Martin Lackmann

The structure of Bcl‐w reveals a role for the C‐terminal residues in modulating biological activity

Pro‐survival Bcl‐2‐related proteins, critical regulators of apoptosis, contain a hydrophobic groove targeted for binding by the BH3 domain of the pro‐apoptotic BH3‐only proteins. The solution structure of the pro‐survival protein Bcl‐w, presented here, reveals that the binding groove is not freely accessible as predicted by previous structures of pro‐survival Bcl‐2‐like molecules. Unexpectedly, the groove appears to be occluded by the C‐terminal residues. Binding and kinetic data suggest that the C‐terminal residues of Bcl‐w and Bcl‐xL modulate pro‐survival activity by regulating ligand access to the groove. Binding of the BH3‐only proteins, critical for cell death initiation, is likely to displace the hydrophobic C‐terminal region of Bcl‐w and Bcl‐xL. Moreover, Bcl‐w does not act only by sequestering the BH3‐only proteins. There fore, pro‐survival Bcl‐2‐like molecules probably control the activation of downstream effectors by a mechanism that remains to be elucidated.

Recruitment of Eph receptors into signaling clusters does not require ephrin contact

Eph receptors and their cell membrane–bound ephrin ligands regulate cell positioning and thereby establish or stabilize patterns of cellular organization. Although it is recognized that ephrin clustering is essential for Eph function, mechanisms that relay information of ephrin density into cell biological responses are poorly understood. We demonstrate by confocal time-lapse and fluorescence resonance energy transfer microscopy that within minutes of binding ephrin-A5–coated beads, EphA3 receptors assemble into large clusters. While remaining positioned around the site of ephrin contact, Eph clusters exceed the size of the interacting ephrin surface severalfold. EphA3 mutants with compromised ephrin-binding capacity, which alone are incapable of cluster formation or phosphorylation, are recruited effectively and become phosphorylated when coexpressed with a functional receptor. Our findings reveal consecutive initiation of ephrin-facilitated Eph clustering and cluster propagation, the latter of which is independent of ephrin contacts and cytosolic Eph signaling functions but involves direct Eph–Eph interactions.

Proapoptotic BH3-only proteins trigger membrane integration of prosurvival Bcl-w and neutralize its activity

Prosurvival Bcl-2–like proteins, like Bcl-w, are thought to function on organelles such as the mitochondrion and to be targeted to them by their hydrophobic COOH-terminal domain. We unexpectedly found, however, that the membrane association of Bcl-w was enhanced during apoptosis. In healthy cells, Bcl-w was loosely attached to the mitochondrial membrane, but it was converted into an integral membrane protein by cytotoxic signals that induce binding of BH3-only proteins, such as Bim, or by the addition of BH3 peptides to lysates. As the structure of Bcl-w has revealed that its COOH-terminal domain occupies the hydrophobic groove where BH3 ligands bind, displacement of that domain by a BH3 ligand would displace the hydrophobic COOH-terminal residues, allowing their insertion into the membrane. To determine whether BH3 ligation is sufficient to induce the enhanced membrane affinity, or to render Bcl-w proapoptotic, we mimicked their complex by tethering the Bim BH3 domain to the NH2 terminus of Bcl-w. The chimera indeed bound avidly to membranes, in a fashion requiring the COOH-terminal domain, but neither promoted nor inhibited apoptosis. These results suggest that ligation of a proapoptotic BH3-only protein alters the conformation of Bcl-w, enhances membrane association, and neutralizes its survival function.

Distinct Subdomains of the EphA3 Receptor Mediate Ligand Binding and Receptor Dimerization

Eph receptor tyrosine kinases and their ligands (ephrins) are highly conserved protein families implicated in patterning events during development, particularly in the nervous system. In a number of functional studies, strict conservation of structure and function across distantly related vertebrate species has been confirmed. In this study we make use of the observation that soluble human EphA3 (HEK) exerts a dominant negative effect on somite formation and axial organization during zebrafish embryogenesis to probe receptor function. Based on exon structure we have dissected the extracellular region of EphA3 receptor into evolutionarily conserved subdomains and used kinetic BIAcore analysis, mRNA injection into zebrafish embryos, and receptor transphosphorylation analysis to study their function. We show that ligand binding is restricted to the N-terminal region encoded by exon III, and we identify an independent, C-terminal receptor-dimerization domain. Recombinant proteins encoding either region in isolation can function as receptor antagonists in zebrafish. We propose a two-step mechanism of Eph receptor activation with distinct ligand binding and ligand-independent receptor-receptor oligomerization events.

S100 protein CP-10 stimulates myeloid cell chemotaxis without activation

Leukocyte recruitment to inflammatory foci is generally associated with cellular activation. Recent evidence suggests that chemotactic agents can be divided into two classes, “classical chemoattractants” such as FMLP, C5a, and IL‐8, which stimulate directed migration and activation events and “pure chemoattractants” such as TGF‐β1 which influence actin polymerisation and movement but not oxidative burst and associated granular enzyme release. The studies reported here demonstrate that the murine S100 chemoattractant protein, CP‐10, belongs to the “non‐classical” group. Despite its potent chemotactic activity for neutrophils and monocytes/macrophages, CP‐10 failed to increase [Ca2+]i in human or mouse PMN, although chemotaxis was inhibited by pertussis toxin, confirming the suggestion of a novel Ca2+‐independent G‐protein‐coupled pathway for post‐receptor signal transduction triggered by “pure chemoattractants.” The co‐ordinated up‐regulation of Mac‐1 and down‐regulation of L‐selectin induced by FMLP on human PMN in vitro was not observed with CP‐10. Quantitative changes in immediate (30 s) actin polymerisation occurred with FMLP and CP‐10‐treated human PMN. The relative F‐actin increases induced in WEHI 265 monocytoid cells by FMLP and CP‐10 was optimal at 60 s and declined over 120 s. F‐actin changes reflected the concentration and potencies of the agonists required to provoke chemotaxis. After 90 min, CP‐10 profoundly altered cell shape and increased both cell size and F‐actin within pseudopodia. These changes are typical of those mediating leukocyte deformability, and CP‐10 may mediate leukocyte retention within microcapillaries and thereby contribute to the initiation of inflammation in vascular beds.

Dissecting the EphA3/Ephrin-A5 Interactions Using a Novel Functional Mutagenesis Screen

The EphA3 receptor tyrosine kinase preferentially binds ephrin-A5, a member of the corresponding subfamily of membrane-associated ligands. Their interaction regulates critical cell communication functions in normal development and may play a role in neoplasia. Here we describe a random mutagenesis approach, which we employed to study the molecular determinants of the EphA3/ephrin-A5 recognition. Selection and functional characterization of EphA3 point mutants with impaired ephrin-A5 binding from a yeast expression library defined three EphA3 surface areas that are essential for the EphA3/ephrin-A5 interaction. Two of these map to regions identified previously in the crystal structure of the homologous EphB2-ephrin-B2 complex as potential ligand/receptor interfaces. In addition, we identify a third EphA3/ephrin-A5 interface that falls outside the structurally characterized interaction domains. Functional analysis of EphA3 mutants reveals that all three Eph/ephrin contact areas are essential for the assembly of signaling-competent, oligomeric receptor-ligand complexes.

Structural Determinants of the Interaction between the erbB2 Receptor and the Src Homology 2 Domain of Grb7

The Src homology 2 (SH2) domain-containing protein Grb7 and the erbB2 receptor tyrosine kinase are overexpressed in a subset of human breast cancers. They also co-immunoprecipitate from cell lysates and associate directly in vitro. Whereas the Grb7 SH2 domain binds strongly to erbB2, the SH2 domain of Grb14, a protein closely related to Grb7, does not. We have investigated the preferred binding site of Grb7 within the erbB2 intracellular domain and the SH2 domain residues that determine the high affinity of Grb7 compared with Grb14 for this site. Phosphopeptide competition and site-directed mutagenesis revealed that Tyr-1139 of erbB2 is the major binding site for the Grb7 SH2 domain, indicating an overlap in binding specificity between the Grb7 and Grb2 SH2 domains. Substituting individual amino acids in the Grb14 SH2 domain with the corresponding residues from Grb7 demonstrated that a Gln to Leu change at the βD6 position imparted high affinity erbB2 interaction, paralleled by a marked increase in affinity for the Tyr-1139 phosphopeptide. The reverse switch at the βD6 position abrogated Grb7 binding to erbB2. This residue therefore represents an important determinant of SH2 domain specificity within the Grb7 family.

Cytoplasmic relaxation of active Eph controls ephrin shedding by ADAM10

Novel imaging strategies reveal a conformational shift in a receptor tyrosine kinase domain that controls ligand shedding by an ADAM metalloprotease.

Signals from Eph and ephrin proteins: a developmental tool kit.

Interactions between Eph receptors and their ligands the ephrin proteins are critically important in many key developmental processes. Emerging evidence also supports a role for these molecules in postembryonic tissues, particularly in pathological processes, including tissue injury and tumor metastasis. We review the signaling mechanisms that allow the 14 Eph and nine ephrin proteins to deliver intracellular signals that regulate cell shape and movement. What emerges is that the initiation of these signals is critically dependent on which Eph and ephrin proteins are expressed, the level of their expression, and, in some cases, which splice variants are expressed. Diversity at the level of initial interaction and in the downstream signaling processes regulated by Eph-ephrin signaling provides a subtle, versatile system of regulation of intercellular adhesion, cell shape, and cell motility. Interactions between Eph receptor family members and their ligands, the ephrins, are critical to many important embryonic patterning events. Evidence is emerging that also implies a role in pathological processes, including cancer and tissue injury. How do 14 Eph and nine ephrin proteins generate the great complexity of signaling mechanisms required to regulate cell movements as diverse as axon guidance, tissue boundary formation, and formation of the vasculature? We review how functional complexity is generated through modular usage of Eph and ephrin components, and discuss recent observations that highlight the unusual versatility of signaling mechanisms provided by this receptor-ligand system.

An early developmental role for Eph-ephrin interaction during vertebrate gastrulation

Eph receptor tyrosine kinases (RTK) and their ephrin ligands are involved in the transmission of signals which regulate cytoskeletal organisation and cell migration, and are expressed in spatially restricted patterns at discrete phases during embryogenesis. Loss of function mutants of Eph RTK or ephrin genes result in defects in neuronal pathfinding or cell migration. In this report we show that soluble forms of human EphA3 and ephrin-A5, acting as dominant negative inhibitors, interfere with early events in zebrafish embryogenesis. Exogenous expression of both proteins results in dose-dependent defects in somite development and organisation of the midbrain-hindbrain boundary and hindbrain. The nature of the defects as well as the distribution and timing of expression of endogenous ligands/receptors for both proteins suggest that Eph-ephrin interaction is required for the organisation of embryonic structures by coordinating the cellular movements of convergence during gastrulation.