Talin-induced integrin binding to extracellular matrix ligands (integrin activation) is the key step to trigger many fundamental cellular processes including cell adhesion, cell migration, and spreading. Talin is widely known to use its N-terminal head domain (talin-H) to bind and activate integrin, but how talin-H operates in the context of full-length talin and its surrounding remains unknown. Here we show that while being capable of inducing integrin activation, talin-H alone exhibits unexpectedly low potency versus a constitutively activated full-length talin. We find that the large C-terminal rod domain of talin (talin-R), which otherwise masks the integrin binding site on talin-H in inactive talin, dramatically enhances the talin-H potency by dimerizing activated talin and bridging it to the integrin co-activator kindlin-2 via the adaptor protein paxillin. These data provide crucial insight into the mechanism of talin and its cooperation with kindlin to promote potent integrin activation, cell adhesion, and signaling.

Coronavirus disease 2019 (COVID-19) is a highly contagious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). It is broadly accepted that SARS-CoV-2 utilizes its spike protein to recognize the extracellular domain of angiotensin-converting enzyme 2 (ACE2) to enter cells for viral infection. However, other mechanisms of SARS-CoV-2 cell entry may occur. We show quantitatively that the SARS-CoV-2 spike protein also binds to the extracellular domain of broadly expressed integrin α5β1 with an affinity comparable to that of SARS-CoV-2 binding to ACE2. More importantly, we provide direct evidence that such binding promotes the internalization of SARS-CoV-2 into non-ACE2 cells in a manner critically dependent upon the activation of the integrin. Our data demonstrate an alternative pathway for the cell entry of SARS-CoV-2, suggesting that upon initial ACE2-mediated invasion of the virus in the respiratory system, which is known to trigger an immune response and secretion of cytokines to activate integrin, the integrin-mediated cell invasion of SARS-CoV-2 into the respiratory system and other organs becomes effective, thereby promoting further infection and progression of COVID-19.

The communication of talin-activated integrin aIIbb3 with cytoskeleton (integrin outside-in signaling) is essential for platelet aggregation, wound healing, and hemostasis. Filamin, a large actin cross-linker and integrin binding partner critical for cell spreading and migration, is implicated as a key regulator of integrin outside-in signaling. However, the current dogma is that filamin, which stabilizes inactive aIIbb3, is displaced from aIIbb3 by talin to promote the integrin activation (inside-out signaling) and how filamin further functions remains unresolved. Here we show that while associating with the inactive aIIbb3, filamin also associates with the talin-bound active aIIbb3 to mediate platelet spreading. FRET-based analysis reveals that while associating with both aIIb and b3 cytoplasmic tails (CTs) to maintain the inactive aIIbb3, filamin is spatiotemporally re-arranged to associate with aIIb CT alone on activated aIIbb3. Consistently, confocal cell imaging indicates that integrin a CT-linked filamin gradually delocalizes from b CT-linked focal adhesion marker - vinculin likely due to the separation of integrin a/b CTs occurring during integrin activation. High-resolution crystal and NMR structure determinations unravel that the activated integrin aIIb CT binds to filamin via a striking a-helix→b-strand transition with strengthened affinity that is dependent on the integrin-activating membrane environment containing enriched phosphatidylinositol 4,5-bisphosphate. These data suggest a novel integrin aIIb CT-filamin-actin linkage that promotes integrin outside-in signaling. Consistently, disruption of such linkage impairs the activation state of aIIbb3, phosphorylation of FAK/Src kinases, and cell migration. Together, our findings advance the fundamental understanding of integrin outside-in signaling with broad implications in blood physiology and pathology.