Integrins promote immunity, embryonic development, wound healing, and hemostasis, and are activated by 'bent/closed' to 'extended/open' conformational changes. Integrin αIIbβ3, being crucial for platelet activation and aggregation, is a therapeutic target for bleeding disorders and thrombosis. Human Platelet Antigen-1a (HPA-1a) on β3 is recognized by pregnancy-associated maternal alloantibodies, potentially causing fetal/neonatal alloimmune thrombocytopenia (FNAIT) and even intracranial hemorrhage or perinatal death. We report the structure of an anti-HPA-1a antibody fragment (Fab 26.4) in complex with integrin αIIbβ3 at high resolution by cryo-electron microscopy. Fab 26.4 binding locks αIIbβ3 in the inactive, bent/closed conformation, is incompatible with integrin extension, and inhibits αIIbβ3-dependent fibrinogen binding and platelet aggregation. Thus, anti-HPA-1a antibodies directly impair integrin activation by preventing required conformational changes. These insights will improve FNAIT diagnostics and treatment, and spark the development of novel allosteric inhibitors against β3 integrins for future therapeutic applications.

BACKGROUND:C3G (RapGEF1) is a guanine nucleotide exchange factor that activates Rap1, a small GTPase implicated in hematologic malignancies. We previously showed that C3G GEF activity is self-repressed via its AIR (autoinhibitory region). A lymphoma-associated missense mutation (Y554H) disrupts this inhibition, resulting in constitutive activation. This study aims to investigate the consequences of C3G dysregulation in B-cell lymphoma. METHODS:Murine C3G mutation Y564H (equivalent to human Y554H) was introduced into the A20 B-cell lymphoma line using CRISPR/Cas9. Rap1 activation, proliferation, apoptosis, ERK1/2 phosphorylation, Rac2 activity, adhesion, migration, invasion, tumorigenicity, and transcriptomic changes were assessed through biochemical assays, in vitro functional studies, in vivo mouse models, and RNA-seq analysis. RESULTS:A20-C3G-Y564H cells exhibited increased Rap1 activation under both basal and stimulated conditions. Hyperactivation of the C3G-Rap1 pathway impaired proliferation, promoted apoptosis, and was associated with reduced ERK1/2 phosphorylation. Furthermore, Rac2 activity was diminished, correlating with altered adhesion properties. Consistently, cell migration and invasion were enhanced, in correspondence with an increased number of metastatic foci in the liver following tail vein injection into syngeneic BALB/c mice. Notably, reduced C3G expression further augmented the metastatic potential of A20 cells. RNA-seq analysis revealed widespread transcriptional changes involving Rac2 signaling, adhesion, and metastatic pathways. CONCLUSIONS:C3G plays a dual role in B-cell lymphoma: it acts as a tumor suppressor by inhibiting growth and promoting apoptosis, but may also facilitate metastasis via enhanced motility. This dual effect likely reflects a functional balance between Rap1 and Rac2 signaling. These findings underscore the complexity of C3G-regulated pathways in B cells and suggest that C3G may serve as a potential novel marker in hematologic malignancies.

Abstract Background The LYP tyrosine phosphatase presents a SNP (1858C > T) that increases the risk of developing autoimmune diseases such as type I diabetes and arthritis. It remains unclear how this SNP affects LYP function and promotes the development of these diseases. The scarce information about LYP substrates is in part responsible for the poor understanding of LYP function. Results In this study, we identify in T lymphocytes several adaptor proteins as potential substrates targeted by LYP, including FYB, SLP-76, HS-1, Vav, SKAP1 and SKAP2. We also show that LYP co-localizes with SLP76 in microclusters, upon TCR engagement. Conclusions These data indicate that LYP may modulate T cell activation by dephosphorylating several adaptor proteins, such as FYB, SLP-76, HS-1, Vav, SKAP1 and SKAP2 upon TCR engagement.

C3G is a guanine nucleotide exchange factor (GEF) that activates Rap1 to promote cell adhesion. Resting C3G is autoinhibited and the GEF activity is released by stimuli that signal through tyrosine kinases. C3G is activated by tyrosine phosphorylation and interaction with Crk adaptor proteins, whose expression is elevated in multiple human cancers. However, the molecular details of C3G activation and the interplay between phosphorylation and Crk interaction are poorly understood. We combined biochemical, biophysical, and cell biology approaches to elucidate the mechanisms of C3G activation. Binding of Crk adaptor proteins to four proline-rich motifs (P1 to P4) in C3G was characterized in vitro using isothermal titration calorimetry and sedimentation velocity, and in Jurkat and HEK293T cells by affinity pull-down assays. The nucleotide exchange activity of C3G over Rap1 was measured using nucleotide-dissociation kinetic assays. Jurkat cells were also used to analyze C3G translocation to the plasma membrane and the C3G-dependent activation of Rap1 upon ligation of T cell receptors. CrkL interacts through its SH3N domain with sites P1 and P2 of inactive C3G in vitro and in Jurkat and HEK293T cells, and these sites are necessary to recruit C3G to the plasma membrane. However, direct stimulation of the GEF activity requires binding of Crk proteins to the P3 and P4 sites. P3 is occluded in resting C3G and is essential for activation, while P4 contributes secondarily towards complete stimulation. Tyrosine phosphorylation of C3G alone causes marginal activation. Instead, phosphorylation primes C3G lowering the concentration of Crk proteins required for activation and increasing the maximum activity. Unexpectedly, optimal activation also requires the interaction of CrkL-SH2 domain with phosphorylated C3G. Our study revealed that phosphorylation of C3G by Src and Crk-binding form a two-factor mechanism that ensures tight control of C3G activation. Additionally, the simultaneous SH2 and SH3N interaction of CrkL with C3G, required for the activation, reveals a novel adaptor-independent function of Crk proteins relevant to understanding their role in physiological signaling and their deregulation in diseases.

FNAIT is a pregnancy-associated condition caused by maternal alloantibodies against paternally-inherited platelet antigens, most frequently HPA-1a on integrin β3. The clinical effects range from no symptoms to fatal intracranial hemorrhage, but underlying pathophysiological determinants are poorly understood. Accumulating evidence suggests that differential antibody-Fc-glycosylation, activation of complement/effector cells, and integrin function-blocking effects contribute to clinical outcome. Furthermore, some antibodies preferentially bind platelet integrin αIIbβ3, but others bind αvβ3 on endothelial cells and trophoblasts. Defects in endothelial cells and angiogenesis may therefore contribute to severe anti-HPA-1a associated FNAIT. Moreover, anti-HPA-1a antibodies may cause placental damage, leading to intrauterine growth restriction. We discuss current insights into diversity and actions of HPA-1a antibodies, gathered from clinical studies, in vitro studies, and mouse models. Assessment of all factors determining severity and progression of anti-HPA-1a-associated FNAIT may importantly improve risk stratification and potentially reveal novel treatment strategies, both for FNAIT and other immunohematological disorders.

An important feature associated with Candida albicans pathogenicity is its ability to switch between yeast and hyphal forms, a process in which CaRas1 plays a key role. CaRas1 is activated by the guanine nucleotide exchange factor (GEF) CaCdc25, triggering hyphal growth-related signaling pathways through its conserved GTP-binding (G)-domain. An important function in hyphal growth has also been proposed for the long hypervariable region downstream the G-domain, whose unusual content of polyglutamine stretches and Q/N repeats make CaRas1 unique within Ras proteins. Despite its biological importance, both the structure of CaRas1 and the molecular basis of its activation by CaCdc25 remain unexplored. Here, we show that CaRas1 has an elongated shape and limited conformational flexibility and that its hypervariable region contains helical structural elements, likely forming an intramolecular coiled-coil. Functional assays disclosed that CaRas1-activation by CaCdc25 is highly efficient, with activities up to 2,000-fold higher than reported for human GEFs. The crystal structure of the CaCdc25 catalytic region revealed an active conformation for the α-helical hairpin, critical for CaRas1-activation, unveiling a specific region exclusive to CTG-clade species. Structural studies on CaRas1/CaCdc25 complexes also revealed an interaction surface clearly distinct from that of homologous human complexes. Furthermore, we identified an inhibitory synthetic peptide, prompting the proposal of a key regulatory mechanism for CaCdc25. To our knowledge, this is the first report of specific inhibition of the CaRas1-activation via targeting its GEF. This, together with their unique pathogen-structural features, disclose a set of novel strategies to specifically block this important virulence-related mechanism. IMPORTANCE Candida albicans is the main causative agent of candidiasis, the commonest fungal infection in humans. The eukaryotic nature of C. albicans and the rapid emergence of antifungal resistance raise the challenge of identifying novel drug targets to battle this prevalent and life-threatening disease. CaRas1 and CaCdc25 are key players in the activation of signaling pathways triggering multiple virulence traits, including the yeast-to-hypha interconversion. The structural similarity of the conserved G-domain of CaRas1 to those of human homologs and the lack of structural information on CaCdc25 has impeded progress in targeting these proteins. The unique structural and functional features for CaRas1 and CaCdc25 presented here, together with the identification of a synthetic peptide capable of specifically inhibiting the GEF activity of CaCdc25, open new possibilities to uncover new antifungal drug targets against C. albicans virulence.

Mutations in the adaptor protein PSTPIP1 cause a spectrum of autoinflammatory diseases, including PAPA and PAMI; however, the mechanism underlying these diseases remains unknown. Most of these mutations lie in PSTPIP1 F-BAR domain, which binds to LYP, a protein tyrosine phosphatase associated with arthritis and lupus. To shed light on the mechanism by which these mutations generate autoinflammatory disorders, we solved the structure of the F-BAR domain of PSTPIP1 alone and bound to the C-terminal homology segment of LYP, revealing a novel mechanism of recognition of Pro-rich motifs by proteins in which a single LYP molecule binds to the PSTPIP1 F-BAR dimer. The residues R228, D246, E250, and E257 of PSTPIP1 that are mutated in immunological diseases directly interact with LYP. These findings link the disruption of the PSTPIP1/LYP interaction to these diseases, and support a critical role for LYP phosphatase in their pathogenesis.

Ras-like protein 1 (CaRas1) is a key regulator of the switch between the yeast and hyphal forms of Candida albicans , a feature associated with pathogenesis. CaRas1 is activated by the guanine nucleotide exchange factor (GEF) CaCdc25, triggering hyphal growth-related signaling pathways through its highly conserved GTP-binding domain (G-domain). An important function in hyphal growth has also been proposed for the long hypervariable region downstream of the G-domain of CaRas1, whose unusual content of polyQ stretches and Q/N repeats make CaRas1 unique within Ras-family proteins. Despite its biological importance, both the structure of CaRas1 and the molecular basis of its activation by CaCdc25 remain unexplored. Here, we show that CaRas1 displays an elongated shape and that its hypervariable region contains helical structural elements with intramolecular coiled-coil propensity and limited conformational flexibility. Functional assays revealed that CaRas1 activation by CaCdc25 is highly efficient, with 5-to 2000-fold higher activity levels than reported for human GEFs. In addition, the threedimensional structure of the catalytic region of CaCdc25, together with the structural characterization of CaRas1/CaCdc25 complexes, unveiled a specific region located in the α-helical hairpin of CaCdc25, critical for CaRas1 activation, where negatively charged substitutions reduce its activity. The unique structural features of the low complexity region of CaRas1 and the distinctive properties of CaRas1 activation by CaCdc25, common in the homologous proteins from CTG-clade species, uncover novel strategies to target key virulence factors in human-infecting fungal pathogens.

Hemidesmosomes (HDs) are specialized multiprotein complexes that connect the keratin cytoskeleton of epithelial cells to the extracellular matrix (ECM). In the skin, these complexes provide stable adhesion of basal keratinocytes to the underlying basement membrane. Integrin α6β4 is a receptor for laminins and plays a vital role in mediating cell adhesion by initiating the assembly of HDs. In addition, α6β4 has been implicated in signal transduction events that regulate diverse cellular processes, including proliferation and survival. In this Review, we detail the role of α6β4 in HD assembly and beyond, and we discuss the molecular mechanisms that regulate its function.

In epithelial cancers, the epidermal growth factor receptor (EGFR) and integrin α6β4 are frequently overexpressed and found to synergistically activate intracellular signaling pathways that promote cell proliferation and migration. In cancer cells, the β4 subunit is phosphorylated at tyrosine residues not normally recognized as kinase substrates; however, the function of these phosphotyrosine residues in cancer cells is a subject of much debate. In EGFR-overexpressing carcinoma cells, we found that the Src family kinase (SFK) inhibitor PP2 reduces β4 tyrosine phosphorylation following the activation of EGFR. However, siRNA mediated knockdown of the SFKs Src, Fyn, Yes and Lyn, individually or in combination, did not affect the EGF-induced phosphorylation of β4. Using phospho-peptide affinity chromatography and mass spectrometry, we found that PLCγ1 binds β4 at the phosphorylated residues Y1422/Y1440, but were unable to verify this interaction in A431 carcinoma cells that overexpress the EGFR. Furthermore, using A431 cells devoid of β4 or reconstituted with phenylalanine specific mutants of β4, the activation of several downstream signaling pathways, including PLCγ/PKC, MAPK and PI3K/Akt, were not substantially affected. We conclude that tyrosine-phosphorylated β4 does not enhance EGFR-mediated signaling in EGFR-overexpressing cells, despite the fact that this integrin subunit is highly tyrosine phosphorylated in these cells.

Abnormally increased signaling by the GTPase RAP1 favors progression of diverse tumors. We have characterized the auto-regulation and activation of C3G (RAPGEF1), an activator of RAP1. This led us to discover mutations in non-Hodgkin's lymphomas that activate C3G-RAP1 constitutively, suggesting that deregulation of C3G may favor the dissemination of tumor cells.

[This corrects the article DOI: 10.1371/journal.pone.0229953.].

C3G is a guanine nucleotide exchange factor (GEF) that regulates cell adhesion and migration by activating the GTPase Rap1. The GEF activity of C3G is stimulated by the adaptor proteins Crk and CrkL and by tyrosine phosphorylation. Here, we uncovered mechanisms of C3G autoinhibition and activation. Specifically, we found that two intramolecular interactions regulate the activity of C3G. First, an autoinhibitory region (AIR) within the central domain of C3G binds to and blocks the catalytic Cdc25H domain. Second, the binding of the protein's N-terminal domain to its Ras exchanger motif (REM) is required for its GEF activity. CrkL activated C3G by displacing the AIR/Cdc25HD interaction. Two missense mutations in the AIR found in non-Hodgkin's lymphomas, Y554H and M555K, disrupted the autoinhibitory mechanism. Expression of C3G-Y554H or C3G-M555K in Ba/F3 pro-B cells caused constitutive activation of Rap1 and, consequently, the integrin LFA-1. Our findings suggest that sustained Rap1 activation by deregulated C3G might promote progression of lymphomas and that designing therapeutics to target C3G might treat these malignancies.

Diffraction images of a crystal of the F-BAR domain of human PSTPIP1 (residues 1-289, Uniprot reference O43586-1) in complex with the CTH of LYP (residues 787-807, Uniprot Q9Y2R2-1). Data were collected on a single crystal at the beamline i03 of the Diamond Light Source synchrotron (Didcot, UK) using radiation of 0.99987 Å wavelength and a PILATUS3 6M detector. The dataset consists of 3 groups, each containing of 1800 images (0.1 degree oscillation per image), collected at three different positions of the same crystal. Crystal belongs to the space group P2(1)2(1)2(1) with unit cell dimensions a=48.0 Å, b=72.0 Å, c=205.0 Å. The asymmetric unit contains an homodimer of the F-BAR domain bound to a LYP-CTH (~53% solvent content), which is the biological complex. Diffraction data was notably anisotropic. The lowest resolution limit was 4.05 Å in the direction b* and the highest limits were 2.11 Å and 2.10 in the directions a* and c*, respectively. The structure derived form these data is published in: Manso, J.A., Marcos, T., Ruiz-Martín, V. Casas J, Alcón P, Sánchez Crespo M, Bayón Y, de Pereda JM, Alonso A PSTPIP1-LYP phosphatase interaction: structural basis and implications for autoinflammatory disorders. Cell. Mol. Life Sci. 79, 131 (2022). https://doi.org/10.1007/s00018-022-04173-w The structure is available at the PDB under the code 7AAM: https://www.ebi.ac.uk/pdbe/entry/pdb/7aam