Talin at a glance

Abstract

Cell migration, growth and differentiation all require the assembly and disassembly of cellular junctions with the extracellular matrix (ECM). These large multiprotein complexes assemble around the integrin family of cell adhesion molecules (transmembrane αβ heterodimers) that are typically linked to the actin cytoskeleton, with the exception of integrin α6β4, which is coupled to intermediate filaments. Talin is one of several proteins that link the cytoplasmic domains of integrin β subunits to actin filaments (others include α-actinin, filamin, tensin, integrin-linked kinase, melusin and skelemin) (Critchley, 2004; Nayal et al., 2004). Moreover, binding of talin to β-integrin cytoplasmic domains triggers a conformational change in the αβ-integrin extracellular domain that increases its affinity for ECM proteins (Calderwood, 2004) and promotes the assembly of focal adhesions (FAs), cell-ECM junctions that are formed by cells in culture. However, studies in flies expressing mutant integrin alleles show that there is not a simple 1:1 relationship between integrins and talin in cell-ECM junctions, and there must be additional mechanisms that recruit talin to these sites (Devenport et al., 2007).Talin (∼270 kDa) comprises a globular N-terminal head region and a flexible rod domain, which can be dissociated by the protease calpain 2. The head contains a FERM (protein 4.1, ezrin, radixin, moesin) domain (subdivided into F1, F2 and F3 subdomains), which has binding sites for the cytoplasmic domains of β-integrins and layilin (a hyaluronan receptor) as well as for filamentous actin (F-actin). The head also binds to two signalling molecules that regulate the dynamics of FA, namely PIPK1γ90 [a splice variant of phosphatidylinositol (4)-phosphate 5-kinase type Iγ] and focal adhesion kinase (FAK), although it is not clear whether binding to FAK is direct. The talin rod contains an additional integrin-binding site, at least two actin-binding sites and several binding sites for vinculin, which itself has multiple partners. Finally, the C-terminal helices of two talin monomers form an anti-parallel dimer, although the relative position of the two subunits within the dimer is uncertain. Yeast two-hybrid and proteomic screens have identified several additional talin-binding proteins (Zaidel-Bar et al., 2007) and numerous phosphorylation sites (Ratnikov et al., 2005).Binding of the talin head to the cytoplasmic domain of integrin β subunits is thought to activate integrins by disrupting a salt bridge between the α- and β-integrin subunits, leading to separation of their cytoplasmic domains (Campbell and Ginsberg, 2004). Expression of the talin head in CHO cells stably expressing the platelet integrin αIIbβ3 resulted in integrin activation, whereas small hairpin RNA (shRNA)-mediated knockdown of talin in these cells compromised activation. Support for the cytoplasmic-domain-separation model comes from fluorescence resonance energy transfer (FRET) studies using the integrin subunits αL and β2 C-terminally tagged with CFP and YFP, respectively, and expressed in K562 cells: the FRET efficiency between the integrin subunits was significantly decreased by the expression of the talin head (Kim et al., 2003).The talin F3 FERM subdomain binds to the more N-terminal of the two NPxY motifs found in β-integrin cytoplasmic domains, and the structure of the F3 subdomain bound to residues 739WDTANNPLYDEA750 in β3-integrin has been determined (underlined residues indicate key binding determinants). F3 has a phosphotyrosine-binding (PTB) fold comprising seven β-strands (S1-S7) and a C-terminal α-helix. β3-integrin binds predominantly to a hydrophobic surface on strand S5, and mutations in S5 markedly reduce binding. The β3-integrin 744NPLY747 sequence adopts a β-turn, and Y747 projects into an acidic pocket in the talin structure, whereas the equivalent region in PTB domains that recognize phosphotyrosine is strongly basic. Phosphorylation of Y747 of β3-integrin could therefore disrupt the talin-integrin interaction, suggesting one way in which the activation of β3-integrin by talin might be regulated. The talin F3 subdomain also contains a hydrophobic pocket, comprising residues R358, A360 and Y377, that is occupied by W739 in the β3-integrin cytoplasmic domain, and alanine substitution of talin R358 markedly reduces integrin binding (Campbell and Ginsberg, 2004). Interestingly, this same surface in talin F3 binds the WVYSPLHY sequence at the C-terminus of PIPK1γ90 and the WVENEIYY sequence in the layilin cytoplasmic domain.Several other PTB-domain proteins bind to the same NPxY motif in integrins, yet do not activate these proteins, implying that talin F3 must make additional contacts with the integrin cytoplasmic domain (Wegener et al., 2007). The flexible loop between β-strands S1 and S2 in talin F3 forms a second hydrophobic pocket that allows the docking of two residues (F727 and F730) of the membrane-proximal helix of β3-integrin. Mut