Structure of Rap1b bound to talin reveals a pathway for triggering integrin activation

Abstract

Activation of transmembrane receptor integrin by talin is essential for inducing cell adhesion. However, the pathway that recruits talin to the membrane, which critically controls talin’s action, remains elusive. Membrane-anchored mammalian small GTPase Rap1 is known to bind talin-F0 domain but the binding was shown to be weak and thus hardly studied. Here we show structurally that talin-F0 binds to human Rap1b like canonical Rap1 effectors despite little sequence homology, and disruption of the binding strongly impairs integrin activation, cell adhesion, and cell spreading. Furthermore, while being weak in conventional binary binding conditions, the Rap1b/talin interaction becomes strong upon attachment of activated Rap1b to vesicular membranes that mimic the agonist-induced microenvironment. These data identify a crucial Rap1-mediated membrane-targeting mechanism for talin to activate integrin. They further broadly caution the analyses of weak protein–protein interactions that may be pivotal for function but neglected in the absence of specific cellular microenvironments.

Rap1 binding and a lipid-dependent helix in talin F1 domain promote integrin activation in tandem

“…1E). These data implicated talin1 F1 as a second Rap1-binding site important in integrin activation; a surprising conclusion in light of a previous report that the F1 domain does not to bind Rap1b (Zhu et al, 2017).…”

“…Furthermore, in Dictyostelium Rap1 directly interacts with the RA domain of talinB (Plak et al, 2016) to enable adhesion during Dictyostelium morphogenesis. Zhu et al confirmed the direct Rap1-talin1 F0 interaction in mammals and showed that membrane-anchored Rap1b in vesicles has enhanced binding to THD, suggesting a mechanism of talin1 recruitment to integrins by Rap1 (Zhu et al, 2017). Quantitative proteomic analyses of murine platelets revealed the high abundance of Rap1b and talin1 (Zeiler et al, 2014).…”

“…Moreover, superimposition of the NMR structures of the talin1 F0 domain with the F1 domain showed that the position of these residues is conserved. We therefore generated a model of the talin1 F1-Rap1 complex by superimposing the talin1 F1 domain with the F0 domain in complex with Rap1b (Zhu et al, 2017). We found that the F1 interface with Rap1b to be very similar to that of F0 and to be compatible with R98 and R118 to form salt bridges with the negatively charged groups on Rap1 surface ( Fig.…”

“…2E. Thus the talin1 F1 domain contains an RA domain that can bind Rap1b with similar affinity to the talin1 F0 domain (Goult et al, 2010;Lagarrigue et al, 2018;Zhu et al, 2017).…”

The tale of two talins – two isoforms to fine‐tune integrin signalling

“…In addition, the F1 subdomain contains a large (~30aa) unstructured insertion, the F1-loop, which, via a cluster of positively charged residues, interacts with PIP2 and is essential for integrin activation [58]. The F0 subdomain has been shown to bind the membrane-tethered small GTPase, Rap1 [58][59][60] and this interaction has been implicated in membrane targeting of talin to the plasma membrane [59]. Interestingly, the additional F0 subdomain and the F1-loop elements of the talin FERM domain are also found in the kindlin family of proteins [58,61] which synergise with talin to activate integrins [62].…”