitutively activated, though local inactivation can occur, as must be the case in migrating cells or cells that round up for division.

FIG 13.8  Mechanisms of integrin activation. (a) Integrin activation is depicted here as a three-step process. Inactive integrin is in a folded state, head down

(1). Though the mechanism that separates the cytosolic domains is not fully understood, binding of talin-1, through the F3 region in the FERM domain, is important. The separation (2) causes an upward switch-blade movement, followed by a shift of the hybrid domain of the -subunit, which moves to the outside position (3). The integrin is now primed and ready to bind ligand (4). (b) The outward movement of the hybrid domain changes the conformation of the 1 domain, creating a ligand binding site. (c) For those integrins that carry an 1 domain, the mechanism of activation is identical, but the change in conformation is indirect. The movement of the hybrid domain causes an effective intrinsic ligand to bind at a site which, when occupied, leads to a conformational change in the 1 domain, creating a binding site for fibronectin. Yellow spots indicate divalent metal ions.Information from Xiong et al.,25 Shimaoka et al.,26 Xiao et al.,31 and Carman and Springer32 (1jv225).

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Primed to active

Ligand binding causes further alterations in the relative positioning of the stalk regions33 so that they recruit proteins that constitute an intracellular signalling complex. Some 50 proteins have been implicated in the localization (transient or stable) of these focal adhesion complexes.34,35 It is not yet clear which proteins actually sense the ligand-bound conformational change and pass the message on into the cell. The clustering of integrins that follows ligand binding (formation of focal adhesion contacts) probably plays an important part in the formation of intracellular signalling complexes, though clustering alone fails to reproduce full outside-in signalling events.36

Finally, amongst the proteins that localize in the focal adhesion complex are ILK (integrin-linked kinase), -actinin, talin, paxillin, and filamin. These are all multidomain proteins that connect integrins with components of the actin cytoskeleton.37 Once again, integrin 4 present in hemidesmosomes is

an exception (see above), being coupled to intermediate filaments through plectin.

Cadherins

In the formation of the early embryo, the compaction of the morula is mediated through Ca2 -dependent adhesion molecules38 (Figure 13.9). Uvomorulin, one of the first to be identified, is instrumental in the transition from a grape-like to a mulberry-like object. The cellular junctions thus formed

FIG 13.9  Role of cadherin in the compaction of the eight-cell stage mouse embryo. Scanning electron microscopy reveals that after three cell divisions (eight-cell stage) mouse embryos change from a uva (grape)-like to a morula (mulberry)-like aggregate. This process, called compaction, enables the cells to attach firmly to each other, manifesting the first signs of polarization. Their morphology now presents distinct basal (contact) and apical (peripheral) membrane surfaces. Cadherins play an important role in the compaction process, they are localized at cell–cell boundaries and their link to the actin cytoskeleton allows both the compaction of the embryo and the profound shape change of the cells. Images in centre panel courtesy of Dr Alexandre, Mons, Belgium.

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FIG 13.10  Cadherin domain structure. (a) Cadherins form a large family of adhesion molecules, characterized by extracellular cadherin (EC) domains and classified on the sequence of EC1. In many cases, the membrane-proximal EC domain is only distantly related and better regarded as the membrane-

proximal extracellular domain. Many cadherins carry a catenin binding domain which links to the actin or intermediate filament cytoskeleton. (b) A ribbon representation of two cadherin repeats (EC1 and EC2 of cadherin-11) with Ca2 binding sites at the domain interface. The interaction between the two cadherins occurs through the N-terminal EC1 domain and is based on a domain swap, in which tryptophan residues, (sticks) play an important role. Ca2 ions indicated as yellow spots. (2a4e43).

domains.43 Anchoring occurs through the insertion of an EC1 side chain into a complementary hydrophobic pocket in the partner molecule and vice versa. This so-called strand exchange exemplifies a more general domain-swapping strategy which enables homophilic interactions between proteins having low affinity yet high specificity.44

Full functionality of the classical cadherins requires structural linkage to the cytoskeleton. To this end, they are organized as a ‘core complex’. This also includes -catenin bound directly to the cytoplasmic domain of cadherin and -catenin bound to the N-terminal region of -catenin. -Catenin is also bound to actin and to several actin binding proteins such as -actinin, ZO-1, vinculin, and formin. All this suggests that the complex plays a role in

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Table 13.2  Human cadherin subfamilies

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Adapted from Nollet et al.42 and from PROSITE.

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