4 D and S3 B), support the theory that talin autoinhibition and the stability of R3 go hand in hand. including migration, differentiation, and proliferation (Geiger and Yamada, 2011). Talin and vinculin are two critical regulators of the mechanical link between integrins and the actin cytoskeleton (Gauthier and Roca-Cusachs, 2018). Structurally, both talin (Goult et al., 2013a) and vinculin (Chorev et al., 2018; Cohen et al., 2005) are thought to exist in dynamic equilibrium between closed (autoinhibited) and open conformations. This has led to an attractive model in which actomyosin-mediated forces are envisaged to induce conformational changes that unmask binding sites in both proteins that support their mutual interaction and association with the contractile actomyosin machinery, plus other binding SR3335 partners (Chorev et al., 2018; del Rio et al., 2009; Sun et al., 2017; Yao et al., 2014, Yao et al., 2016). For vinculin, force is thought to overcome the strong autoinhibitory interaction (= 15 mitochondria from five cells. Results are representative of three independent repeats. (D) FLAP curves of PAGFP-talinFL at FAs coexpressed with either mCh-vinFL or mCh-vinT12. Note the reduced turnover of talin at FAs when coexpressed with vinT12. Error bars represent SEM; = 92 (vinFL) or 68 SR3335 (vinT12) FAs, from 10C15 cells. Data are pooled from three independent experiments. Active vinculin binds talin without forces The lack of recruitment of vinculin to talin in the absence of force (Fig. 1 D) is in line with previously reported in vitro single-molecule stretching experiments, which concluded that the two proteins do not interact before tension being applied across talin (del Rio et al., 2009; Yao et al., 2014). Importantly, these experiments were performed using a vinculin peptide (aa 1C258) with an exposed talin-binding site, which is hidden in the full-length vinculin protein (Cohen et al., 2005). Therefore, we hypothesized that in the absence of force, talin should not interact even with a vinculin construct with an exposed talin-binding site. To test this hypothesis, we coexpressed GFP-talinFL with a constitutively active (opened) form of full-length CCR1 vinculin (vinT12; Cohen et al., 2005) as well as truncated forms of vinculin (vin258 and vin880) that have exposed talin-binding sites but lack the actin-binding site located in the vinculin tail region (Carisey et al., 2013). Each vinculin construct was tagged with cBAK for mitochondrial targeting and mCherry for visualization. Surprisingly, GFP-talinFL bound to all of the vinculin constructs (Fig. 2 A and Fig. S1 B). Moreover, the interaction occurred in the presence of the actomyosin inhibitors blebbistatin or Y-27632, and also the actin polymerization inhibitor cytochalasin D (Fig. 2 B), demonstrating that actomyosin-mediated forces are not essential for talinFL to bind activated vinculin. Similarly, activated vinculin (vinT12) at mitochondria also recruited a talinFL construct bearing mutations that compromise the two actin-binding sites (ABS2 and ABS3) in the talin rod (Atherton et al., 2015; Kumar et al., 2016; Fig. 2 C). Open in a separate window Figure 2. Active vinculin can bind talin independently of force. (A) Coexpression of active mCh-vinT12-cBAK with GFP-talinFL in NIH3T3 cells shows that the two constructs colocalize at mitochondria. (B) This interaction occurs in the presence of Y-27632 (50 M), blebbistatin (50 M), or cytochalasin D (Cyto D; 2.5 g ml?1). (C) mCh-vinT12-cBAK also SR3335 recruited a talin construct that has mutations in both actin binding sites in the talin rod (ABS2 and ABS3; GFP-talinABS2+ABS3mut) SR3335 in NIH3T3 cells. Scale bars in ACC indicate 10 m. (D) FLAP experiments in NIH3T3 cells coexpressing mCh-vinT12-cBAK and photoactivatable (PA) GFP-talinFL show that there is minimal loss of fluorescence over time after activation, indicating a very strong interaction between the two proteins. Error bars represent SEM; = 11 mitochondria from 5 cells. Results are representative of three independent experiments. Scale bar indicates 5 m. In FAs, increased engagement of.