LCP1 preferentially binds clasped αMß2 integrin and attenuates leukocyte adhesion under flow.

Integrins are α/ß heterodimers that interconvert between inactive and active states. In the active state the α/ß cytoplasmic domains recruit integrin-activating proteins and separate the transmembrane and cytoplasmic (TMcyto) domains (unclasped TMcyto). Conversely, in the inactive state the α/ß TMcyto domains bind integrin-inactivating proteins, resulting in the association of the TMcyto domains (clasped TMcyto). Here, we report the isolation of integrin cytoplasmic tail interactors using either lipid bicelle-incorporated integrin TMcyto domains (α5, αM, αIIb, ß1, ß2 and ß3 integrin TMcyto) or a clasped, lipid bicelle-incorporated αMß2 TMcyto. Among the proteins found to preferentially bind clasped rather than the isolated αM and ß2 subunits was L-plastin (LCP1, also known as plastin-2), which binds to and maintains the inactive state of αMß2 integrin in vivo and thereby regulates leukocyte adhesion to integrin ligands under flow. Our findings offer a global view on cytoplasmic proteins interacting with different integrins and provide evidence for the existence of conformation-specific integrin interactors.

PPFIA1 drives active α5ß1 integrin recycling and controls fibronectin fibrillogenesis and vascular morphogenesis.

Basolateral polymerization of cellular fibronectin (FN) into a meshwork drives endothelial cell (EC) polarity and vascular remodelling. However, mechanisms coordinating α5ß1 integrin-mediated extracellular FN endocytosis and exocytosis of newly synthesized FN remain elusive. Here we show that, on Rab21-elicited internalization, FN-bound/active α5ß1 is recycled to the EC surface. We identify a pathway, comprising the regulators of post-Golgi carrier formation PI4KB and AP-1A, the small GTPase Rab11B, the surface tyrosine phosphatase receptor PTPRF and its adaptor PPFIA1, which we propose acts as a funnel combining FN secretion and recycling of active α5ß1 integrin from the trans-Golgi network (TGN) to the EC surface, thus allowing FN fibrillogenesis. In this framework, PPFIA1 interacts with active α5ß1 integrin and localizes close to EC adhesions where post-Golgi carriers are targeted. We show that PPFIA1 is required for FN polymerization-dependent vascular morphogenesis, both in vitro and in the developing zebrafish embryo.

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

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.

An EGFR/Src-dependent ß4 integrin/FAK complex contributes to malignancy of breast cancer.

ß4 integrin and focal adhesion kinase (FAK) are often associated with a poor prognosis in cancer patients, and their signaling events have recently been linked to malignant outcomes. Here, we demonstrate, for the first time, physical and functional interactions between ß4 integrin and FAK that influence breast cancer malignancy. An amino-terminal linker within FAK is essential for its binding with the cytodomain of ß4 integrin. Moreover, EGFR/Src-signaling triggers the tyrosine phosphorylation of ß4 integrin, which, in turn, recruits FAK to ß4 integrin and leads to FAK activation and signaling. Upon disruption of the ß4 integrin/FAK complex, tumorigenesis and metastasis in triple-negative breast cancer were markedly reduced. Importantly, the concomitant overexpression of ß4 integrin and FAK significantly correlates with malignant potential in patients with triple-negative breast cancer. This study describes a pro-metastatic EGFR/Src-dependent ß4 integrin/FAK complex that is involved in breast cancer malignancy and is a novel therapeutic target for triple-negative breast cancer.

Integrin α3ß1 regulates kidney collecting duct development via TRAF6-dependent K63-linked polyubiquitination of Akt.

The collecting system of the kidney develops from the ureteric bud (UB), which undergoes branching morphogenesis, a process regulated by multiple factors, including integrin-extracellular matrix interactions. The laminin (LM)-binding integrin α3ß1 is crucial for this developmental program; however, the LM types and LM/integrin α3ß1-dependent signaling pathways are poorly defined. We show that α3 chain-containing LMs promote normal UB branching morphogenesis and that LM-332 is a better substrate than LM-511 for stimulating integrin α3ß1-dependent collecting duct cell functions. We demonstrate that integrin α3ß1-mediated cell adhesion to LM-332 modulates Akt activation in the developing collecting system and that Akt activation is PI3K independent but requires decreased PTEN activity and K63-linked polyubiquitination. We identified the ubiquitin-modifying enzyme TRAF6 as an interactor with the integrin ß1 subunit and regulator of integrin α3ß1-dependent Akt activation. Finally, we established that the developmental defects of TRAF6- and integrin α3-null mouse kidneys are similar. Thus K63-linked polyubiquitination plays a previously unrecognized role in integrin α3ß1-dependent cell signaling required for UB development and may represent a novel mechanism whereby integrins regulate signaling pathways.

Sorting nexin 31 binds multiple ß integrin cytoplasmic domains and regulates ß1 integrin surface levels and stability.

Trafficking of α5ß1 integrin to lysosomes and its subsequent degradation is influenced by ligand occupancy and the binding of SNX17 via its protein 4.1, ezrin, radixin, moesin (FERM) domain to the membrane-distal NPxY motif in the cytoplasmic domain of ß1 integrin in early endosomes. Two other sorting nexin (SNX) family members, namely SNX27 and SNX31, share with SNX17 next to their obligate phox domain a FERM domain, which may enable them to bind ß integrin tails. Here we report that, in addition to SNX17, SNX31 but not SNX27 binds several ß integrin tails in early endosomes in a PI3 (phosphatidylinositide 3)-kinase-dependent manner. Similarly like SNX17, binding of SNX31 with ß1 integrin tails in early endosomes occurs between the FERM domain and the membrane-distal NPxY motif in the ß1 integrin cytoplasmic domain. Furthermore, expression of SNX31 rescues ß1 integrin surface levels and stability in SNX17-depleted cells. In contrast to SNX17, expression of SNX31 is restricted and found highly expressed in bladder and melanoma tissue. Altogether, these results demonstrate that SNX31 is an endosomal regulator of ß integrins with a restricted expression pattern.

Sorting nexin 17 prevents lysosomal degradation of ß1 integrins by binding to the ß1-integrin tail.

Integrin functions are controlled by regulating their affinity for ligand, and by the efficient recycling of intact integrins through endosomes. Here we demonstrate that the Kindlin-binding site in the ß1-integrin cytoplasmic domain serves as a molecular switch enabling the sequential binding of two FERM-domain-containing proteins in different cellular compartments. When ß1 integrins are at the plasma membrane, Kindlins control ligand-binding affinity. However, when they are internalized, Kindlins dissociate from integrins and sorting nexin 17 (SNX17) is recruited to free ß1-integrin tails in early endosomes to prevent ß1-integrin degradation, leading to their recycling back to the cell surface. Our results identify SNX17 as a ß1-integrin-tail-binding protein that interacts with the free Kindlin-binding site in endosomes to stabilize ß1 integrins, resulting in their recycling to the cell surface where they can be reused.