Diversity of polyproline recognition by EVH1 domains

Insights into the Interaction Landscape of the EVH1 Domain of Mena

The Enabled/VASP homology 1 (EVH1) domain is a small module that interacts with proline-rich stretches in its ligands and is found in various signaling and scaffolding proteins. Mena, the mammalian homologue of Ena, is involved in diverse actin-associated events, such as membrane dynamics, bacterial motility, and tumor intravasation and extravasation. Two-dimensional (2D) 1H-15N HSQC NMR was used to study Mena EVH1 binding properties, defining the amino acids involved in ligand recognition for the physiological ligands ActA and PCARE, and a synthetic polyproline-inspired small molecule (hereafter inhibitor 6c). Chemical shift perturbations indicated that proline-rich segments bind in the conserved EVH1 hydrophobic cleft. The PCARE-derived peptide elicited more perturbations compared to the ActA-derived peptide, consistent with a previous report of a structural alteration in the solvent-exposed β7-β8 loop. Unexpectedly, EVH1 and the proline-rich segment of PTP1B did not exhibit NMR chemical shift perturbations; however, the high-resolution crystal structure implicated the conserved EVH1 hydrophobic cleft in ligand recognition. Intrinsic steady-state fluorescence and fluorescence polarization assays indicate that residues outside the proline-rich segment enhance the ligand affinity for EVH1 (Kd = 3-8 μM). Inhibitor 6c displayed tighter binding (Kd ∼ 0.3 μM) and occupies the same EVH1 cleft as physiological ligands. These studies revealed that the EVH1 domain enhances ligand affinity through recognition of residues flanking the proline-rich segments. Additionally, a synthetic inhibitor binds more tightly to the EVH1 domain than natural ligands, occupying the same hydrophobic cleft.

Elaboration of the Homer1 recognition landscape reveals incomplete divergence of paralogous EVH1 domains

Short sequences that mediate interactions with modular binding domains are ubiquitous throughout eukaryotic proteomes. Networks of short linear motifs (SLiMs) and their corresponding binding domains orchestrate many cellular processes, and the low mutational barrier to evolving novel interactions provides a way for biological systems to rapidly sample selectable phenotypes. Mapping SLiM binding specificity and the rules that govern SLiM evolution is fundamental to uncovering the pathways regulated by these networks and developing the tools to manipulate them. We used high-throughput screening of the human proteome to identify sequences that bind to the Enabled/VASP homology 1 (EVH1) domain of the postsynaptic density scaffolding protein Homer1. This expanded our understanding of the determinants of Homer EVH1 binding preferences and defined a new motif that can facilitate the discovery of additional Homer-mediated interactions. Interestingly, the Homer1 EVH1 domain preferentially binds to sequences containing an N-terminally overlapping motif that is bound by the paralogous family of Ena/VASP actin polymerases, and many of these sequences can bind to EVH1 domains from both protein families. We provide evidence from orthologous EVH1 domains in pre-metazoan organisms that the overlap in human Ena/VASP and Homer binding preferences corresponds to an incomplete divergence from a common Ena/VASP ancestor. Given this overlap in binding profiles, promiscuous sequences that can be recognized by both families either achieve specificity through extrinsic regulatory strategies or may provide functional benefits via multi-specificity. This may explain why these paralogs incompletely diverged despite the accessibility of further diverged isoforms.

Elaboration of the Homer1 Recognition Landscape Reveals Incomplete Divergence of Paralogous EVH1 Domains

Short sequences that mediate interactions with modular binding domains are ubiquitous throughout eukaryotic proteomes. Networks of Short Linear Motifs (SLiMs) and their corresponding binding domains orchestrate many cellular processes, and the low mutational barrier to evolving novel interactions provides a way for biological systems to rapidly sample selectable phenotypes. Mapping SLiM binding specificity and the rules that govern SLiM evolution is fundamental to uncovering the pathways regulated by these networks and developing the tools to manipulate them. We used high-throughput screening of the human proteome to identify sequences that bind to the Enabled/VASP homology 1 (EVH1) domain of the postsynaptic density scaffolding protein Homer1. In doing so, we expanded current understanding of the determinants of Homer EVH1 binding preferences and defined a new motif that can facilitate the discovery of additional Homer-mediated interactions. Interestingly, the Homer1 EVH1 domain preferentially binds to sequences containing an N-terminally overlapping motif that is bound by the paralogous family of Ena/VASP actin polymerases, and many of these sequences can bind to EVH1 domains from both protein families. We provide evidence from orthologous EVH1 domains in pre-metazoan organisms that the overlap in human Ena/VASP and Homer binding preferences corresponds to an incomplete divergence from a common Ena/VASP ancestor. Given this overlap in binding profiles, promiscuous sequences that can be recognized by both families either achieve specificity through extrinsic regulatory strategies or may provide functional benefits via multi-specificity. This may explain why these paralogs incompletely diverged despite the accessibility of further diverged isoforms.

Conserved proline residues prevent dimerization and aggregation in the β-lactamase BlaC

Evolution leads to conservation of amino acid residues in protein families. Conserved proline residues are usually considered to ensure the correct folding and to stabilize the three-dimensional structure. Surprisingly, proline residues that are highly conserved in class A β-lactamases were found to tolerate various substitutions without large losses in enzyme activity. We investigated the roles of three conserved prolines at positions 107, 226, and 258 in the β-lactamase BlaC from Mycobacterium tuberculosis and found that mutations can lead to dimerization of the enzyme and an overall less stable protein that is prone to aggregate over time. For the variant Pro107Thr, the crystal structure shows dimer formation resembling domain swapping. It is concluded that the proline substitutions loosen the structure, enhancing multimerization. Even though the enzyme does not lose its properties without the conserved proline residues, the prolines ensure the long-term structural integrity of the enzyme.

Cardiovascular Functions of Ena/VASP Proteins: Past, Present and Beyond

Actin binding proteins are of crucial importance for the spatiotemporal regulation of actin cytoskeletal dynamics, thereby mediating a tremendous range of cellular processes. Since their initial discovery more than 30 years ago, the enabled/vasodilator-stimulated phosphoprotein (Ena/VASP) family has evolved as one of the most fascinating and versatile family of actin regulating proteins. The proteins directly enhance actin filament assembly, but they also organize higher order actin networks and link kinase signaling pathways to actin filament assembly. Thereby, Ena/VASP proteins regulate dynamic cellular processes ranging from membrane protrusions and trafficking, and cell-cell and cell-matrix adhesions, to the generation of mechanical tension and contractile force. Important insights have been gained into the physiological functions of Ena/VASP proteins in platelets, leukocytes, endothelial cells, smooth muscle cells and cardiomyocytes. In this review, we summarize the unique and redundant functions of Ena/VASP proteins in cardiovascular cells and discuss the underlying molecular mechanisms.

Two cold shock domain containing proteins trigger the development of infectious Trypanosoma brucei

Cold shock proteins are members of a family of DNA- and RNA-binding proteins with one or more evolutionarily conserved cold shock domain (CSD). These proteins have a wide variety of biological functions, including DNA-damage repair, mRNA stability, and regulation of transcription, splicing and translation. We previously identified two CSD containing proteins, CSD1 and CSD2, in the protozoan parasite Trypanosoma brucei to be required for RBP6-driven metacyclic production, albeit at different steps of the developmental program. During metacyclogenesis T. brucei undergoes major morphological and metabolic changes that culminate in the establishment of quiescent metacyclic parasites and the acquisition of mammalian infectivity. To investigate the specific role of CSD1 and CSD2 in this process, we ectopically expressed CSD1 or CSD2 in non-infectious procyclic parasites and discovered that each protein is sufficient to produce infectious metacyclic parasites in 24 hours. Domain truncation assays determined that the N-terminal domain, but not the C-terminal domain, of CSD1 and CSD2 was required for metacyclic development. Furthermore, conserved amino acid residues in the CSD of CSD1 and CSD2, known to be important for binding nucleic acids, were found to be necessary for metacyclic production. Using single-end enhanced crosslinking and immunoprecipitation (seCLIP) we identified the specific binding motif of CSD1 and CSD2 as "ANACAU" and the bound mRNAs were enriched for biological processes, including lipid metabolism, microtubule-based movement and nucleocytoplasmic transport that are likely involved in the transition to bloodstream form-like cells.

Chromosome Translocation t(10;19)(q26;q13) in a CIC-sarcoma

BACKGROUND/AIM

CIC-sarcomas are characterized by rearrangements of the capicua transcriptional repressor (CIC) gene on chromosome subband 19q13.2, generating chimeras in which CIC is the 5'-end partner. Most reported CIC-sarcomas have been detected using PCR amplifications together with Sanger sequencing, high throughput sequencing, and fluorescence in situ hybridization (FISH). Only a few CIC-rearranged tumors have been characterized cytogenetically. Here, we describe the cytogenetic and molecular genetic features of a CIC-sarcoma carrying a t(10;19)(q26;q13), a chromosomal rearrangement not previously detected in such neoplasms.

MATERIALS AND METHODS

A round cell sarcoma removed from the right thigh of a 57-year-old man was investigated by G-banding cytogenetics, FISH, PCR and Sanger sequencing.

RESULTS

The tumor cells had three cytogenetically related clones with the translocations t(9;18)(q22;q21) and t(10;19)(q26;q13) common to all of them. FISH with a BAC probe containing the CIC gene hybridized to the normal chromosome 19, to der(10)t(10;19), and to der(19)t(10;19). PCR using tumor cDNA as template together with Sanger sequencing detected two CIC::DUX4 fusion transcripts which both had a stop TAG codon immediately after the fusion point. Both transcripts are predicted to encode truncated CIC polypeptides lacking the carboxy terminal part of the native protein. This missing part is crucial for CIC's DNA binding capacity and interaction with other proteins.

CONCLUSION

In addition to demonstrating that CIC rearrangement in sarcomas can occur via the microscopically visible translocation t(10;19)(q26;q13), the findings in the present case provide evidence that the missing part in CIC-truncated proteins has important functions whose loss may be important in tumorigenesis.

PRMT4-mediated arginine methylation promotes tyrosine phosphorylation of VEGFR-2 and regulates filopodia protrusions

Through tightly controlled multilayer mechanisms, vascular endothelial growth factor receptor-2 (VEGFR-2) activation and its downstream signal transduction govern vasculogenesis and pathological angiogenesis, such as tumor angiogenesis. Therefore, it is critical to understand the molecular mechanisms governing VEGFR-2 signal transduction. We report that protein arginine methyltransferase 4 (PRMT4) via its highly conserved EVH1 and PH domain-like N-terminal domain binds to VEGFR-2 and mediates methylation of the juxtamembrane arginine 817 (R817) on VEGFR-2. Methylation of R817 selectively increases phosphorylation of tyrosine 820 (Y820). Phosphorylation of Y820 facilitates the c-Src binding with VEGFR-2 via Src homology domain 2 (SH2). Interfering with the methylation of R817 or phosphorylation of Y820 inhibits VEGFR-2-induced filopodia protrusions, a process that is critical for the core angiogenic responses of VEGFR-2. Methylation of R817 is an important previously unrecognized mechanism of the angiogenic signaling of VEGFR-2, with implications for the development of novel-targeted VEGFR-2 inhibitors.

•

Arginine 817 methylation regulates phosphorylation of Y820 on VEGFR-2

•

Phosphorylation of Y820 recruits c-Src kinase to VEGFR-2, leading to its activation

•

VEGFR-2/c-Src axis mediates filopodia protrusions in endothelial cells

Protein Interaction Domains: Structural Features and Drug Discovery Applications (Part 2)

BACKGROUND

Proteins present a modular organization made up of several domains. Apart from the domains playing catalytic functions, many others are crucial to recruit interactors. The latter domains can be defined as "PIDs" (Protein Interaction Domains) and are responsible for pivotal outcomes in signal transduction and a certain array of normal physiological and disease-related pathways. Targeting such PIDs with small molecules and peptides able to modulate their interaction networks, may represent a valuable route to discover novel therapeutics.

OBJECTIVE

This work represents a continuation of a very recent review describing PIDs able to recognize post-translationally modified peptide segments. On the contrary, the second part concerns with PIDs that interact with simple peptide sequences provided with standard amino acids.

METHODS

Crucial structural information on different domain subfamilies and their interactomes was gained by a wide search in different online available databases (including the PDB (Protein Data Bank), the Pfam (Protein family), and the SMART (Simple Modular Architecture Research Tool)). Pubmed was also searched to explore the most recent literature related to the topic.

RESULTS AND CONCLUSION

PIDs are multifaceted: they have all diverse structural features and can recognize several consensus sequences. PIDs can be linked to different diseases onset and progression, like cancer or viral infections and find applications in the personalized medicine field. Many efforts have been centered on peptide/peptidomimetic inhibitors of PIDs mediated interactions but much more work needs to be conducted to improve drug-likeness and interaction affinities of identified compounds.

Novel perspectives of target-binding by the evolutionarily conserved PP4 phosphatase

Protein phosphatase 4 (PP4) is an evolutionarily conserved and essential Ser/Thr phosphatase that regulates cell division, development and DNA repair in eukaryotes. The major form of PP4, present from yeast to human, is the PP4c-R2-R3 heterotrimeric complex. The R3 subunit is responsible for substrate-recognition via its EVH1 domain. In typical EVH1 domains, conserved phenylalanine, tyrosine and tryptophan residues form the specific recognition site for their target's proline-rich sequences. Here, we identify novel binding partners of the EVH1 domain of the Drosophila R3 subunit, Falafel, and demonstrate that instead of binding to proline-rich sequences this EVH1 variant specifically recognizes atypical ligands, namely the FxxP and MxPP short linear consensus motifs. This interaction is dependent on an exclusively conserved leucine that replaces the phenylalanine invariant of all canonical EVH1 domains. We propose that the EVH1 domain of PP4 represents a new class of the EVH1 family that can accommodate low proline content sequences, such as the FxxP motif. Finally, our data implicate the conserved Smk-1 domain of Falafel in target-binding. These findings greatly enhance our understanding of the substrate-recognition mechanisms and function of PP4.

SPRED proteins and their roles in signal transduction, development, and malignancy

The roles of SPRED proteins in signaling, development, and cancer are becoming increasingly recognized. SPRED proteins comprise an N-terminal EVH-1 domain, a central c-Kit-binding domain, and C-terminal SROUTY domain. They negatively regulate signaling from tyrosine kinases to the Ras-MAPK pathway. SPRED1 binds directly to both c-KIT and to the RasGAP, neurofibromin, whose function is completely dependent on this interaction. Loss-of-function mutations in SPRED1 occur in human cancers and cause the developmental disorder, Legius syndrome. Genetic ablation of SPRED genes in mice leads to behavioral problems, dwarfism, and multiple other phenotypes including increased risk of leukemia. In this review, we summarize and discuss biochemical, structural, and biological functions of these proteins including their roles in normal cell growth and differentiation and in human disease.

A proline-rich motif in the large intracellular loop of the glycine receptor α1 subunit interacts with the Pleckstrin homology domain of collybistin

Introduction

The inhibitory glycine receptor (GlyR), a mediator of fast synaptic inhibition, is located and held at neuronal synapses through the anchoring proteins gephyrin and collybistin. Stable localization of neurotransmitter receptors is essential for synaptic function. In case of GlyRs, only beta subunits were known until now to mediate synaptic anchoring.

Objectives

We identified a poly-proline II helix (PPII) in position 365–373 of the intra-cellular TM3-4 loop of the human GlyRα1 subunit as a novel potential synaptic anchoring site. The potential role of the PPII helix as synaptic anchoring site was tested.

Methods

Glycine receptors and collybistin variants were generated and recombinantly expressed in HEK293 cells and cultured neurons. Receptor function was assessed using patch-clamp electrophysiology, protein-protein interaction was studied using co-immuno-precipitation and pulldown experiments.

Results

Recombinantly expressed collybistin bound to isolated GlyRα1 TM3-4 loops in GST-pulldown assays. When the five proline residues P365A, P366A, P367A, P369A, P373A (GlyRα1^P1-5A) located in the GlyRα1-PPII helix were replaced by alanines, the PPII secondary structure was disrupted. Recombinant GlyRα1^P1-5A mutant subunits displayed normal cell surface expression and wildtype-like ion channel function, but binding to collybistin was abolished. The GlyRα1-collybistin interaction was independently confirmed by o-immunoprecipitation assays using full-length GlyRα1 subunits. Surprisingly, the interaction was not mediated by the SH3 domain of collybistin, but by its Pleckstrin homology (PH) domain. The mutation GlyRα1^P366L, identified in a hyperekplexia patient, is also disrupting the PPII helix, and caused reduced collybistin binding.

Conclusion

Our data suggest a novel interaction between α1 GlyR subunits and collybistin, which is physiologically relevant in vitro and in vivo and may contribute to postsynaptic anchoring of glycine receptors.

Transfer of HTLV-1 p8 and Gag to target T-cells depends on VASP, a novel interaction partner of p8

The Human T-cell leukemia virus type 1 (HTLV-1) orf I-encoded accessory protein p8 is cleaved from its precursor p12, and both proteins contribute to viral persistence. p8 induces cellular protrusions, which are thought to facilitate transfer of p8 to target cells and virus transmission. Host factors interacting with p8 and mediating p8 transfer are unknown. Here, we report that vasodilator-stimulated phosphoprotein (VASP), which promotes actin filament elongation, is a novel interaction partner of p8 and important for p8 and HTLV-1 Gag cell-to-cell transfer. VASP contains an Ena/VASP homology 1 (EVH1) domain that targets the protein to focal adhesions. Bioinformatics identified a short stretch in p8 (amino acids (aa) 24–45) which may mediate interactions with the EVH1 domain of VASP. Co-immunoprecipitations confirmed interactions of VASP:p8 in 293T, Jurkat and HTLV-1-infected MT-2 cells. Co-precipitation of VASP:p8 could be significantly blocked by peptides mimicking aa 26–37 of p8. Mutational studies revealed that the EVH1-domain of VASP is necessary, but not sufficient for the interaction with p8. Further, deletion of the VASP G- and F-actin binding domains significantly diminished co-precipitation of p8. Imaging identified areas of partial co-localization of VASP with p8 at the plasma membrane and in protrusive structures, which was confirmed by proximity ligation assays. Co-culture experiments revealed that p8 is transferred between Jurkat T-cells via VASP-containing conduits. Imaging and flow cytometry revealed that repression of both endogenous and overexpressed VASP by RNA interference or by CRISPR/Cas9 reduced p8 transfer to the cell surface and to target Jurkat T-cells. Stable repression of VASP by RNA interference in chronically infected MT-2 cells impaired both p8 and HTLV-1 Gag transfer to target Jurkat T-cells, while virus release was unaffected. Thus, we identified VASP as a novel interaction partner of p8, which is important for transfer of HTLV-1 p8 and Gag to target T-cells.

The delta-retrovirus Human T-cell leukemia virus type 1 encodes the accessory protein p8, which is generated by proteolytic cleavage from p12. Earlier work has shown that p8 enhances the formation of cellular conduits between T-cells, is transferred through these conduits to target T-cells and increases HTLV-1 transmission. It was suggested that p8 dampens T-cell responses in target T-cells, thus facilitating HTLV-1 infection. Our work sheds light on the mechanism of p8 transfer to target T-cells. We show that vasodilator-stimulated phosphoprotein (VASP), a novel interaction partner of p8, contributes to transfer of p8 to target T-cells. Mechanistically, VASP is crucial for recruitment of p8 to the cell surface. Since VASP is known to promote elongation of actin filaments by preventing them from capping, interactions of p8 with VASP are an elegant strategy to exploit the host cell machinery for being transported to the cell surface, and as a consequence, to other cells. Given that VASP is also important for cell-to-cell transfer of the HTLV-1 Gag protein, our work proposes that VASP is a new cellular target to counteract HTLV-1 cell-to-cell transmission.

Structural Insights into the SPRED1-Neurofibromin-KRAS Complex and Disruption of SPRED1-Neurofibromin Interaction by Oncogenic EGFR

Sprouty-related, EVH1 domain-containing (SPRED) proteins negatively regulate RAS/mitogen-activated protein kinase (MAPK) signaling following growth factor stimulation. This inhibition of RAS is thought to occur primarily through SPRED1 binding and recruitment of neurofibromin, a RasGAP, to the plasma membrane. Here, we report the structure of neurofibromin (GTPase-activating protein [GAP]-related domain) complexed with SPRED1 (EVH1 domain) and KRAS. The structure provides insight into how the membrane targeting of neurofibromin by SPRED1 allows simultaneous interaction with activated KRAS. SPRED1 and NF1 loss-of-function mutations occur across multiple cancer types and developmental diseases. Analysis of the neurofibromin-SPRED1 interface provides a rationale for mutations observed in Legius syndrome and suggests why SPRED1 can bind to neurofibromin but no other RasGAPs. We show that oncogenic EGFR(L858R) signaling leads to the phosphorylation of SPRED1 on serine 105, disrupting the SPRED1-neurofibromin complex. The structural, biochemical, and biological results provide new mechanistic insights about how SPRED1 interacts with neurofibromin and regulates active KRAS levels in normal and pathologic conditions.

The Disordered Cellular Multi-Tasker WIP and Its Protein-Protein Interactions: A Structural View

WASp-interacting protein (WIP), a regulator of actin cytoskeleton assembly and remodeling, is a cellular multi-tasker and a key member of a network of protein-protein interactions, with significant impact on health and disease. Here, we attempt to complement the well-established understanding of WIP function from cell biology studies, summarized in several reviews, with a structural description of WIP interactions, highlighting works that present a molecular view of WIP's protein-protein interactions. This provides a deeper understanding of the mechanisms by which WIP mediates its biological functions. The fully disordered WIP also serves as an intriguing example of how intrinsically disordered proteins (IDPs) exert their function. WIP consists of consecutive small functional domains and motifs that interact with a host of cellular partners, with a striking preponderance of proline-rich motif capable of interactions with several well-recognized binding partners; indeed, over 30% of the WIP primary structure are proline residues. We focus on the binding motifs and binding interfaces of three important WIP segments, the actin-binding N-terminal domain, the central domain that binds SH3 domains of various interaction partners, and the WASp-binding C-terminal domain. Beyond the obvious importance of a more fundamental understanding of the biology of this central cellular player, this approach carries an immediate and highly beneficial effect on drug-design efforts targeting WIP and its binding partners. These factors make the value of such structural studies, challenging as they are, readily apparent.

A Consensus Binding Motif for the PP4 Protein Phosphatase

Dynamic protein phosphorylation constitutes a fundamental regulatory mechanism in all organisms. Phosphoprotein phosphatase 4 (PP4) is a conserved and essential nuclear serine and threonine phosphatase. Despite the importance of PP4, general principles of substrate selection are unknown, hampering the study of signal regulation by this phosphatase. Here, we identify and thoroughly characterize a general PP4 consensus-binding motif, the FxxP motif. X-ray crystallography studies reveal that FxxP motifs bind to a conserved pocket in the PP4 regulatory subunit PPP4R3. Systems-wide in silico searches integrated with proteomic analysis of PP4 interacting proteins allow us to identify numerous FxxP motifs in proteins controlling a range of fundamental cellular processes. We identify an FxxP motif in the cohesin release factor WAPL and show that this regulates WAPL phosphorylation status and is required for efficient cohesin release. Collectively our work uncovers basic principles of PP4 specificity with broad implications for understanding phosphorylation-mediated signaling in cells.

Membrane-associated mucins of the ocular surface: New genes, new protein functions and new biological roles in human and mouse

The mucosal glycocalyx of the ocular surface constitutes the point of interaction between the tear film and the apical epithelial cells. Membrane-associated mucins (MAMs) are the defining molecules of the glycocalyx in all mucosal epithelia. Long recognized for their biophysical properties of hydration, lubrication, anti-adhesion and repulsion, MAMs maintain the wet ocular surface, lubricate the blink, stabilize the tear film and create a physical barrier to the outside world. However, it is increasingly appreciated that MAMs also function as cell surface receptors that transduce information from the outside to the inside of the cell. A number of excellent review articles have provided perspective on the field as it has progressed since 1987, when molecular cloning of the first MAM was reported. The current article provides an update for the ocular surface, placing it into the broad context of findings made in other organ systems, and including new genes, new protein functions and new biological roles. We discuss the epithelial tissue-equivalent with mucosal differentiation, the key model system making these advances possible. In addition, we make the first systematic comparison of MAMs in human and mouse, establishing the basis for using knockout mice for investigations with the complexity of an in vivo system. Lastly, we discuss findings from human genetics/genomics, which are providing clues to new MAM roles previously unimagined. Taken together, this information allows us to generate hypotheses for the next stage of investigation to expand our knowledge of MAM function in intracellular signaling and roles unique to the ocular surface.

P-Bodies: Composition, Properties, and Functions

Processing bodies (P-bodies) are cytoplasmic ribonucleoprotein (RNP) granules primarily composed of translationally repressed mRNAs and proteins related to mRNA decay, suggesting roles in post-transcriptional regulation. P-bodies are conserved in eukaryotic cells and exhibit properties of liquid droplets. However, the function of P-bodies in translational repression and/or mRNA decay remains contentious. Here we review recent advances in our understanding of the molecular composition of P-bodies, the interactions and processes that regulate P-body liquid-liquid phase separation (LLPS), and the cellular localization of mRNA decay machinery, in the context of how these discoveries refine models of P-body function.

New Structural Insights into Formation of the Key Actin Regulating WIP-WASp Complex Determined by NMR and Molecular Imaging

Wiskott-Aldrich syndrome protein (WASp) is exclusively expressed in hematopoietic cells and responsible for actin-dependent processes, including cellular activation, migration, and invasiveness. The C-terminal domain of WASp-Interacting Protein (WIP) binds to WASp and regulates its activity by shielding it from degradation in a phosphorylation dependent manner as we previously demonstrated. Mutations in the WAS-encoding gene lead to the primary immunodeficiencies Wiskott-Aldrich syndrome (WAS) and X-linked thrombocytopenia (XLT). Here, we shed a first structural light upon this function of WIP using nuclear magnetic resonance (NMR) and in vivo molecular imaging. Coexpression of fragments WASp(20-158) and WIP(442-492) allowed the purification and structural characterization of a natively folded complex, determined to form a characteristic pleckstrin homology domain with a mixed α/β-fold and central two-winged β-sheet. The WIP-derived peptide, unstructured in its free form, wraps around and interacts with WASp through short structural elements. Förster resonance energy transfer (FRET) and biochemical experiments demonstrated that, of these elements, WIP residues 454-456 are the major contributor to WASp affinity, and the previously overlooked residues 449-451 were found to have the largest effect upon WASp ubiquitylation and, presumably, degradation. Results obtained from this complementary combination of technologies link WIP-WASp affinity to protection from degradation. Our findings about the nature of WIP·WASp complex formation are relevant for ongoing efforts to understand hematopoietic cell behavior, paving the way for new therapeutic approaches to WAS and XLT.

NMR resonance assignments of the EVH1 domain of neurofibromin's recruitment factor Spred1

Neurofibromin and Sprouty-related EVH1 domain-containing protein 1 (Spred1) both act as negative regulators of the mitogen-activated protein kinase pathway and are associated with the rare diseases Neurofibromatosis type 1 and Legius syndrome, respectively. Spred1 recruits the major GTPase activating protein (GAP) neurofibromin from the cytosol to the membrane in order to inactivate the small G protein Ras. These functions are dependent on the N-terminal EVH1 domain and the C-terminal Sprouty domain of Spred1 whereas the former specifically recognizes the GAP related domain of neurofibromin and the latter is responsible for membrane targeting. Within the GAP domain, Spred1 binding depends on the GAPex portion which is dispensable for Ras inactivation. In a first step towards the characterization of the Neurofibromin Spred1 interface in solution we assigned backbone and side chain ¹H, ¹³C, and ¹⁵N chemical shifts of the Spred1 derived EVH1 domain. Our chemical shift data analysis indicate seven consecutive β-strands followed by a C-terminal α-helix which is in agreement with the previously reported crystal structure of Spred1(EVH1). Our data provide a framework for further analysis of the function of patient-derived mutations associated with rare diseases.

Mille viae in eukaryotic mRNA decapping

Cellular mRNA levels are regulated via rates of transcription and decay. Since the removal of the mRNA 5'-cap by the decapping enzyme DCP2 is generally an irreversible step towards decay, it requires regulation. Control of DCP2 activity is likely effected by two interdependent means: by conformational control of the DCP2-DCP1 complex, and by assembly control of the decapping network, an array of mutually interacting effector proteins. Here, we compare three recent and conformationally distinct crystal structures of the DCP2-DCP1 decapping complex in the presence of substrate analogs and decapping enhancers and we discuss alternative substrate recognition modes for the catalytic domain of DCP2. Together with structure-based insight into decapping network assembly, we propose that DCP2-mediated decapping follows more than one path.

TANGO1/cTAGE5 receptor as a polyvalent template for assembly of large COPII coats

The supramolecular cargo procollagen is loaded into coat protein complex II (COPII)-coated carriers at endoplasmic reticulum (ER) exit sites by the receptor molecule TANGO1/cTAGE5. Electron microscopy studies have identified a tubular carrier of suitable dimensions that is molded by a distinctive helical array of the COPII inner coat protein Sec23/24•Sar1; the helical arrangement is absent from canonical COPII-coated small vesicles. In this study, we combined X-ray crystallographic and biochemical analysis to characterize the association of TANGO1/cTAGE5 with COPII proteins. The affinity for Sec23 is concentrated in the proline-rich domains (PRDs) of TANGO1 and cTAGE5, but Sec23 recognizes merely a PPP motif. The PRDs contain repeated PPP motifs separated by proline-rich linkers, so a single TANGO1/cTAGE5 receptor can bind multiple copies of coat protein in a close-packed array. We propose that TANGO1/cTAGE5 promotes the accretion of inner coat proteins to the helical lattice. Furthermore, we show that PPP motifs in the outer coat protein Sec31 also bind to Sec23, suggesting that stepwise COPII coat assembly will ultimately displace TANGO1/cTAGE5 and compartmentalize its operation to the base of the growing COPII tubule.

The neurofibromin recruitment factor Spred1 binds to the GAP related domain without affecting Ras inactivation

Neurofibromatosis type 1 (NF1) and Legius syndrome are related diseases with partially overlapping symptoms caused by alterations of the tumor suppressor genes NF1 (encoding the protein neurofibromin) and SPRED1 (encoding sprouty-related, EVH1 domain-containing protein 1, Spred1), respectively. Both proteins are negative regulators of Ras/MAPK signaling with neurofibromin functioning as a Ras-specific GTPase activating protein (GAP) and Spred1 acting on hitherto undefined components of the pathway. Importantly, neurofibromin has been identified as a key protein in the development of cancer, as it is genetically altered in a large number of sporadic human malignancies unrelated to NF1. Spred1 has previously been demonstrated to interact with neurofibromin via its N-terminal Ena/VASP Homology 1 (EVH1) domain and to mediate membrane translocation of its target dependent on its C-terminal Sprouty domain. However, the region of neurofibromin required for the interaction with Spred1 has remained unclear. Here we show that the EVH1 domain of Spred1 binds to the noncatalytic (GAPex) portion of the GAP-related domain (GRD) of neurofibromin. Binding is compatible with simultaneous binding of Ras and does not interfere with GAP activity. Our study points to a potential targeting function of the GAPex subdomain of neurofibromin that is present in all known canonical RasGAPs.

A requirement for filopodia extension toward Slit during Robo-mediated axon repulsion

Axons navigate long distances through complex 3D environments to interconnect the nervous system during development. Although the precise spatiotemporal effects of most axon guidance cues remain poorly characterized, a prevailing model posits that attractive guidance cues stimulate actin polymerization in neuronal growth cones whereas repulsive cues induce actin disassembly. Contrary to this model, we find that the repulsive guidance cue Slit stimulates the formation and elongation of actin-based filopodia from mouse dorsal root ganglion growth cones. Surprisingly, filopodia form and elongate toward sources of Slit, a response that we find is required for subsequent axonal repulsion away from Slit. Mechanistically, Slit evokes changes in filopodium dynamics by increasing direct binding of its receptor, Robo, to members of the actin-regulatory Ena/VASP family. Perturbing filopodium dynamics pharmacologically or genetically disrupts Slit-mediated repulsion and produces severe axon guidance defects in vivo. Thus, Slit locally stimulates directional filopodial extension, a process that is required for subsequent axonal repulsion downstream of the Robo receptor.

Centromeric binding and activity of Protein Phosphatase 4

The cell division cycle requires tight coupling between protein phosphorylation and dephosphorylation. However, understanding the cell cycle roles of multimeric protein phosphatases has been limited by the lack of knowledge of how their diverse regulatory subunits target highly conserved catalytic subunits to their sites of action. Phosphoprotein phosphatase 4 (PP4) has been recently shown to participate in the regulation of cell cycle progression. We now find that the EVH1 domain of the regulatory subunit 3 of Drosophila PP4, Falafel (Flfl), directly interacts with the centromeric protein C (CENP-C). Unlike other EVH1 domains that interact with proline-rich ligands, the crystal structure of the Flfl amino-terminal EVH1 domain bound to a CENP-C peptide reveals a new target-recognition mode for the phosphatase subunit. We also show that binding of Flfl to CENP-C is required to bring PP4 activity to centromeres to maintain CENP-C and attached core kinetochore proteins at chromosomes during mitosis.

Ena/VASP proteins cooperate with the WAVE complex to regulate the actin cytoskeleton

Ena/VASP proteins and the WAVE regulatory complex (WRC) regulate cell motility by virtue of their ability to independently promote actin polymerization. We demonstrate that Ena/VASP and the WRC control actin polymerization in a cooperative manner through the interaction of the Ena/VASP EVH1 domain with an extended proline rich motif in Abi. This interaction increases cell migration and enables VASP to cooperatively enhance WRC stimulation of Arp2/3 complex-mediated actin assembly in vitro in the presence of Rac. Loss of this interaction in Drosophila macrophages results in defects in lamellipodia formation, cell spreading, and redistribution of Ena to the tips of filopodia-like extensions. Rescue experiments of abi mutants also reveals a physiological requirement for the Abi:Ena interaction in photoreceptor axon targeting and oogenesis. Our data demonstrate that the activities of Ena/VASP and the WRC are intimately linked to ensure optimal control of actin polymerization during cell migration and development.

A new mechanism in the binding between Homer3 EVH1 domain and inositol 1,4,5 trisphosphate receptor suppressor domain

The suppressor domain of inositol 1,4,5 trisphosphate receptor (IP3R) has critical roles in regulating the calcium channel by interacting with many binding partners. The residue 49-53 (PPKKF) of the suppressor domain was suggested to be a canonical Homer EVH1 domain binding site and is also the first a part of calmodulin (CaM) binding site. As CaM-binding of the suppressor domain has been shown to involve large-scale conformational changes, we studied the binding characteristics of the Homer EVH1-suppressor domain with NMR spectroscopy and biochemical pull-down assays for mutants. Our data show that the suppressor domain employs the PPKKF motif in a similar but subtly different way compared to previously characterized interactions, and that the suppressor domain does not undergo large-scale conformational changes. Chemical shift assignments of the Homer3 EVH1 domain found that a new set of residues, located at the opposite side of the previously reported binding site, is also involved in binding, which was confirmed by mutant binding assays. Further analysis suggests that F40 in the new binding sites may have a critical role as a conformational lock-switch in Homer-target binding. The proposed mechanism is implicated in the signaling network involving calcium channels.

Sulfopeptide probes of the CXCR4/CXCL12 interface reveal oligomer-specific contacts and chemokine allostery

Tyrosine sulfation is a post-translational modification that enhances protein-protein interactions and may identify druggable sites in the extracellular space. The G protein-coupled receptor CXCR4 is a prototypical example with three potential sulfation sites at positions 7, 12, and 21. Each receptor sulfotyrosine participates in specific contacts with its chemokine ligand in the structure of a soluble, dimeric CXCL12:CXCR4(1-38) complex, but their relative importance for CXCR4 binding and activation by the monomeric chemokine remains undefined. NMR titrations with short sulfopeptides showed that the tyrosine motifs of CXCR4 varied widely in their contributions to CXCL12 binding affinity and site specificity. Whereas the Tyr21 sulfopeptide bound the same site as in previously solved structures, the Tyr7 and Tyr12 sulfopeptides interacted nonspecifically. Surprisingly, the unsulfated Tyr7 peptide occupied a hydrophobic site on the CXCL12 monomer that is inaccessible in the CXCL12 dimer. Functional analysis of CXCR4 mutants validated the relative importance of individual CXCR4 sulfotyrosine modifications (Tyr21 > Tyr12 > Tyr7) for CXCL12 binding and receptor activation. Biophysical measurements also revealed a cooperative relationship between sulfopeptide binding at the Tyr21 site and CXCL12 dimerization, the first example of allosteric behavior in a chemokine. Future ligands that occupy the sTyr21 recognition site may act as both competitive inhibitors of receptor binding and allosteric modulators of chemokine function. Together, our data suggests that sulfation does not ubiquitously enhance complex affinity and that distinct patterns of tyrosine sulfation could encode oligomer selectivity, implying another layer of regulation for chemokine signaling.

Proline: the distribution, frequency, positioning, and common functional roles of proline and polyproline sequences in the human proteome

Proline is an anomalous amino acid. Its nitrogen atom is covalently locked within a ring, thus it is the only proteinogenic amino acid with a constrained phi angle. Sequences of three consecutive prolines can fold into polyproline helices, structures that join alpha helices and beta pleats as architectural motifs in protein configuration. Triproline helices are participants in protein-protein signaling interactions. Longer spans of repeat prolines also occur, containing as many as 27 consecutive proline residues. Little is known about the frequency, positioning, and functional significance of these proline sequences. Therefore we have undertaken a systematic bioinformatics study of proline residues in proteins. We analyzed the distribution and frequency of 687,434 proline residues among 18,666 human proteins, identifying single residues, dimers, trimers, and longer repeats. Proline accounts for 6.3% of the 10,882,808 protein amino acids. Of all proline residues, 4.4% are in trimers or longer spans. We detected patterns that influence function based on proline location, spacing, and concentration. We propose a classification based on proline-rich, polyproline-rich, and proline-poor status. Whereas singlet proline residues are often found in proteins that display recurring architectural patterns, trimers or longer proline sequences tend be associated with the absence of repetitive structural motifs. Spans of 6 or more are associated with DNA/RNA processing, actin, and developmental processes. We also suggest a role for proline in Kruppel-type zinc finger protein control of DNA expression, and in the nucleation and translocation of actin by the formin complex.

Structural analysis of SHARPIN, a subunit of a large multi-protein E3 ubiquitin ligase, reveals a novel dimerization function for the pleckstrin homology superfold

SHARPIN (SHANK-associated RH domain interacting protein) is part of a large multi-protein E3 ubiquitin ligase complex called LUBAC (linear ubiquitin chain assembly complex), which catalyzes the formation of linear ubiquitin chains and regulates immune and apoptopic signaling pathways. The C-terminal half of SHARPIN contains ubiquitin-like domain and Npl4-zinc finger domains that mediate the interaction with the LUBAC subunit HOIP and ubiquitin, respectively. In contrast, the N-terminal region does not show any homology with known protein interaction domains but has been suggested to be responsible for self-association of SHARPIN, presumably via a coiled-coil region. We have determined the crystal structure of the N-terminal portion of SHARPIN, which adopts the highly conserved pleckstrin homology superfold that is often used as a scaffold to create protein interaction modules. We show that in SHARPIN, this domain does not appear to be used as a ligand recognition domain because it lacks many of the surface properties that are present in other pleckstrin homology fold-based interaction modules. Instead, it acts as a dimerization module extending the functional applications of this superfold.

Proline rich motifs as drug targets in immune mediated disorders

The current version of the human immunome network consists of nearly 1400 interactions involving approximately 600 proteins. Intermolecular interactions mediated by proline-rich motifs (PRMs) are observed in many facets of the immune response. The proline-rich regions are known to preferentially adopt a polyproline type II helical conformation, an extended structure that facilitates transient intermolecular interactions such as signal transduction, antigen recognition, cell-cell communication and cytoskeletal organization. The propensity of both the side chain and the backbone carbonyls of the polyproline type II helix to participate in the interface interaction makes it an excellent recognition motif. An advantage of such distinct chemical features is that the interactions can be discriminatory even in the absence of high affinities. Indeed, the immune response is mediated by well-orchestrated low-affinity short-duration intermolecular interactions. The proline-rich regions are predominantly localized in the solvent-exposed regions such as the loops, intrinsically disordered regions, or between domains that constitute the intermolecular interface. Peptide mimics of the PRM have been suggested as potential antagonists of intermolecular interactions. In this paper, we discuss novel PRM-mediated interactions in the human immunome that potentially serve as attractive targets for immunomodulation and drug development for inflammatory and autoimmune pathologies.

Scaffolding protein Homer 1c mediates hypertrophic responses downstream of Gq in cardiomyocytes

Activation of the heterotrimeric G protein, Gq, causes cardiomyocyte hypertrophy in vivo and in cell models. Responses to activated Gq in cardiomyocytes are mediated exclusively by phospholipase Cβ1b (PLCβ1b), because it localizes at the sarcolemma by binding to Shank3, a high-molecular-weight (MW) scaffolding protein. Shank3 can bind to the Homer family of low-MW scaffolding proteins that fine tune Ca(2+) signaling by facilitating crosstalk between Ca(2+) channels at the cell surface with those on intracellular Ca(2+) stores. Activation of α(1)-adrenergic receptors, expression of constitutively active Gαq (GαqQL), or PLCβ1b initiated cardiomyocyte hypertrophy and increased Homer 1c mRNA expression, by 1.6 ± 0.18-, 1.9 ± 0.17-, and 1.5 ± 0.07-fold, respectively (means ± se, 6 independent experiments, P<0.05). Expression of Homer 1c induced an increase in cardiomyocyte area from 853 ± 27 to 1146 ± 31 μm(2) (P<0.05); furthermore, expression of dominant-negative Homer (Homer 1a) reversed the increase in cell size caused by α(1)-adrenergic agonist or PLCβ1b treatment (1503±48 to 996±28 and 1626±48 to 828±31 μm(2), respectively, P<0.05). Homer proteins were localized near the sarcolemma, associated with Shank3 and phospholipase Cβ1b. We conclude that Gq-mediated hypertrophy involves activation of PLCβ1b scaffolded onto a Shank3/Homer complex. Signaling downstream of Homer 1c is necessary and sufficient for Gq-initiated hypertrophy.

Assignment of PolyProline II conformation and analysis of sequence--structure relationship

BACKGROUND

Secondary structures are elements of great importance in structural biology, biochemistry and bioinformatics. They are broadly composed of two repetitive structures namely α-helices and β-sheets, apart from turns, and the rest is associated to coil. These repetitive secondary structures have specific and conserved biophysical and geometric properties. PolyProline II (PPII) helix is yet another interesting repetitive structure which is less frequent and not usually associated with stabilizing interactions. Recent studies have shown that PPII frequency is higher than expected, and they could have an important role in protein-protein interactions.

METHODOLOGY/PRINCIPAL FINDINGS

A major factor that limits the study of PPII is that its assignment cannot be carried out with the most commonly used secondary structure assignment methods (SSAMs). The purpose of this work is to propose a PPII assignment methodology that can be defined in the frame of DSSP secondary structure assignment. Considering the ambiguity in PPII assignments by different methods, a consensus assignment strategy was utilized. To define the most consensual rule of PPII assignment, three SSAMs that can assign PPII, were compared and analyzed. The assignment rule was defined to have a maximum coverage of all assignments made by these SSAMs. Not many constraints were added to the assignment and only PPII helices of at least 2 residues length are defined.

CONCLUSIONS/SIGNIFICANCE

The simple rules designed in this study for characterizing PPII conformation, lead to the assignment of 5% of all amino as PPII. Sequence-structure relationships associated with PPII, defined by the different SSAMs, underline few striking differences. A specific study of amino acid preferences in their N and C-cap regions was carried out as their solvent accessibility and contact patterns. Thus the assignment of PPII can be coupled with DSSP and thus opens a simple way for further analysis in this field.

A novel immunity system for bacterial nucleic acid degrading toxins and its recruitment in various eukaryotic and DNA viral systems

The use of nucleases as toxins for defense, offense or addiction of selfish elements is widely encountered across all life forms. Using sensitive sequence profile analysis methods, we characterize a novel superfamily (the SUKH superfamily) that unites a diverse group of proteins including Smi1/Knr4, PGs2, FBXO3, SKIP16, Syd, herpesviral US22, IRS1 and TRS1, and their bacterial homologs. Using contextual analysis we present evidence that the bacterial members of this superfamily are potential immunity proteins for a variety of toxin systems that also include the recently characterized contact-dependent inhibition (CDI) systems of proteobacteria. By analyzing the toxin proteins encoded in the neighborhood of the SUKH superfamily we predict that they possess domains belonging to diverse nuclease and nucleic acid deaminase families. These include at least eight distinct types of DNases belonging to HNH/EndoVII- and restriction endonuclease-fold, and RNases of the EndoU-like and colicin E3-like cytotoxic RNases-folds. The N-terminal domains of these toxins indicate that they are extruded by several distinct secretory mechanisms such as the two-partner system (shared with the CDI systems) in proteobacteria, ESAT-6/WXG-like ATP-dependent secretory systems in Gram-positive bacteria and the conventional Sec-dependent system in several bacterial lineages. The hedgehog-intein domain might also release a subset of toxic nuclease domains through auto-proteolytic action. Unlike classical colicin-like nuclease toxins, the overwhelming majority of toxin systems with the SUKH superfamily is chromosomally encoded and appears to have diversified through a recombination process combining different C-terminal nuclease domains to N-terminal secretion-related domains. Across the bacterial superkingdom these systems might participate in discriminating `self’ or kin from `non-self’ or non-kin strains. Using structural analysis we demonstrate that the SUKH domain possesses a versatile scaffold that can be used to bind a wide range of protein partners. In eukaryotes it appears to have been recruited as an adaptor to regulate modification of proteins by ubiquitination or polyglutamylation. Similarly, another widespread immunity protein from these toxin systems, namely the suppressor of fused (SuFu) superfamily has been recruited for comparable roles in eukaryotes. In animal DNA viruses, such as herpesviruses, poxviruses, iridoviruses and adenoviruses, the ability of the SUKH domain to bind diverse targets has been deployed to counter diverse anti-viral responses by interacting with specific host proteins.

Dcp1 links coactivators of mRNA decapping to Dcp2 by proline recognition

Cap hydrolysis is a critical step in several eukaryotic mRNA decay pathways and is carried out by the evolutionarily conserved decapping complex containing Dcp2 at the catalytic core. In yeast, Dcp1 is an essential activator of decapping and coactivators such as Edc1 and Edc2 are thought to enhance activity, though their mechanism remains elusive. Using kinetic analysis we show that a crucial function of Dcp1 is to couple the binding of coactivators of decapping to activation of Dcp2. Edc1 and Edc2 bind Dcp1 via its EVH1 proline recognition site and stimulate decapping by 1000-fold, affecting both the K(M) for mRNA and rate of the catalytic step. The C-terminus of Edc1 is necessary and sufficient to enhance the catalytic step, while the remainder of the protein likely increases mRNA binding to the decapping complex. Lesions in the Dcp1 EVH1 domain or the Edc1 proline-rich sequence are sufficient to block stimulation. These results identify a new role of Dcp1, which is to link the binding of coactivators to substrate recognition and activation of Dcp2.

Energetics of Src homology domain interactions in receptor tyrosine kinase-mediated signaling

Intracellular signaling from receptor tyrosine kinases (RTK) on extracellular stimulation is fundamental to all cellular processes. The protein-protein interactions which form the basis of this signaling are mediated through a limited number of polypeptide domains. For signal transduction without corruption, based on a model where signaling pathways are considered as linear bimolecular relays, these interactions have to be highly specific. This is particularly the case when one considers that any cell may have copies of similar binding domains found in numerous proteins. In this work, an overview of the thermodynamics of binding of two of the most common of these domains (SH2 and SH3 domains) is given. This, coupled with insight from high-resolution structural detail, provides a comprehensive survey of how recognition of cognate binding sites for these domains occurs. Based on the data presented, we conclude that specificity offered by these interactions of SH2 and SH3 domains is limited and not sufficient to enforce mutual exclusivity in RTK-mediated signaling. This may explain the current lack of success in pharmaceutical intervention to inhibit the interactions of these domains when they are responsible for aberrant signaling and the resulting disease states such as cancer.

Interleukin-1 Receptor-Associated Kinase-1 (IRAK-1) functionally associates with PKCepsilon and VASP in the regulation of macrophage migration

Macrophage migration is mediated by complex cellular signaling processes and cytoskeleton re-arrangement. In particular, recent advances indicate that the innate immunity signaling process plays a key role in the regulation of macrophage migration. In this report, we have provided evidence demonstrating the involvement of a key innate immunity signaling kinase, Interleukin-1 Receptor-Associated Kinase-1 (IRAK-1) as a critical modulator of macrophage migration. Macrophage migration induced by phorbol 12-myristate 13-acetate (PMA) is significantly attenuated in IRAK-1(-/-) macrophages as compared to wild type macrophages. Mechanistically, we demonstrated that IRAK-1 works downstream of PKCepsilon and upstream of VASP, a member of Ena/VASP family proteins. IRAK-1 forms a close complex with PKCepsilon as well as VASP, and participates in PMA-induced phosphorylation of VASP. Notably, IRAK-1 contains a novel EVH1 domain binding motif (L(167)WPPPP) within its N-terminus, which is responsible for its interaction with VASP. The mutant IRAK-1 (L167A/W168A) fails to associate with VASP. Our findings provide a novel facet regarding the molecular signaling process regulating macrophage migration.