Structure-function analysis of the WIP role in T cell receptor-stimulated NFAT activation: evidence that WIP-WASP dissociation is not required and that the WIP NH2 terminus is inhibitory

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.

Tyrosine phosphorylation of WIP releases bound WASP and impairs podosome assembly in macrophages

Podosomes are integrin-containing adhesion structures commonly found in migrating leukocytes of the monocytic lineage. The actin cytoskeletal organisation of podosomes is based on a WASP- and Arp2/3-mediated mechanism. WASP also associates with a second protein, WIP (also known as WIPF1), and they co-localise in podosome cores. Here, we report for the first time that WIP can be phosphorylated on tyrosine residues and that tyrosine phosphorylation of WIP is a trigger for release of WASP from the WIP-WASP complex. Using a knockdown approach together with expression of WIP phosphomimics, we show that in the absence of WIP-WASP binding, cellular WASP is rapidly degraded, leading to disruption of podosomes and a failure of cells to degrade an underlying matrix. In the absence of tyrosine phosphorylation, the WIP-WASP complex remains intact and podosome lifetimes are extended. A screen of candidate kinases and inhibitor-based assays identified Bruton's tyrosine kinase (Btk) as a regulator of WIP tyrosine phosphorylation. We conclude that tyrosine phosphorylation of WIP is a crucial regulator of WASP stability and function as an actin-nucleation-promoting factor.

Molecular mechanisms and functional implications of polarized actin remodeling at the T cell immunological synapse

Transient,specialized cell-cell interactions play a central role in leukocyte function by enabling specific intercellular communication in the context of a highly dynamic systems level response. The dramatic structural changes required for the formation of these contacts are driven by rapid and precise cytoskeletal remodeling events. In recent years, the immunological synapse that forms between a T lymphocyte and its antigen-presenting target cell has emerged as an important model system for understanding immune cell interactions. In this review, we discuss how regulators of the cortical actin cytoskeleton control synaptic architecture and in this way specify T cell function.

WIP: more than a WASp-interacting protein

WIP plays an important role in the remodeling of the actin cytoskeleton, which controls cellular activation, proliferation, and function. WIP regulates actin polymerization by linking the actin machinery to signaling cascades. WIP binding to WASp and to its homolog, N-WASp, which are central activators of the actin-nucleating complex Arp2/3, regulates their cellular distribution, function, and stability. By binding to WASp, WIP protects it from degradation and thus, is crucial for WASp retention. Indeed, most mutations that result in WAS, an X-linked immunodeficiency caused by defective/absent WASp activity, are located in the WIP-binding region of WASp. In addition, by binding directly to actin, WIP promotes the formation and stabilization of actin filaments. WASp-independent activities of WIP constitute a new research frontier and are discussed extensively in this article. Here, we review the current information on WIP in human and mouse systems, focusing on its associated proteins, its molecular-regulatory mechanisms, and its role as a key regulator of actin-based processes in the immune system.

Triple-color FRET analysis reveals conformational changes in the WIP-WASp actin-regulating complex

Wiskott-Aldrich syndrome protein (WASp) is a key regulator of the actin cytoskeletal machinery. Binding of WASp-interacting protein (WIP) to WASp modulates WASp activity and protects it from degradation. Formation of the WIP-WASp complex is crucial for the adaptive immune response. We found that WIP and WASp interacted in cells through two distinct molecular interfaces. One interaction occurred between the WASp-homology-1 (WH1) domain of WASp and the carboxyl-terminal domain of WIP that depended on the phosphorylation status of WIP, which is phosphorylated by protein kinase C θ (PKCθ) in response to T cell receptor activation. The other interaction occurred between the verprolin homology, central hydrophobic region, and acidic region (VCA) domain of WASp and the amino-terminal domain of WIP. This latter interaction required actin, because it was inhibited by latrunculin A, which sequesters actin monomers. With triple-color fluorescence resonance energy transfer (3FRET) technology, we demonstrated that the WASp activation mechanism involved dissociation of the first interaction, while leaving the second interaction intact. This conformation exposed the ubiquitylation site on WASp, leading to degradation of WASp. Together, these data suggest that the activation and degradation of WASp are delicately balanced and depend on the phosphorylation state of WIP. Our molecular analysis of the WIP-WASp interaction provides insight into the regulation of actin-dependent processes.

NMR determines transient structure and dynamics in the disordered C-terminal domain of WASp interacting protein

WASp-interacting protein (WIP) is a 503-residue proline-rich polypeptide expressed in human T cells. The WIP C-terminal domain binds to Wiskott-Aldrich syndrome protein (WASp) and regulates its activation and degradation, and the WIP-WASp interaction has been shown to be critical for actin polymerization and implicated in the onset of WAS and X-linked thrombocytopenia. WIP is predicted to be an intrinsically disordered protein, a class of polypeptides that are of great interest because they violate the traditional structure-function paradigm. In this first (to our knowledge) study of WIP in its unbound state, we used NMR to investigate the biophysical behavior of WIP(C), a C-terminal domain fragment of WIP that includes residues 407-503 and contains the WASp-binding site. In light of the poor spectral dispersion exhibited by WIP(C) and the high occurrence (25%) of proline residues, we employed 5D-NMR(13)C-detected NMR experiments with nonuniform sampling to accomplish full resonance assignment. Secondary chemical-shift analysis, (15)N relaxation rates, and protection from solvent exchange all concurred in detecting transient structure located in motifs that span the WASp-binding site. Residues 446-456 exhibited a propensity for helical conformation, and an extended conformation followed by a short, capped helix was observed for residues 468-478. The (13)C-detected approach allows chemical-shift assignment in the WIP(C) polyproline stretches and thus sheds light on their conformation and dynamics. The effects of temperature on chemical shifts referenced to a denatured sample of the polypeptide demonstrate that heating reduces the structural character of WIP(C). Thus, we conclude that the disordered WIP(C) fragment is comprised of regions with latent structure connected by flexible loops, an architecture with implications for binding affinity and function.

WIP: WASP-interacting proteins at invadopodia and podosomes

Regulated cell invasion resulting from migratory and matrix-degrading events is an essential step in physiological processes such as the inflammatory response and tissue repair. Cell invasion is also thought to be a critical parameter in pathological conditions such as cancer metastasis. The migration of normal and cancer cells is largely driven by the actin cytoskeleton, which controls cell shape, adhesion and contractility. Podosomes and invadopodia are actin-rich protrusions that drive invasion in normal and cancer cells. These structures protrude from the basal region of the cell facing the extracellular matrix, where they adhere to and degrade the matrix, thus facilitating invasive migration. WASP (Wiskott-Aldrich syndrome protein) and WIP (WASP-interacting protein) localise to the actin rich core of podosomes and play a critical role in their formation. More recently, studies performed on microarray data sets from cancer patients of several tumour categories show a strong correlation between reduced WIP expression and improved prognosis. In this article, we identify endogenous WIP at the distal tips of cancer cell invasive protrusions and we summarise recent advances in the study of the roles of WIP- and WASP-protein families during migration and invasion of normal and cancer cells related to podosome and invadopodium generation.

WIP remodeling actin behind the scenes: how WIP reshapes immune and other functions

Actin polymerization is a fundamental cellular process regulating immune cell functions and the immune response. The Wiskott-Aldrich syndrome protein (WASp) is an actin nucleation promoting factor, which is exclusively expressed in hematopoietic cells, where it plays a key regulatory role in cytoskeletal dynamics. WASp interacting protein (WIP) was first discovered as the binding partner of WASp, through the use of the yeast two hybrid system. WIP was later identified as a chaperone of WASp, necessary for its stability. Mutations occurring at the WASp homology 1 domain (WH1), which serves as the WIP binding site, were found to cause the Wiskott-Aldrich syndrome (WAS) and X-linked thrombocytopenia (XLT). WAS manifests as an immune deficiency characterized by eczema, thrombocytopenia, recurrent infections, and hematopoietic malignancies, demonstrating the importance of WIP for WASp complex formation and for a proper immune response. WIP deficiency was found to lead to different abnormalities in the activity of various lymphocytes, suggesting differential cell-dependent roles for WIP. Additionally, WIP deficiency causes cellular abnormalities not found in WASp-deficient cells, indicating that WIP fulfills roles beyond stabilizing WASp. Indeed, WIP was shown to interact with various binding partners, including the signaling proteins Nck, CrkL and cortactin. Recent studies have demonstrated that WIP also takes part in non immune cellular processes such as cancer invasion and metastasis, in addition to cell subversion by intracellular pathogens. Understanding of numerous functions of WIP can enhance our current understanding of activation and function of immune and other cell types.

Ubiquitylation-dependent negative regulation of WASp is essential for actin cytoskeleton dynamics

The Wiskott-Aldrich syndrome protein (WASp) is a key regulator of actin dynamics during cell motility and adhesion, and mutations in its gene are responsible for Wiskott-Aldrich syndrome (WAS). Here, we demonstrate that WASp is ubiquitylated following T-cell antigen receptor (TCR) activation. WASp phosphorylation at tyrosine 291 results in recruitment of the E3 ligase Cbl-b, which, together with c-Cbl, carries out WASp ubiquitylation. Lysine residues 76 and 81, located at the WASp WH1 domain, which contains the vast majority of WASp gene mutations, serve as the ubiquitylation sites. Disruption of WASp ubiquitylation causes WASp accumulation and alters actin dynamics and the formation of actin-dependent structures. Our data suggest that regulated degradation of activated WASp might be an efficient strategy by which the duration and localization of actin rearrangement and the intensity of T-cell activation are controlled.

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.

WIP is a negative regulator of neuronal maturation and synaptic activity

Wiskott-Aldrich syndrome protein (WASP) -interacting protein (WIP) is an actin-binding protein involved in the regulation of actin polymerization in cells, such as fibroblasts and lymphocytes. Despite its recognized function in non-neuronal cells, the role of WIP in the central nervous system has not been examined previously. We used WIP-deficient mice to examine WIP function both in vivo and in vitro. We report here that WIP(-)(/-) hippocampal neurons exhibit enlargement of somas as well as overgrowth of neuritic and dendritic branches that are more evident in early developmental stages. Dendritic arborization and synaptogenesis, which includes generation of postsynaptic dendritic spines, are actin-dependent processes that occur in parallel at later stages. WIP deficiency also increases the amplitude and frequency of miniature excitatory postsynaptic currents, suggesting that WIP(-)(/-) neurons have more mature synapses than wild-type neurons. These findings reveal WIP as a previously unreported regulator of neuronal maturation and synaptic activity.

SWAP-70-like adapter of T cells: a novel Lck-regulated guanine nucleotide exchange factor coordinating actin cytoskeleton reorganization and Ca2+ signaling in T cells

SWAP-70-like adapter of T cells (SLAT) is a recently identified guanine nucleotide exchange factor (GEF) for Cdc42 and Rac1, which is highly expressed in both thymocytes and peripheral T cells. Here, we present and discuss findings resulting from biochemical and genetic analyses aimed at unveiling the role of SLAT in CD4+ T-cell development, activation, and T-helper (Th) cell differentiation. Slat(-/-) mice display a developmental defect at one of the earliest stages of thymocyte differentiation, the double negative 1 (DN1) stage, leading to decreased peripheral T-cell numbers. Slat(-/-) peripheral CD4+ T cells demonstrate impaired T-cell receptor/CD28-induced proliferation and IL-2 production. Moreover, SLAT positively regulates the development of Th1 and Th2 inflammatory responses by controlling Ca2+/NFAT signaling. SLAT is also a positive regulator of the recently emerging Th subset, i.e., Th17 cells, as evidenced by its critical role in Th17 cell-mediated central nervous system inflammation. Furthermore, TCR engagement induces SLAT translocation to the immunological synapse, a process mediated by its Lck-dependent phosphorylation, which thereafter facilitates the triggering of SLAT GEF activity towards Cdc42 and Rac1, leading to NFAT activation and Th1/Th2 differentiation. Future work will aim to dissect the interacting partners of SLAT and may thus shed light on the poorly understood events that coordinate and link actin cytoskeleton reorganization to Ca2+ signaling and gene transcription in T cells.

Recent advances in the biology of WASP and WIP

WASP, the product of the gene mutated in Wiskott-Aldrich syndrome, is expressed only in hematopoietic cells and is the archetype of a family of proteins that include N-WASP and Scar/WAVE. WASP plays a critical role in T cell activation and actin reorganization. WASP has multiple protein-interacting domains. Through its N-terminal EVH1 domain WASP binds to its partner WASP interacting protein (WIP) and through its C-terminal end it interacts with and activates the Arp2/3 complex. In lymphocytes, most of WASP is sequestered with WIP and binding to WIP is essential for the stability of WASP. The central proline-rich region of WASP serves as docking site to several adaptor proteins. Through these multiple interactions WASP integrates many cellular signals to actin cytoskeleton remodeling. In this review, we have summarized recent developments in the biology of WASP and the role of WIP in regulating WASP function. We also discuss WASP-independent functions of WIP.

Phosphorylation of WASp is a key regulator of activity and stability in vivo

The Wiskott-Aldrich syndrome protein (WASp) is a key cytoskeletal regulator in hematopoietic cells. Covalent modification of a conserved tyrosine by phosphorylation has emerged as an important potential determinant of activity, although the physiological significance remains uncertain. In a murine knockin model, mutation resulting in inability to phosphorylate Y293 (Y293F) mimicked many features of complete WASp-deficiency. Although a phosphomimicking mutant Y293E conferred enhanced actin-polymerization, the cellular phenotype was similar due to functional dysregulation. Furthermore, steady-state levels of Y293E-WASp were markedly reduced compared to wild-type WASp and Y293F-WASp, although partially recoverable by treatment of cells with proteasome inhibitors. Consequently, tyrosine phosphorylation plays a critical role in normal activation of WASp in vivo, and is indispensible for multiple tasks including proliferation, phagocytosis, chemotaxis, and assembly of adhesion structures. Furthermore, it may target WASp for proteasome-mediated degradation, thereby providing a default mechanism for self-limiting stimulation of the Arp2/3 complex.

Wiskott-Aldrich syndrome protein is an effector of Kit signaling

The pleiotropic receptor tyrosine kinase Kit can provide cytoskeletal signals that define cell shape, positioning, and migration, but the underlying mechanisms are less well understood. In this study, we provide evidence that Kit signals through Wiskott-Aldrich syndrome protein (WASP), the central hematopoietic actin nucleation-promoting factor and regulator of the cytoskeleton. Kit ligand (KL) stimulation resulted in transient tyrosine phosphorylation of WASP, as well as interacting proteins WASP-interacting protein and Arp2/3. KL-induced filopodia in bone marrow-derived mast cells (BMMCs) were significantly decreased in number and size in the absence of WASP. KL-dependent regulation of intracellular Ca(2+) levels was aberrant in WASP-deficient BMMCs. When BMMCs were derived from WASP-heterozygous female mice using KL as a growth factor, the cultures eventually developed from a mixture of WASP-positive and -negative populations into a homogenous WASP-positive culture derived from the WASP-positive progenitors. Thus, WASP expression conferred a selective advantage to the development of Kit-dependent hematopoiesis consistent with the selective advantage of WASP-positive hematopoietic cells observed in WAS-heterozygous female humans. Finally, KL-mediated gene expression in wild-type and WASP-deficient BMMCs was compared and revealed that approximately 30% of all Kit-induced changes were WASP dependent. The results indicate that Kit signaling through WASP is necessary for normal Kit-mediated filopodia formation, cell survival, and gene expression, and provide new insight into the mechanism in which WASP exerts a strong selective pressure in hematopoiesis.

Diversity of polyproline recognition by EVH1 domains

Enabled/VASP Homology-1 (EVH1) domains function primarily as interaction modules that link signaling proteins by binding to proline-rich sequences. EVH1 domains are ~115 residues in length and adopt the pleckstrin homology (PH) fold. Four different protein families contain EVH1 domains: Ena/VASP, Homer, WASP and SPRED. Except for the SPRED domains, for which no binding partners are known, EVH1 domains use a conserved hydrophobic cleft to bind a four-residue motif containing 2-4 prolines. Conserved aromatic residues, including an invariant tryptophan, create a wedge-shaped groove on the EVH1 surface that matches the triangular profile of a polyproline type II helix. Hydrophobic residues adjacent to the polyproline motif dock into complementary sites on the EVH1 domain to enhance ligand binding specificity. Pseudosymmetry in the polyproline type II helix allows peptide ligands to bind in either of two N-to-C terminal orientations, depending on interactions between sequences flanking the prolines and the EVH1 domain. EVH1 domains also recognize non-proline motifs, as illustrated by the structure of an EVH1:LIM3 complex and the extended EVH1 ligands of the verprolin family.

Tyrosine-phosphorylation-dependent translocation of the SLAT protein to the immunological synapse is required for NFAT transcription factor activation

SWAP-70-like adaptor of T cells (SLAT) is a guanine nucleotide exchange factor for Rho GTPases that regulates the development of T helper 1 (Th1) and Th2 cell inflammatory responses by controlling the Ca(2+)-NFAT signaling pathway. However, the mechanism used by SLAT to regulate these events is unknown. Here, we report that the T cell receptor (TCR)-induced translocation of SLAT to the immunological synapse required Lck-mediated phosphorylation of two tyrosine residues located in an immunoreceptor tyrosine-based activation motif-like sequence but was independent of the SLAT PH domain. This subcellular relocalization was coupled to, and necessary for, activation of the NFAT pathway. Furthermore, membrane targeting of the SLAT Dbl-homology (catalytic) domain was sufficient to trigger TCR-mediated NFAT activation and Th1 and Th2 differentiation in a Cdc42-dependent manner. Therefore, tyrosine-phosphorylation-mediated relocalization of SLAT to the site of antigen recognition is required for SLAT to exert its pivotal role in NFAT-dependent CD4(+) T cell differentiation.

T cell activation and the cytoskeleton: you can't have one without the other

More than a quarter of a century has passed since the observation that T cells rapidly polarize their actin and microtubule cytoskeletal systems toward antigen-presenting cells during activation. Since this initial discovery, several receptors on T cells (e.g., T cell receptor [TCR], co-receptors, integrins, and chemokine receptors) have been identified to regulate these two cytoskeletal networks through complex signaling pathways, which are still being elucidated. There is now an undeniable body of biochemical, pharmacological, and genetic evidence indicating that regulators of actin and microtubule dynamics are crucial for T cell activation and effector functions. In fact, the actin cytoskeleton participates in the initial clustering of TCR-major histocompatibility complex or peptide complexes, formation and stabilization of the immune synapse, integrin-mediated adhesion, and receptor sequestration, whereas both the actin and microtubule cytoskeletons regulate the establishment of cell polarity, cell migration, and directed secretion of cytokines and cytolytic granules. Over the past several years, we have begun to more thoroughly understand the contributions of specific actin-regulatory and actin-nucleating proteins that govern these processes. Herein, we discuss our current understanding of how activating receptors on T lymphocytes regulate the actin and microtubule cytoskeletons, and how in turn, these distinct but integrated cytoskeletal networks coordinate T cell immune responses.