Allosteric inhibition of the nonMyristoylated c-Abl tyrosine kinase by phosphopeptides derived from Abi1/Hssh3bp1

Proximity proteomics reveals role of Abelson interactor 1 in the regulation of TAK1/RIPK1 signaling

Dysregulation of the adaptor protein Abelson interactor 1 (ABI1) is linked to malignant transformation. To interrogate the role of ABI1 in cancer development, we mapped the ABI1 interactome using proximity-dependent labeling (PDL) with biotin followed by mass spectrometry. Using a novel PDL data filtering strategy, considering both peptide spectral matches and peak areas of detected peptides, we identified 212 ABI1 proximal interactors. These included WAVE2 complex components such as CYFIP1, NCKAP1, or WASF1, confirming the known role of ABI1 in the regulation of actin-polymerization-dependent processes. We also identified proteins associated with the TAK1-IKK pathway, including TAK1, TAB2, and RIPK1, denoting a newly identified function of ABI1 in TAK1-NF-κB inflammatory signaling. Functional assays using TNFα-stimulated, ABI1-overexpressing or ABI1-deficient cells showed effects on the TAK1-NF-kB pathway-dependent signaling to RIPK1, with ABI1-knockout cells being less susceptible to TNFα-induced, RIPK1-mediated, TAK1-dependent apoptosis. In sum, our PDL-based strategy enabled mapping of the ABI1 proximal interactome, thus revealing a previously unknown role of this adaptor protein in TAK1/RIPK1-based regulation of cell death and survival.

Abl kinases can function as suppressors of tumor progression and metastasis

Introduction

Abl family kinases function as proto-oncogenes in various leukemias, and pro-tumor functions have been discovered for Abl kinases in many solid tumors as well. However, a growing body of evidence indicates that Abl kinases can function to suppress tumor cell proliferation and motility and tumor growth in vivo in some settings.

Methods

To investigate the role of Abl kinases in tumor progression, we used RNAi to generate Abl-deficient cells in a model of androgen receptor-indifferent, metastatic prostate cancer. The effect of Abl kinase depletion on tumor progression and metastasis was studied in an in vivo orthotopic model, and tumor cell motility, 3D growth, and signaling was studied in vitro.

Results

Reduced Abl family kinase expression resulted in a highly aggressive, metastatic phenotype in vivo that was associated with AKT pathway activation, increased growth on 3D collagen matrix, and enhanced cell motility in vitro. Inhibiting AKT pathway signaling abolished the increased 3D growth of Abl-deficient cells, while treatment with the Abl kinase inhibitor, imatinib, promoted 3D growth of multiple additional tumor cell types. Moreover, Abl kinase inhibition also promoted soft-agar colony formation by pre-malignant fibroblasts.

Conclusions

Collectively, our data reveal that Abl family kinases can function to suppress malignant cell phenotypes in vitro, and tumor progression and metastasis in vivo.

Src family kinases, adaptor proteins and the actin cytoskeleton in epithelial-to-mesenchymal transition

Over a century of scientific inquiry since the discovery of v-SRC but still no final judgement on SRC function. However, a significant body of work has defined Src family kinases as key players in tumor progression, invasion and metastasis in human cancer. With the ever-growing evidence supporting the role of epithelial-mesenchymal transition (EMT) in invasion and metastasis, so does our understanding of the role SFKs play in mediating these processes. Here we describe some key mechanisms through which Src family kinases play critical role in epithelial homeostasis and how their function is essential for the propagation of invasive signals. Video abstract.

PTEN dephosphorylates Abi1 to promote epithelial morphogenesis

The tumor suppressor PTEN is essential for early development. Its lipid phosphatase activity converts PIP₃ to PIP₂ and antagonizes the PI3K–Akt pathway. In this study, we demonstrate that PTEN’s protein phosphatase activity is required for epiblast epithelial differentiation and polarization. This is accomplished by reconstitution of PTEN-null embryoid bodies with PTEN mutants that lack only PTEN’s lipid phosphatase activity or both PTEN’s lipid and protein phosphatase activities. Phosphotyrosine antibody immunoprecipitation and mass spectrometry were used to identify Abi1, a core component of the WASP-family verprolin homologous protein (WAVE) regulatory complex (WRC), as a new PTEN substrate. We demonstrate that PTEN dephosphorylation of Abi1 at Y213 and S216 results in Abi1 degradation through the calpain pathway. This leads to down-regulation of the WRC and reorganization of the actin cytoskeleton. The latter is critical to the transformation of nonpolar pluripotent stem cells into the polarized epiblast epithelium. Our findings establish a link between PTEN and WAVE-Arp2/3–regulated actin cytoskeletal dynamics in epithelial morphogenesis.

PTEN suppresses epithelial-mesenchymal transition and cancer stem cell activity by downregulating Abi1

The epithelial–mesenchymal transition (EMT) is an embryonic program frequently reactivated during cancer progression and is implicated in cancer invasion and metastasis. Cancer cells can also acquire stem cell properties to self-renew and give rise to new tumors through the EMT. Inactivation of the tumor suppressor PTEN has been shown to induce the EMT, but the underlying molecular mechanisms are less understood. In this study, we reconstituted PTEN-deficient breast cancer cells with wild-type and mutant PTEN, demonstrating that restoration of PTEN expression converted cancer cells with mesenchymal traits to an epithelial phenotype and inhibited cancer stem cell (CSC) activity. The protein rather than the lipid phosphatase activity of PTEN accounts for the reversal of the EMT. PTEN dephosphorylates and downregulates Abi1 in breast cancer cells. Gain- and loss-of-function analysis indicates that upregulation of Abi1 mediates PTEN loss-induced EMT and CSC activity. These results suggest that PTEN may suppress breast cancer invasion and metastasis via dephosphorylating and downregulating Abi1.

Abi1 loss drives prostate tumorigenesis through activation of EMT and non-canonical WNT signaling

Background

Prostate cancer development involves various mechanisms, which are poorly understood but pointing to epithelial mesenchymal transition (EMT) as the key mechanism in progression to metastatic disease. ABI1, a member of WAVE complex and actin cytoskeleton regulator and adaptor protein, acts as tumor suppressor in prostate cancer but the role of ABI1 in EMT is not clear.

Methods

To investigate the molecular mechanism by which loss of ABI1 contributes to tumor progression, we disrupted the ABI1 gene in the benign prostate epithelial RWPE-1 cell line and determined its phenotype. Levels of ABI1 expression in prostate organoid tumor cell lines was evaluated by Western blotting and RNA sequencing. ABI1 expression and its association with prostate tumor grade was evaluated in a TMA cohort of 505 patients and metastatic cell lines.

Results

Low ABI1 expression is associated with biochemical recurrence, metastasis and death (p = 0.038). Moreover, ABI1 expression was significantly decreased in Gleason pattern 5 vs. pattern 4 (p = 0.0025) and 3 (p = 0.0012), indicating an association between low ABI1 expression and highly invasive prostate tumors. Disruption of ABI1 gene in RWPE-1 cell line resulted in gain of an invasive phenotype, which was characterized by a loss of cell-cell adhesion markers and increased migratory ability of RWPE-1 spheroids. Through RNA sequencing and protein expression analysis, we discovered that ABI1 loss leads to activation of non-canonical WNT signaling and EMT pathways, which are rescued by re-expression of ABI1. Furthermore, an increase in STAT3 phosphorylation upon ABI1 inactivation and the evidence of a high-affinity interaction between the FYN SH2 domain and ABI1 pY421 support a model in which ABI1 acts as a gatekeeper of non-canonical WNT-EMT pathway activation downstream of the FZD2 receptor.

Conclusions

ABI1 controls prostate tumor progression and epithelial plasticity through regulation of EMT-WNT pathway. Here we discovered that ABI1 inhibits EMT through suppressing FYN-STAT3 activation downstream from non-canonical WNT signaling thus providing a novel mechanism of prostate tumor suppression.

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0410-y) contains supplementary material, which is available to authorized users.

The Src module: an ancient scaffold in the evolution of cytoplasmic tyrosine kinases

Tyrosine kinases were first discovered as the protein products of viral oncogenes. We now know that this large family of metazoan enzymes includes nearly one hundred structurally diverse members. Tyrosine kinases are broadly classified into two groups: the transmembrane receptor tyrosine kinases, which sense extracellular stimuli, and the cytoplasmic tyrosine kinases, which contain modular ligand-binding domains and propagate intracellular signals. Several families of cytoplasmic tyrosine kinases have in common a core architecture, the "Src module," composed of a Src-homology 3 (SH3) domain, a Src-homology 2 (SH2) domain, and a kinase domain. Each of these families is defined by additional elaborations on this core architecture. Structural, functional, and evolutionary studies have revealed a unifying set of principles underlying the activity and regulation of tyrosine kinases built on the Src module. The discovery of these conserved properties has shaped our knowledge of the workings of protein kinases in general, and it has had important implications for our understanding of kinase dysregulation in disease and the development of effective kinase-targeted therapies.

Bone marrow-specific loss of ABI1 induces myeloproliferative neoplasm with features resembling human myelofibrosis

Although the pathogenesis of primary myelofibrosis (PMF) and other myeloproliferative neoplasms (MPNs) is linked to constitutive activation of the JAK-STAT pathway, JAK inhibitors have neither curative nor MPN-stem cell-eradicating potential, indicating that other targetable mechanisms are contributing to the pathophysiology of MPNs. We previously demonstrated that Abelson interactor 1 (Abi-1), a negative regulator of Abelson kinase 1, functions as a tumor suppressor. Here we present data showing that bone marrow-specific deletion of Abi1 in a novel mouse model leads to development of an MPN-like phenotype resembling human PMF. Abi1 loss resulted in a significant increase in the activity of the Src family kinases (SFKs), STAT3, and NF-κB signaling. We also observed impairment of hematopoietic stem cell self-renewal and fitness, as evidenced in noncompetitive and competitive bone marrow transplant experiments. CD34⁺ hematopoietic progenitors and granulocytes from patients with PMF showed decreased levels of ABI1 transcript as well as increased activity of SFKs, STAT3, and NF-κB. In aggregate, our data link the loss of Abi-1 function to hyperactive SFKs/STAT3/NF-κB signaling and suggest that this signaling axis may represent a regulatory module involved in the molecular pathophysiology of PMF.

Reciprocal regulation of Abl kinase by Crk Y251 and Abi1 controls invasive phenotypes in glioblastoma

Crk is the prototypical member of a class of Src homology 2 (SH2) and Src homology 3 (SH3) domain-containing adaptor proteins that positively regulate cell motility via the activation of Rac1 and, in certain tumor types such as GBM, can promote cell invasion and metastasis by mechanisms that are not well understood. Here we demonstrate that Crk, via its phosphorylation at Tyr251, promotes invasive behavior of tumor cells, is a prominent feature in GBM, and correlating with aggressive glioma grade IV staging and overall poor survival outcomes. At the molecular level, Tyr251 phosphorylation of Crk is negatively regulated by Abi1, which competes for Crk binding to Abl and attenuates Abl transactivation. Together, these results show that Crk and Abi1 have reciprocal biological effects and act as a molecular rheostat to control Abl activation and cell invasion. Finally, these data suggest that Crk Tyr251 phosphorylation regulate invasive cell phenotypes and may serve as a biomarker for aggressive GBM.

The NESH/Abi-3-based WAVE2 complex is functionally distinct from the Abi-1-based WAVE2 complex

BACKGROUND

Abl interactor (Abi) family proteins play significant roles in actin cytoskeleton organization through participation in the WAVE complex. Mammals possess three Abi proteins: Abi-1, Abi-2, and NESH/Abi-3. Abi-1 and Abi-2 were originally identified as Abl tyrosine kinase-binding proteins. It has been disclosed that Abi-1 acts as a bridge between c-Abl and WAVE2, and c-Abl-mediated WAVE2 phosphorylation promotes actin remodeling. We showed previously that NESH/Abi-3 is present in the WAVE2 complex, but neither binds to c-Abl nor promotes c-Abl-mediated phosphorylation of WAVE2.

RESULTS

In this study, we characterized NESH/Abi-3 in more detail, and compared its properties with those of Abi-1 and Abi-2. NESH/Abi-3 was ectopically expressed in NIH3T3 cells, in which Abi-1, but not NESH/Abi-3, is expressed. The expression of NESH/Abi-3 caused degradation of endogenous Abi-1, which led to the formation of a NESH/Abi-3-based WAVE2 complex. When these cells were plated on fibronectin-coated dishes, the translocation of WAVE2 to the plasma membrane was significantly reduced and the formation of peripheral lamellipodial structures was disturbed, suggesting that the NESH/Abi-3-based WAVE2 complex was unable to help produce lamellipodial protrusions. Next, Abi-1, Abi-2, or NESH/Abi-3 was expressed in v-src-transformed NIH3T3 cells. Only in NESH/Abi-3-expressed cells did treatment with an Abl kinase inhibitor, imatinib mesylate, or siRNA-mediated knockdown of c-Abl promote the formation of invadopodia, which are ventral membrane protrusions with extracellular matrix degradation activity. Structural studies showed that a linker region between the proline-rich regions and the Src homology 3 (SH3) domain of Abi-1 is crucial for its interaction with c-Abl and c-Abl-mediated phosphorylation of WAVE2.

CONCLUSIONS

The NESH/Abi-3-based WAVE2 complex is functionally distinct from the Abi-1-based one, and NESH/Abi-3 may be involved in the formation of ventral protrusions under certain conditions.

Structure and dynamic regulation of Abl kinases

The c-abl proto-oncogene encodes a unique protein-tyrosine kinase (Abl) distinct from c-Src, c-Fes, and other cytoplasmic tyrosine kinases. In normal cells, Abl plays prominent roles in cellular responses to genotoxic stress as well as in the regulation of the actin cytoskeleton. Abl is also well known in the context of Bcr-Abl, the oncogenic fusion protein characteristic of chronic myelogenous leukemia. Selective inhibitors of Bcr-Abl, of which imatinib is the prototype, have had a tremendous impact on clinical outcomes in chronic myelogenous leukemia and revolutionized the field of targeted cancer therapy. In this minireview, we focus on the structural organization and dynamics of Abl kinases and how these features influence inhibitor sensitivity.

Crk and ABI1: binary molecular switches that regulate abl tyrosine kinase and signaling to the cytoskeleton

The nonreceptor tyrosine kinases Abl and Arg are among the most well-characterized tyrosine kinases in the human genome. The activation of Abl by N-terminal fusions with Bcr (Bcr-Abl) or Gag (v-Abl) is responsible for chronic myeloid leukemia or Ph+ acute lymphoblastic leukemia and mouse leukemia virus, respectively. In addition, aberrant Abl and Arg activation downstream of several oncogenic growth factor receptors contributes to the development and progression of a variety of human cancers, often associated with poor clinical outcome, drug resistance, and tumor invasion and metastasis. Abl activation can occur by a variety of mechanisms that include domain interactions involving structural remodeling of autoinhibited conformations as well as direct phosphorylation by upstream kinases and phosphatases. Constitutive activation of Abl plays a significant role in regulating the actin cytoskeleton by modulating cell adhesion, motility, and invadopodia. This review addresses the role of Abl and Arg in tumor progression with particular emphasis on the roles of Crk and Abi1 adapter proteins as distinct molecular switches for Abl transactivation. These insights, combined with new insights into the structure of these kinases, provide the rationale to envision that Crk and Abi1 fine-tune Abl regulation to control signaling to the cytoskeleton.

Disruption of Abi1/Hssh3bp1 expression induces prostatic intraepithelial neoplasia in the conditional Abi1/Hssh3bp1 KO mice

Prostate cancer is one of the leading causes of cancer-related deaths in the United States and a leading diagnosed non-skin cancer in American men. Genetic mutations underlying prostate tumorigenesis include alterations of tumor suppressor genes. We tested the tumor suppressor hypothesis for ABI1/hSSH3BP1 by searching for gene mutations in primary prostate tumors from patients, and by analyzing the consequences of prostate-specific disruption of the mouse Abi1/Hssh3bp1 ortholog. We sequenced the ABI1/hSSH3BP1 gene and identified recurring mutations in 6 out of 35 prostate tumors. Moreover, complementation and anchorage-independent growth, proliferation, cellular adhesion and xenograft assays using the LNCaP cell line, which contains a loss-of-function Abi1 mutation, and a stably expressed wild-type or mutated ABI gene, were consistent with the tumor suppressor hypothesis. To test the hypothesis further, we disrupted the gene in the mouse prostate by breeding the Abi1 floxed strain with the probasin promoter-driven Cre recombinase strain. Histopathological evaluation of mice indicated development of prostatic intraepithelial neoplasia (PIN) in Abi1/Hssh3bp1 knockout mouse as early as the eighth month, but no progression beyond PIN was observed in mice as old as 12 months. Observed decreased levels of E-cadherin, β-catenin and WAVE2 in mouse prostate suggest abnormal cellular adhesion as the mechanism underlying PIN development owing to Abi1 disruption. Analysis of syngeneic cell lines point to the possibility that upregulation of phospho-Akt underlies the enhanced cellular proliferation phenotype of cells lacking Abi1. This study provides proof-of-concept for the hypothesis that Abi1 downregulation has a role in the development of prostate cancer.

SH3 domains: modules of protein-protein interactions

Src homology 3 (SH3) domains are involved in the regulation of important cellular pathways, such as cell proliferation, migration and cytoskeletal modifications. Recognition of polyproline and a number of noncanonical sequences by SH3 domains has been extensively studied by crystallography, nuclear magnetic resonance and other methods. High-affinity peptides that bind SH3 domains are used in drug development as candidates for anticancer treatment. This review summarizes the latest achievements in deciphering structural determinants of SH3 function.

Abi1, a critical molecule coordinating actin cytoskeleton reorganization with PI-3 kinase and growth signaling

Coordination of actin cytoskeletal reorganization with growth and proliferation signals is a key cellular process that is not fully understood. PI-3 kinase is one of the central nodes for distributing growth and proliferation signals downstream from growth factor receptors to the nucleus. Although PI-3 kinase function has been associated with actin cytoskeleton remodeling, satisfactory explanations of the mechanisms mediating this regulation have been elusive. Here we propose that interaction of the Abi1 protein with the p85 regulatory subunit of PI-3 kinase represents the link between growth receptor signaling and actin cytoskeleton remodeling. This function of Abi1, which involves WAVE complex, was initially observed in macropinocytosis, and may explain the coincident dysregulation of PI-3 kinase and actin cytoskeleton in cancer.

Functional mechanisms and roles of adaptor proteins in abl-regulated cytoskeletal actin dynamics

Abl is a nonreceptor tyrosine kinase and plays an essential role in the modeling and remodeling of F-actin by transducing extracellular signals. Abl and its paralog, Arg, are unique among the tyrosine kinase family in that they contain an unusual extended C-terminal half consisting of multiple functional domains. This structural characteristic may underlie the role of Abl as a mediator of upstream signals to downstream signaling machineries involved in actin dynamics. Indeed, a group of SH3-containing accessory proteins, or adaptor proteins, have been identified that bind to a proline-rich domain of the C-terminal portion of Abl and modulate its kinase activity, substrate recognition, and intracellular localization. Moreover, the existence of signaling cascade and biological outcomes unique to each adaptor protein has been demonstrated. In this paper, we summarize functional roles and mechanisms of adaptor proteins in Abl-regulated actin dynamics, mainly focusing on a family of adaptor proteins, Abi. The mechanism of Abl's activation and downstream signaling mediated by Abi is described in comparison with those by another adaptor protein, Crk.

Phosphorylation of Crk on tyrosine 251 in the RT loop of the SH3C domain promotes Abl kinase transactivation

Here, we report the identification and characterization of a novel tyrosine phosphorylation site in the carboxy-terminal Src Homology 3 (SH3) (SH3C) domain of the Crk adaptor protein. Y251 is located in the highly conserved RT loop structure of the SH3C, a region of Crk involved in the allosteric regulation of the Abl kinase. Exploiting kinase assays to show that Y251 is phosphorylated by Abl in vitro, we generated affinity-purified antisera against phosphorylated Y251 in Crk and showed that Abl induces phosphorylation at Y251 in vivo, and that the kinetics of phosphorylation at Y251 and the negative regulatory Y221 site in vitro are similar. Y251 on endogenous Crk was robustly phosphorylated in chronic myelogenous leukemia cell lines and in A431 and MDA-MB-468 cells stimulated with epidermal growth factor. Using streptavidin-biotin pull downs and unbiased high-throughput Src Homology 2 (SH2) profiling approaches, we found that a pY251 phosphopeptide binds specifically to a subset of SH2 domains, including Abl and Arg SH2, and that binding of pY251 to Abl SH2 induces transactivation of Abl 1b. Finally, the Y251F Crk mutant significantly abrogates Abl transactivation in vitro and in vivo. These studies point to a yet unrealized positive regulatory role resulting from tyrosine phosphorylation of Crk, and identify a novel mechanism by which an adaptor protein activates a non-receptor tyrosine kinase by SH2 domain displacement.

Essential role for Abi1 in embryonic survival and WAVE2 complex integrity

Abl interactor 1 (Abi1) plays a critical function in actin cytoskeleton dynamics through participation in the WAVE2 complex. To gain a better understanding of the specific role of Abi1, we generated a conditional Abi1-KO mouse model and MEFs lacking Abi1 expression. Abi1-KO cells displayed defective regulation of the actin cytoskeleton, and this dysregulation was ascribed to altered activity of the WAVE2 complex. Changes in motility of Abi1-KO cells were manifested by a decreased migration rate and distance but increased directional persistence. Although these phenotypes did not correlate with peripheral ruffling, which was unaffected, Abi1-KO cells exhibited decreased dorsal ruffling. Western blotting analysis of Abi1-KO cell lysates indicated reduced levels of the WAVE complex components WAVE1 and WAVE2, Nap1, and Sra-1/PIR121. Although relative Abi2 levels were more than doubled in Abi1-KO cells, the absolute Abi2 expression in these cells amounted only to a fifth of Abi1 levels in the control cell line. This finding suggests that the presence of Abi1 is critical for the integrity and stability of WAVE complex and that Abi2 levels are not sufficiently increased to compensate fully for the loss of Abi1 in KO cells and to restore the integrity and function of the WAVE complex. The essential function of Abi1 in WAVE complexes and their regulation might explain the observed embryonic lethality of Abi1-deficient embryos, which survived until approximately embryonic day 11.5 and displayed malformations in the developing heart and brain. Cells lacking Abi1 and the conditional Abi1-KO mouse will serve as critical models for defining Abi1 function.

Identification and functional analysis of a new phosphorylation site (Y398) in the SH3 domain of Abi-1

Abi-1 is an adaptor protein for Abelson kinase (c-Abl), and Abi-1 promotes the Abl-mediated phosphorylation of Mammalian Enabled (Mena) by binding both c-Abl and Mena. Here, we identified a new phosphorylation site (Y398) in the SH3 domain of Abi-1, and disruption of Y398, combined with the previously identified phosphorylation site Y213, significantly weakens the binding of Abi-1 to c-Abl. The SH3 domain of Abi-1 and the proline-rich domain of c-Abl are involved in this interaction. Abi-1 phosphorylation at both sites stimulates the phosphorylation of Mena through the activation of c-Abl kinase. The phosphorylation of Abi-1 also plays a role in enhancing the adhesion of Bcr-Abl-transformed leukemic cells.

ABL tyrosine kinases: evolution of function, regulation, and specificity

ABL-family proteins comprise one of the best conserved branches of the tyrosine kinases. Each ABL protein contains an SH3-SH2-TK (Src homology 3-Src homology 2-tyrosine kinase) domain cassette, which confers autoregulated kinase activity and is common among nonreceptor tyrosine kinases. This cassette is coupled to an actin-binding and -bundling domain, which makes ABL proteins capable of connecting phosphoregulation with actin-filament reorganization. Two vertebrate paralogs, ABL1 and ABL2, have evolved to perform specialized functions. ABL1 includes nuclear localization signals and a DNA binding domain through which it mediates DNA damage-repair functions, whereas ABL2 has additional binding capacity for actin and for microtubules to enhance its cytoskeletal remodeling functions. Several types of posttranslational modifications control ABL catalytic activity, subcellular localization, and stability, with consequences for both cytoplasmic and nuclear ABL functions. Binding partners provide additional regulation of ABL catalytic activity, substrate specificity, and downstream signaling. Information on ABL regulatory mechanisms is being mined to provide new therapeutic strategies against hematopoietic malignancies caused by BCR-ABL1 and related leukemogenic proteins.

Abi1/Hssh3bp1 pY213 links Abl kinase signaling to p85 regulatory subunit of PI-3 kinase in regulation of macropinocytosis in LNCaP cells

Macropinocytosis is regulated by Abl kinase via an unknown mechanism. We previously demonstrated that Abl kinase activity is, itself, regulated by Abi1 subsequent to Abl kinase phosphorylation of Abi1 tyrosine 213 (pY213) [1]. Here we show that blocking phosphorylation of Y213 abrogated the ability of Abl to regulate macropinocytosis, implicating Abi1 pY213 as a key regulator of macropinocytosis. Results from screening the human SH2 domain library and mapping the interaction site between Abi1 and the p85 regulatory domain of PI-3 kinase, coupled with data from cells transfected with loss-of-function p85 mutants, support the hypothesis that macropinocytosis is regulated by interactions between Abi1 pY213 and the C-terminal SH2 domain of p85-thereby linking Abl kinase signaling to p85-dependent regulation of macropinocytosis.

Differential regulation of macropinocytosis by Abi1/Hssh3bp1 isoforms

Background

Macropinocytosis, which is a constitutive cellular process of fluid and macromolecule uptake, is regulated by actin cytoskeleton rearrangements near the plasma membrane. Activation of Rac1, which is proposed to act upstream of the actin polymerization regulatory Wave 2 complex, has been found to correlate with enhanced macropinocytosis. One of the components of the Wave 2 complex is Abi1. Multiple, alternatively spliced isoforms of Abi1 are expressed in mammalian cells, but the functional significance of the various isoforms is unknown.

Principal Findings

Here, using flow cytometric assay analysis for Alexa Fluor 647, we demonstrate that Abi1 isoforms 2 and 3 differentially regulate macropinocytosis. LNCaP cells expressing isoform 3 had increased macropinocytic uptake that correlated with enhanced cell spreading and higher Rac1 activation in comparison to cells expressing isoform 2. Isoform 2 expressing cells had decreased macropinocytic uptake, but demonstrated greater sensitivity to Rac1 activation. Moreover, more isoform 2 was localized within the cytoplasm in comparison to isoform 3, which was more associated with the plasma membrane. Activated Rac1 was found to specifically bind to a site in exon 10 of isoform 2 in vitro. Because of alternative mRNA splicing, exon 10 is absent from isoform 3, precluding similar binding of activated Rac1. Both isoforms, however, bound to inactive Rac1 through the same non-exon 10 site. Thus, Abi1 isoform 3-containing Wave 2 complex exhibited a differential binding to activated vs. inactive Rac1, whereas isoform 2-containing Wave 2 complex bound activated or inactive Rac1 comparably.

Conclusion

Based on these observations, we postulate that Abi1 isoforms differentially regulate macropinocytosis as a consequence of their different relative affinities for activated Rac1 in Wave 2 complex. These findings also raise the possibility that isoform-specific roles occur in other Abi1 functions.

Grading the commercial optical biosensor literature-Class of 2008: 'The Mighty Binders'

Optical biosensor technology continues to be the method of choice for label-free, real-time interaction analysis. But when it comes to improving the quality of the biosensor literature, education should be fundamental. Of the 1413 articles published in 2008, less than 30% would pass the requirements for high-school chemistry. To teach by example, we spotlight 10 papers that illustrate how to implement the technology properly. Then we grade every paper published in 2008 on a scale from A to F and outline what features make a biosensor article fabulous, middling or abysmal. To help improve the quality of published data, we focus on a few experimental, analysis and presentation mistakes that are alarmingly common. With the literature as a guide, we want to ensure that no user is left behind.

Regulation of cell migration and morphogenesis by Abl-family kinases: emerging mechanisms and physiological contexts

The Abl-family non-receptor tyrosine kinases are essential regulators of the cytoskeleton. They transduce diverse extracellular cues into cytoskeletal rearrangements that have dramatic effects on cell motility and morphogenesis. Recent biochemical and genetic studies have revealed several mechanisms that Abl-family kinases use to mediate these effects. Abl-family kinases stimulate actin polymerization through the activation of cortactin, hematopoietic lineage cell-specific protein (HS1), WASp- and WAVE-family proteins, and Rac1. They also attenuate cell contractility by inhibiting RhoA and altering adhesion dynamics. These pathways impinge on several physiological processes, including development and maintenance of the nervous and immune systems, and epithelial morphogenesis. Elucidating how Abl-family kinases are regulated, and where and when they coordinate cytoskeletal changes, is essential for garnering a better understanding of these complex processes.

Enhancement of ABL kinase catalytic efficiency by a direct binding regulator is independent of other regulatory mechanisms

ABL family tyrosine kinases are tightly regulated by autoinhibition and phosphorylation mechanisms. These kinases maintain an inactive conformation through intramolecular interactions involving SH3 and SH2 domains. RIN1, a downstream effector of RAS, binds to the ABL SH3 and SH2 domains and stimulates ABL tyrosine kinase activity. RIN1 binding to the ABL2 kinase resulted in a large decrease in Km and a small increase in Vmax toward an ABL consensus substrate peptide. The enzyme efficiency (k(cat)/Km) was increased more than 5-fold by RIN1. In addition, RIN1 strongly enhanced ABL-mediated phosphorylation of CRK, PSTPIP1, and DOK1, all established ABL substrates but with unique protein structures and distinct target sequences. Importantly RIN1-mediated stimulation of ABL kinase activity was independent of activation by SRC-mediated phosphorylation. RIN1 increased the kinase activity of both ABL1 and ABL2, and this occurred in the presence or absence of ABL regulatory domains outside the SH3-SH2-tyrosine kinase domain core. We further demonstrate that a catalytic site mutation associated with broad drug resistance, ABL1T315I, remains responsive to stimulation by RIN1. These findings are consistent with an allosteric kinase activation mechanism by which RIN1 binding promotes a more accessible ABL catalytic site through relief of autoinhibition. Direct disruption of RIN1 binding may therefore be a useful strategy to suppress the activity of normal and oncogenic ABL, including inhibitor-resistant mutants that confound current therapeutic strategies. Stimulation through derepression may be applicable to many other tyrosine kinases autoinhibited by coupled SH3 and SH2 domains.

Tyrosine phosphorylation in the SH3 domain disrupts negative regulatory interactions within the c-Abl kinase core

Recent studies have shown that trans-phosphorylation of the Abl SH3 domain at Tyr89 by Src-family kinases is required for the full transforming activity of Bcr-Abl. Tyr89 localizes to a binding surface of the SH3 domain that engages the SH2-kinase linker in the crystal structure of the c-Abl core. Displacement of SH3 from the linker is likely to influence efficient downregulation of c-Abl. Hydrogen-deuterium exchange (HX) and mass spectrometry (MS) were used to investigate whether Tyr89 phosphorylation affects the ability of the SH3 domain to interact intramolecularly with the SH2-kinase linker in cis as well as other peptide ligands in trans. HX MS analysis of SH3 binding showed that when various Abl constructs were phosphorylated at Tyr89 by the Src-family kinase Hck, SH3 was unable to engage a high-affinity ligand in trans and that interaction with the linker in cis was reduced dramatically in a construct containing the SH3 and SH2 domains plus the linker. Phosphorylation of the Abl SH3 domain on Tyr89 also interfered with binding to the negative regulatory protein Abi-1 in trans. Site-directed mutagenesis of Tyr89 and Tyr245, another tyrosine phosphorylation site located in the linker that may also influence SH3 binding, implicated Tyr89 as the key residue necessary for disrupting regulation after phosphorylation. These results imply that phosphorylation at Tyr89 by Src-family kinases prevents engagement of the Abl SH3 domain with its intramolecular binding partner leading to enhanced Abl kinase activity and cellular signaling.