ILK binding to β1 Integrin is indirectly mediated by Kindlin-2
Integrin-linked kinase (ILK) and kindlin-2 (K2) are key components of focal adhesions (FAs) that regulate cell-matrix adhesion and integrin signaling. Both proteins directly bind each other, but how they influence each others localization to FAs and binding to integrins remains a subject of ongoing debate. Here, we establish a sensitive workflow to study protein-protein interactions in cells by combining methods from biochemistry, cell biology and super-resolution microscopy. Together with an analytical framework this approach allowed us to distinguish direct from indirect molecular interactions and construct detailed interaction networks. Disrupting the ILK-K2 interaction reduced ILK localization to FAs and compromised integrin function, whereas K2 recruitment was unaffected. Our interdisciplinary approach also revealed that ILK does not directly bind {beta}1-integrin cytosolic domains in vitro and in cells. Instead, ILK was recruited to integrins exclusively through a K2-dependent mechanism, primarily via K2 bridging ILK and {beta}1 integrins. These findings define the hierarchical relationship between ILK and K2 in FAs and highlight the essential role of K2-mediated ILK recruitment for integrin adhesion and signaling. Significance StatementHow cells anchor to their environment is a fundamental question in biology. Integrins provide such a connection by bridging the extracellular matrix and the cytoskeleton. A central regulator of the integrin machine is Integrin-linked kinase (ILK). How ILK is recruited to {beta}1 integrins is hotly debated since its discovery more than 30 years ago. By integrating cell biology and biochemistry with super-resolution DNA-PAINT microscopy and a novel spatial analysis framework, we demonstrate that ILK does not bind directly to integrin cytoplasmic tails. Instead, we found that ILK is recruited by kindlin-2 (K2) to active, adhesion plaque-resident integrins. This work resolves a long-standing controversy in cell biology and establishes a versatile workflow for distinguishing direct from indirect protein-protein interactions in situ.