Kank2 activates talin, reduces force transduction across integrins and induces central adhesion formation

Abstract

Integrin-based adhesions play critical roles in cell migration. Talin activates integrins and flexibly connects integrins to the actomyosin cytoskeleton, thereby serving as a ‘molecular clutch’ that transmits forces to the extracellular matrix to drive cell migration. Here we identify the evolutionarily conserved Kank protein family as novel components of focal adhesions (FAs). Kank proteins accumulate at the lateral border of FAs, which we term the FA belt, and in central sliding adhesions, where they directly bind the talin rod domain through the Kank amino-terminal (KN) motif and induce talin and integrin activation. In addition, Kank proteins diminish the talin–actomyosin linkage, which curbs force transmission across integrins, leading to reduced integrin–ligand bond strength, slippage between integrin and ligand, central adhesion formation and sliding, and reduced cell migration speed. Our data identify Kank proteins as talin activators that decrease the grip between the integrin–talin complex and actomyosin to regulate cell migration velocity.

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

“…This led on to the identification of the talin‐binding sequence in RIAM 24, 29 as an LD‐motif, and the identification of paxillin as a novel talin ligand 94. More recently, the KANK (kidney ankyrin repeat‐containing) proteins have been identified as LD‐motif‐containing ligands 16, 17, binding to a conserved face on the R7 5‐helix bundle. The ability of 5‐helix bundles to bind LD‐motif proteins greatly expands the number of potential ligand‐binding sites in talin.…”

Talin – the master of integrin adhesions

“…Kank binding to talin is important both for the behaviour of focal adhesions and microtubules. In fibroblasts, Kank2 promotes the dynamic exchange of integrin and talin, sliding of adhesions, and activates talin and integrins (Sun et al, 2016). In HeLa cells, Kank1 binding to talin is required for the normal reduction of microtubule growth at the cell periphery (Bouchet et al, 2016).…”

“…A novel aspect of how talin is involved in adhesion-site architecture emerged recently with the discovery that talin recruits Kank1 and Kank2, as well as their associated 'cortical microtubule stabilisation complexes' to the periphery of focal adhesions by direct binding of talin R7 to the KN motif (Bouchet et al, 2016;Sun et al, 2016). Keeping Kank proteins at the edge of focal adhesions requires balancing interactions with talin inside the adhesion and other partners outside.…”

“…It is not yet clear where the equilibrium between closed and open talin conformations lies, that is, whether talin has to be converted into an active conformation to function or whether its activity is just stimulated by factors that push the equilibrium toward an open conformation. Known activators include the phosphoinositide PIP2 (Martel et al, 2001), as well as the Rap1 effector RIAM and the heterotrimeric G protein Gα13, both of which bind F3 and displace R9 (Yang et al, 2014;Schiemer et al, 2016), and Kank2, which binds R7 to activate talin (Sun et al, 2016). Both RIAM and PIP2 also contribute to talin activation by bringing it to the membrane Goult et al, 2010;Chang et al, 2014).…”

“…As a VBS consists of a single α-helix, many of the 62 α-helices of talin could be VBSs; indeed, an in vitro analysis identified ten strong VBSs and a further nine weaker ones . Binding sites have also been characterised for RIAM, paxillin, α-synemin, DLC1, Kank1 and Kank2 (Sun et al, 2008(Sun et al, , 2016Lee et al, 2009;Gingras et al, 2010;Li et al, 2011;Goult et al, 2013b;Bouchet et al, 2016;Zacharchenko et al, 2016). Furthermore, talin is required for recruitment of all IAPs examined to date in Drosophila (e.g.…”