For a cell to move forward it must convert chemical energy into mechanical propulsion. Force produced by actin polymerization can generate traction across the plasma membrane by transmission through integrins to their ligands. However, the role this force plays in integrin activation is unknown. Here we show that integrin activity and cytoskeletal dynamics are reciprocally linked, where actin-dependent force itself appears to regulate integrin activity. We generated fluorescent tension-sensing constructs of integrin αLβ2 (LFA-1) to visualize intramolecular tension during cell migration. Using quantitative imaging of migrating T cells, we correlate tension in the αL or β2 subunit with cell and actin dynamics. We find that actin engagement produces tension within the β2 subunit to induce and stabilize an active integrin conformational state and that this requires intact talin and kindlin motifs. This supports a general mechanism where localized actin polymerization can coordinate activation of the complex machinery required for cell migration.

Several patients have been reported to have variant dominant forms of Glanzmann thrombasthenia, associated with macrothrombocytopenia and caused by gain-of-function mutations of ITGB3 or ITGA2B leading to reduced surface expression and constitutive activation of integrin αIIbβ3. The mechanisms leading to a bleeding phenotype of these patients have never been addressed. The aim of this study was to unravel the mechanism by which ITGB3 mutations causing activation of αIIbβ3 lead to platelet dysfunction and macrothrombocytopenia. Using platelets from two patients carrying the β3 del647-686 mutation and Chinese hamster ovary cells expressing different αIIbβ3-activating mutations, we showed that reduced surface expression of αIIbβ3 is due to receptor internalization. Moreover, we demonstrated that permanent triggering of αIIbβ3-mediated outside-in signaling causes an impairment of cytoskeletal reorganization arresting actin turnover at the stage of polymerization. The induction of actin polymerization by jasplakinolide, a natural toxin that promotes actin nucleation and prevents depolymerization of stress fibers, in control platelets produced an impairment of platelet function similar to that of patients with variant forms of dominant Glanzmann thrombasthenia. del647-686β3-transduced murine megakaryocytes generated proplatelets with a reduced number of large tips and asymmetric barbell-proplatelets, suggesting that impaired cytoskeletal rearrangement is the cause of macrothrombocytopenia. These data show that impaired cytoskeletal remodeling caused by a constitutively activated αIIbβ3 is the main effector of platelet dysfunction and macrothrombocytopenia, and thus of bleeding, in variant forms of dominant Glanzmann thrombasthenia.

Malaria parasite infection is initiated by the mosquito-transmitted sporozoite stage, a highly motile invasive cell that targets hepatocytes in the liver for infection. A promising approach to developing a malaria vaccine is the use of proteins located on the sporozoite surface as antigens to elicit humoral immune responses that prevent the establishment of infection. Very little of the P. falciparum genome has been considered as potential vaccine targets, and candidate vaccines have been almost exclusively based on single antigens, generating the need for novel target identification. The most advanced malaria vaccine to date, RTS,S, a subunit vaccine consisting of a portion of the major surface protein circumsporozoite protein (CSP), conferred limited protection in Phase III trials, falling short of community-established vaccine efficacy goals. In striking contrast to the limited protection seen in current vaccine trials, sterilizing immunity can be achieved by immunization with radiation-attenuated sporozoites, suggesting that more potent protection may be achievable with a multivalent protein vaccine. Here, we provide the most comprehensive analysis to date of proteins located on the surface of or secreted by Plasmodium falciparum salivary gland sporozoites. We used chemical labeling to isolate surface-exposed proteins on sporozoites and identified these proteins by mass spectrometry. We validated several of these targets and also provide evidence that components of the inner membrane complex are in fact surface-exposed and accessible to antibodies in live sporozoites. Finally, our mass spectrometry data provide the first direct evidence that the Plasmodium surface proteins CSP and TRAP are glycosylated in sporozoites, a finding that could impact the selection of vaccine antigens.

We review the evolution and structure of members of the transforming growth factor β (TGF-β) family, antagonistic or agonistic modulators, and receptors that regulate TGF-β signaling in extracellular environments. The growth factor (GF) domain common to all family members and many of their antagonists evolved from a common cystine knot growth factor (CKGF) domain. The CKGF superfamily comprises six distinct families in primitive metazoans, including the TGF-β and Dan families. Compared with Wnt/Frizzled and Notch/Delta families that also specify body axes, cell fate, tissues, and other families that contain CKGF domains that evolved in parallel, the TGF-β family was the most fruitful in evolution. Complexes between the prodomains and GFs of the TGF-β family suggest a new paradigm for regulating GF release by conversion from closed- to open-arm procomplex conformations. Ternary complexes of the final step in extracellular signaling show how TGF-β GF dimers bind type I and type II receptors on the cell surface, and enable understanding of much of the specificity and promiscuity in extracellular signaling. However, structures suggest that when GFs bind repulsive guidance molecule (RGM) family coreceptors, type I receptors do not bind until reaching an intracellular, membrane-enveloped compartment, blurring the line between extra- and intracellular signaling. Modulator protein structures show how structurally diverse antagonists including follistatins, noggin, and members of the chordin family bind GFs to regulate signaling; complexes with the Dan family remain elusive. Much work is needed to understand how these molecular components assemble to form signaling hubs in extracellular environments in vivo.

Whether β1 integrin ectodomains visit conformational states similarly to β2 and β3 integrins has not been characterized. Furthermore, despite a wealth of activating and inhibitory antibodies to β1 integrins, the conformational states that these antibodies stabilize, and the relation of these conformations to function, remain incompletely characterized. Using negative-stain electron microscopy, we show that the integrin α5β1 ectodomain adopts extended-closed and extended-open conformations as well as a bent conformation. Antibodies SNAKA51, 8E3, N29, and 9EG7 bind to different domains in the α5 or β1 legs, activate, and stabilize extended ectodomain conformations. Antibodies 12G10 and HUTS-4 bind to the β1 βI domain and hybrid domains, respectively, activate, and stabilize the open headpiece conformation. Antibody TS2/16 binds a similar epitope as 12G10, activates, and appears to stabilize an open βI domain conformation without requiring extension or hybrid domain swing-out. mAb13 and SG/19 bind to the βI domain and βI-hybrid domain interface, respectively, inhibit, and stabilize the closed conformation of the headpiece. The effects of the antibodies on cell adhesion to fibronectin substrates suggest that the extended-open conformation of α5β1 is adhesive and that the extended-closed and bent-closed conformations are nonadhesive. The functional effects and binding sites of antibodies and fibronectin were consistent with their ability in binding to α5β1 on cell surfaces to cross-enhance or inhibit one another by competitive or noncompetitive (allosteric) mechanisms.

High-resolution crystal structures of the headpiece of lymphocyte function-associated antigen-1 (integrin αLβ2) reveal how the αI domain interacts with its platform formed by the α-subunit β-propeller and β-subunit βI domains. The αLβ2 structures compared with αXβ2 structures show that the αI domain, tethered through its N-linker and a disulfide to a stable β-ribbon pillar near the center of the platform, can undergo remarkable pivoting and tilting motions that appear buffered by N-glycan decorations that differ between αL and αX subunits. Rerefined β2 integrin structures reveal details including pyroglutamic acid at the β2 N terminus and bending within the EGF1 domain. Allostery is relayed to the αI domain by an internal ligand that binds to a pocket at the interface between the β-propeller and βI domains. Marked differences between the αL and αX subunit β-propeller domains concentrate near the binding pocket and αI domain interfaces. Remarkably, movement in allostery in the βI domain of specificity determining loop 1 (SDL1) causes concerted movement of SDL2 and thereby tightens the binding pocket for the internal ligand.

The platelet integrin αIIbβ3 binds to a KQAGDV motif at the fibrinogen γ-chain C-terminus and to RGD motifs present in loops in many extracellular matrix proteins. These ligands bind in a groove between the integrin α and β subunits; the basic Lys or Arg sidechain hydrogen bonds to the αIIb-subunit and the acidic Asp sidechain coordinates to a metal ion held by the β3-subunit. Ligand binding induces headpiece opening, with conformational change in the β-subunit. During this opening, RGD slides in the ligand-binding pocket towards αIIb, with movement of the βI-domain β1-α1 loop toward αIIb, enabling formation of direct, charged hydrogen bonds between the Arg sidechain and αIIb. Here we test whether ligand interactions with β3 suffice for stable ligand binding and headpiece opening. We find that the AGDV tetrapeptide from KQAGDV binds to the αIIbβ3 headpiece with affinity comparable to the RGDSP peptide from fibronectin. AGDV induced complete headpiece opening in solution as shown by increase in hydrodynamic radius. Soaking of AGDV into closed αIIbβ3 headpiece crystals induced intermediate states similarly to RGDSP. AGDV has very little contact with the α subunit. Furthermore, as measured by epitope exposure, AGDV, like the fibrinogen γ C-terminal peptide and RGD, caused integrin extension on the cell surface. Thus, pushing by the β3 subunit on Asp is sufficient for headpiece opening and ligand sliding, and no pulling by the αIIb subunit on Arg is required.

Copyright © 2015, The American Society for Biochemistry and Molecular Biology.