Individual Cas phosphorylation sites are dispensable for processive phosphorylation by Src and anchorage-independent cell growth

Effects of heterologous kinase domains on growth factor receptor specificity

The kinase domains of receptor tyrosine kinases (RTKs) are highly conserved, yet they are able to discriminate among potential substrates to selectively activate downstream signaling pathways. In this study, we tested the importance of catalytic domain specificity by creating two series of chimeric RTKs. In one set, the kinase domain of insulin-like growth factor I receptor (IGF1R) was replaced by the kinase domains from insulin receptor (IR), macrophage stimulating protein 1 receptor/Ron (Ron) or Src. In the other set of chimeras, the kinase domain of epidermal growth factor receptor (EGFR) was similarly replaced by the kinase domains of IR, Ron, or Src. We expressed the wild-type and chimeric forms of the receptors in mammalian cells. For some signaling events, such as recognition of IRS1, the identity of the tyrosine kinase catalytic domain did not appear to be crucial. In contrast, recognition of some sites, such as the C-terminal autophosphorylation sites on EGFR, did depend on the identity of the kinase domain. Our data also showed that ligand dependence was lost when the native kinase domains were replaced by Src, suggesting that the identity of the kinase domains could be important for proper receptor regulation. Overall, the results are consistent with the idea that the fidelity of RTK signaling depends on co-localization and targeting with substrates, as well as on the intrinsic specificity of the kinase domain.

ACK1 and BRK non-receptor tyrosine kinase deficiencies are associated with familial systemic lupus and involved in efferocytosis

Systemic Lupus Erythematosus (SLE) is an autoimmune disease, the pathophysiology and genetic basis of which are incompletely understood. Using a forward genetic screen in multiplex families with systemic lupus erythematosus (SLE) we identified an association between SLE and compound heterozygous deleterious variants in the non-receptor tyrosine kinases (NRTKs) ACK1 and BRK. Experimental blockade of ACK1 or BRK increased circulating autoantibodies in vivo in mice and exacerbated glomerular IgG deposits in an SLE mouse model. Mechanistically, non-receptor tyrosine kinases (NRTKs) regulate activation, migration, and proliferation of immune cells. We found that the patients' ACK1 and BRK variants impair efferocytosis, the MERTK-mediated anti-inflammatory response to apoptotic cells, in human induced Pluripotent Stem Cell (hiPSC)-derived macrophages, which may contribute to SLE pathogenesis. Overall, our data suggest that ACK1 and BRK deficiencies are associated with human SLE and impair efferocytosis in macrophages.

Independent effects of Src kinase and podoplanin on anchorage independent cell growth and migration

The Src tyrosine kinase is a strong tumor promotor. Over a century of research has elucidated fundamental mechanisms that drive its oncogenic potential. Src phosphorylates effector proteins to promote hallmarks of tumor progression. For example, Src associates with the Cas focal adhesion adaptor protein to promote anchorage independent cell growth. In addition, Src phosphorylates Cas to induce Pdpn expression to promote cell migration. Pdpn is a transmembrane receptor that can independently increase cell migration in the absence of oncogenic Src kinase activity. However, to our knowledge, effects of Src kinase activity on anchorage independent cell growth and migration have not been examined in the absence of Pdpn expression. Here, we analyzed the effects of an inducible Src kinase construct in knockout cells with and without exogenous Pdpn expression on cell morphology migration and anchorage independent growth. We report that Src promoted anchorage independent cell growth in the absence of Pdpn expression. In contrast, Src was not able to promote cell migration in the absence of Pdpn expression. In addition, continued Src kinase activity was required for cells to assume a transformed morphology since cells reverted to a nontransformed morphology upon cessation of Src kinase activity. We also used phosphoproteomic analysis to identify 28 proteins that are phosphorylated in Src transformed cells in a Pdpn dependent manner. Taken together, these data indicate that Src utilizes Pdpn to promote transformed cell growth and motility in complementary, but parallel, as opposed to serial, pathways.

PTPN12/PTP-PEST Regulates Phosphorylation-Dependent Ubiquitination and Stability of Focal Adhesion Substrates in Invasive Glioblastoma Cells

Glioblastoma (GBM) is an invasive brain cancer with tumor cells that disperse from the primary mass, escaping surgical resection and invariably giving rise to lethal recurrent lesions. Here we report that PTP-PEST, a cytoplasmic protein tyrosine phosphatase, controls GBM cell invasion by physically bridging the focal adhesion protein Crk-associated substrate (Cas) to valosin-containing protein (Vcp), an ATP-dependent protein segregase that selectively extracts ubiquitinated proteins from multiprotein complexes and targets them for degradation via the ubiquitin proteasome system. Both Cas and Vcp are substrates for PTP-PEST, with the phosphorylation status of tyrosine 805 (Y805) in Vcp impacting affinity for Cas in focal adhesions and controlling ubiquitination levels and protein stability. Perturbing PTP-PEST-mediated phosphorylation of Cas and Vcp led to alterations in GBM cell-invasive growth in vitro and in preclinical mouse models. Collectively, these data reveal a novel regulatory mechanism involving PTP-PEST, Vcp, and Cas that dynamically balances phosphorylation-dependent ubiquitination of key focal proteins involved in GBM cell invasion.Significance: PTP-PEST balances GBM cell growth and invasion by interacting with the ATP-dependent ubiquitin segregase Vcp/p97 and regulating phosphorylation and stability of the focal adhesion protein p130Cas.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/78/14/3809/F1.large.jpg Cancer Res; 78(14); 3809-22. ©2018 AACR.

The miR-24-3p/p130Cas: a novel axis regulating the migration and invasion of cancer cells

MicroRNAs (miRNAs) are small non-coding RNAs that negatively regulate gene expression by suppressing translation or facilitating mRNA decay. Differential expression of miRNAs is involved in the pathogenesis of several diseases including cancer. Here, we investigated the role of-miR-24-3p as a downregulated miRNA in metastatic cancer. miR-24-3p was decreased in metastatic cancer and lower expression of miR-24-3p was related to poor survival of cancer patients. Consistently, ectopic expression of miR-24-3p suppressed the cell migration, invasion, and proliferation of MCF7, Hep3B, B16F10, SK-Hep1, and PC-3 cells by directly targeting p130Cas. Stable expression of p130Cas restored miR-24-3p-mediated inhibition of cell migration and invasion. These results suggest that miR-24-3p functions as a tumor suppressor and the miR-24-3p/p130Cas axis is a novel factor of cancer progression by regulating cell migration and invasion.

A pre-metazoan origin of the CRK gene family and co-opted signaling network

CRK and CRKL adapter proteins play essential roles in development and cancer through their SRC homology 2 and 3 (SH2 and SH3) domains. To gain insight into the origin of their shared functions, we have investigated their evolutionary history. We propose a term, crk/crkl ancestral (crka), for orthologs in invertebrates before the divergence of CRK and CRKL in the vertebrate ancestor. We have isolated two orthologs expressed in the choanoflagellate Monosiga brevicollis, a unicellular relative to the metazoans. Consistent with its highly-conserved three-dimensional structure, the SH2 domain of M. brevicollis crka1 can bind to the mammalian CRK/CRKL SH2 binding consensus phospho-YxxP, and to the SRC substrate/focal adhesion protein BCAR1 (p130^CAS) in the presence of activated SRC. These results demonstrate an ancient origin of the CRK/CRKL SH2-target recognition specificity. Although BCAR1 orthologs exist only in metazoans as identified by an N-terminal SH3 domain, YxxP motifs, and a C-terminal FAT-like domain, some pre-metazoan transmembrane proteins include several YxxP repeats in their cytosolic region, suggesting that they are remotely related to the BCAR1 substrate domain. Since the tyrosine kinase SRC also has a pre-metazoan origin, co-option of BCAR1-related sequences may have rewired the crka-dependent network to mediate adhesion signals in the metazoan ancestor.

The ubiquitin-proteasome system regulates focal adhesions at the leading edge of migrating cells

Cell migration requires the cyclical assembly and disassembly of focal adhesions. Adhesion induces phosphorylation of focal adhesion proteins, including Cas (Crk-associated substrate/p130Cas/BCAR1). However, Cas phosphorylation stimulates adhesion turnover. This raises the question of how adhesion assembly occurs against opposition from phospho-Cas. Here we show that suppressor of cytokine signaling 6 (SOCS6) and Cullin 5, two components of the CRL5^SOCS6 ubiquitin ligase, inhibit Cas-dependent focal adhesion turnover at the front but not rear of migrating epithelial cells. The front focal adhesions contain phospho-Cas which recruits SOCS6. If SOCS6 cannot access focal adhesions, or if cullins or the proteasome are inhibited, adhesion disassembly is stimulated. This suggests that the localized targeting of phospho-Cas within adhesions by CRL5^SOCS6 and concurrent cullin and proteasome activity provide a negative feedback loop, ensuring that adhesion assembly predominates over disassembly at the leading edge. By this mechanism, ubiquitination provides a new level of spatio-temporal control over cell migration.

DOI:http://dx.doi.org/10.7554/eLife.17440.001

Animal cells can move in the body, for example to heal a wound, by protruding a leading edge forwards, attaching it to the surroundings and then pulling against these new attachments while disassembling the older ones. Mechanical forces regulate the assembly and disassembly of these attachments, known as focal adhesions, and so do signals from outside the cell that are transmitted to the adhesions via specialized proteins. However, it was not clear how the assembly and disassembly of adhesions is coordinated.

CRL5 is a ubiquitin ligase, an enzyme that can mark other proteins for destruction. Cells migrate more quickly if CRL5 is inhibited, and so Teckchandani and Cooper set out to uncover whether CRL5 affects the assembly and disassembly of focal adhesions. The experiments showed that human cells lacking a crucial component of the CRL5 complex, SOCS6, disassemble adhesions faster than normal cells, but only at their leading edge and not at the rear.

Teckchandani and Cooper also found that SOCS6 localizes to the leading edge by binding to a focal adhesion protein called Cas. Shortly after the attachments assemble, the Cas protein becomes tagged with a phosphate group and then acts to promote the adhesion to disassemble. Further experiments indicated that Cas was marked by the CRL5 complex and possibly destroyed while in or very close to the leading edge adhesions, slowing their disassembly.

Together, these findings suggest that by binding Cas, SOCS6 regulates the turnover of adhesions, specifically by inhibiting disassembly and allowing adhesions to grow at the leading edge. Since SOCS6 is not present in adhesions outside of the leading edge, this may help explain how the older adhesions are disassembled. Future studies could next focus on the exact sequence of events that occur in focal adhesions after the CRL5 complex binds to Cas as the cell migrates.

DOI:http://dx.doi.org/10.7554/eLife.17440.002

Ezrin Is Associated with Disease Progression in Ovarian Carcinoma

Objective

Ezrin and p130Cas are structural proteins with an important role in signaling pathways and have been shown to promote cancer dissemination. We previously reported on overexpression of both ezrin and p130Cas in breast carcinoma effusions compared to primary carcinomas. Since ovarian and breast carcinomas share the ability to disseminate by forming malignant effusions, we sought to study the role of these molecules in ovarian carcinoma (OC).

Methods

OC cell lines were cultured in two different 3-dimensional conditions, on alginate scaffolds and as spheroids, which served as models for solid tumor and malignant effusions, respectively. shRNA was used to reduce protein expression in the cells. The malignant potential was evaluated by chemo-invasion assay, branching capacity on Matrigel and rate of proliferation. Subsequently, clinical specimens of high-grade serous carcinoma effusions, ovarian tumors and solid metastases were analyzed for ezrin and p130Cas expression.

Results

Higher ezrin expression was found in cells composing the spheroids compared to their counterparts cultured on alginate scaffold and in clinical samples of malignant effusions compared to solid tumors. In addition, reduced Ezrin expression impaired the invasion ability and the branching capacity of OC cells to a greater extent than reduced p130Cas expression. However, ezrin and p130Cas expression in effusions was unrelated to clinical outcome.

Conclusions

The 3-dimensional cell cultures were found to mimic the different tumor sites and be applicable as a model. The in vitro results concur with the clinical specimen analysis, suggesting that in OC, the role of ezrin in disease progression is more pronounced than that of p130Cas.

JNK Signaling: Regulation and Functions Based on Complex Protein-Protein Partnerships

The c-Jun N-terminal kinases (JNKs), as members of the mitogen-activated protein kinase (MAPK) family, mediate eukaryotic cell responses to a wide range of abiotic and biotic stress insults. JNKs also regulate important physiological processes, including neuronal functions, immunological actions, and embryonic development, via their impact on gene expression, cytoskeletal protein dynamics, and cell death/survival pathways. Although the JNK pathway has been under study for >20 years, its complexity is still perplexing, with multiple protein partners of JNKs underlying the diversity of actions. Here we review the current knowledge of JNK structure and isoforms as well as the partnerships of JNKs with a range of intracellular proteins. Many of these proteins are direct substrates of the JNKs. We analyzed almost 100 of these target proteins in detail within a framework of their classification based on their regulation by JNKs. Examples of these JNK substrates include a diverse assortment of nuclear transcription factors (Jun, ATF2, Myc, Elk1), cytoplasmic proteins involved in cytoskeleton regulation (DCX, Tau, WDR62) or vesicular transport (JIP1, JIP3), cell membrane receptors (BMPR2), and mitochondrial proteins (Mcl1, Bim). In addition, because upstream signaling components impact JNK activity, we critically assessed the involvement of signaling scaffolds and the roles of feedback mechanisms in the JNK pathway. Despite a clarification of many regulatory events in JNK-dependent signaling during the past decade, many other structural and mechanistic insights are just beginning to be revealed. These advances open new opportunities to understand the role of JNK signaling in diverse physiological and pathophysiological states.

PKA and CDK5 can phosphorylate specific serines on the intracellular domain of podoplanin (PDPN) to inhibit cell motility

Podoplanin (PDPN) is a transmembrane glycoprotein that promotes tumor cell migration, invasion, and cancer metastasis. In fact, PDPN expression is induced in many types of cancer. Thus, PDPN has emerged as a functionally relevant cancer biomarker and chemotherapeutic target. PDPN contains 2 intracellular serine residues that are conserved between species ranging from mouse to humans. Recent studies indicate that protein kinase A (PKA) can phosphorylate PDPN in order to inhibit cell migration. However, the number and identification of specific residues phosphorylated by PKA have not been defined. In addition, roles of other kinases that may phosphorylate PDPN to control cell migration have not been investigated. We report here that cyclin dependent kinase 5 (CDK5) can phosphorylate PDPN in addition to PKA. Moreover, results from this study indicate that PKA and CDK5 cooperate to phosphorylate PDPN on both intracellular serine residues to decrease cell motility. These results provide new insight into PDPN phosphorylation dynamics and the role of PDPN in cell motility. Understanding novel mechanisms of PDPN intracellular signaling could assist with designing novel targeted chemotherapeutic agents and procedures.

Association of the breast cancer antiestrogen resistance protein 1 (BCAR1) and BCAR3 scaffolding proteins in cell signaling and antiestrogen resistance

Most breast cancers are estrogen receptor-positive and treated with antiestrogens, but aberrant signaling networks can induce drug resistance. One of these networks involves the scaffolding protein BCAR1/p130CAS, which regulates cell growth and migration/invasion. A less investigated scaffolding protein that also confers antiestrogen resistance is the SH2 domain-containing protein BCAR3. BCAR1 and BCAR3 bind tightly to each other through their C-terminal domains, thus potentially connecting their associated signaling networks. However, recent studies using BCAR1 and BCAR3 interaction mutants concluded that association between the two proteins is not critical for many of their interrelated activities regulating breast cancer malignancy. We report that these previously used BCAR mutations fail to cause adequate loss-of-function of the complex. By using structure-based BCAR1 and BCAR3 mutants that lack the ability to interact, we show that BCAR3-induced antiestrogen resistance in MCF7 breast cancer cells critically depends on its ability to bind BCAR1. Interaction with BCAR3 increases the levels of phosphorylated BCAR1, ultimately potentiating BCAR1-dependent antiestrogen resistance. Furthermore, antiestrogen resistance in cells overexpressing BCAR1/BCAR3 correlates with increased ERK1/2 activity. Inhibiting ERK1/2 through overexpression of the regulatory protein PEA15 negates the resistance, revealing a key role for ERK1/2 in BCAR1/BCAR3-induced antiestrogen resistance. Reverse-phase protein array data show that PEA15 levels in invasive breast cancers correlate with patient survival, suggesting that PEA15 can override ERK1/2 activation by BCAR1/BCAR3 and other upstream regulators. We further uncovered that the BCAR3-related NSP3 can also promote antiestrogen resistance. Thus, strategies to disrupt BCAR1-BCAR3/NSP3 complexes and associated signaling networks could ultimately lead to new breast cancer therapies.

In vitro membrane reconstitution of the T-cell receptor proximal signaling network

T-cell receptor (TCR) phosphorylation is controlled by a complex network that includes Lck, a Src family kinase (SFK), the tyrosine phosphatase CD45, and the Lck-inhibitory kinase Csk. How these competing phosphorylation and dephosphorylation reactions are modulated to produce T-cell triggering is not fully understood. Here we reconstituted this signaling network using purified enzymes on liposomes, recapitulating the membrane environment in which they normally interact. We demonstrate that Lck's enzymatic activity can be regulated over a ~10-fold range by controlling its phosphorylation state. By varying kinase and phosphatase concentrations, we constructed phase diagrams that reveal ultrasensitivity in the transition from the quiescent to the phosphorylated state and demonstrate that coclustering TCR-Lck or detaching Csk from the membrane can trigger TCR phosphorylation. Our results provide insight into the mechanism of TCR signaling as well as other signaling pathways involving SFKs.

Identification and functional characterization of p130Cas as a substrate of protein tyrosine phosphatase nonreceptor 14

Protein tyrosine phosphatase nonreceptor type 14 (PTPN14) is frequently mutated in a variety of human cancers. However, the cell signaling pathways regulated by PTPN14 largely remain to be elucidated. Here, we identify a list of potential substrates of PTPN14 using a phospho-proteomic approach. We show that p130 Crk-associated substrate (p130Cas) is a direct substrate of PTPN14 and that PTPN14 specifically regulates p130Cas phosphorylation at tyrosine residue 128 (Y128) in colorectal cancer (CRC) cells. We engineered CRC cells homozygous for a p130Cas Y128F knock-in mutant and found that these cells exhibit significantly reduced migration and colony formation, impaired anchorage-independent growth, slower xenograft tumor growth in nude mice and have decreased phosphorylation of AKT. Furthermore, we demonstrate that SRC phosphorylates p130Cas Y128 and that CRC cell lines harboring high levels of pY128Cas are more sensitive to SRC family kinase inhibitor Dasatinib. These findings suggest that p130Cas Y128 phosphorylation may be exploited as a predictive marker for Dasatinib response in cancer patients. In aggregate, our studies reveal a novel signaling pathway that has an important role in colorectal tumorigenesis.

SRC points the way to biomarkers and chemotherapeutic targets

The role of Src in tumorigenesis has been extensively studied since the work of Peyton Rous over a hundred years ago. Src is a non-receptor tyrosine kinase that plays key roles in signaling pathways controlling tumor cell growth and migration. Src regulates the activities of numerous molecules to induce cell transformation. However, transformed cells do not always migrate and realize their tumorigenic potential. They can be normalized by surrounding nontransformed cells by a process called contact normalization. Tumor cells need to override contact normalization to become malignant or metastatic. In this review, we discuss the role of Src in cell migration and contact normalization, with emphasis on Cas and Abl pathways. This paradigm illuminates several chemotherapeutic targets and may lead to the identification of new biomarkers and the development of effective anticancer treatments.

Lack of Csk-mediated negative regulation in a unicellular SRC kinase

Phosphotyrosine-based signaling plays a vital role in cellular communication in multicellular organisms. Unexpectedly, unicellular choanoflagellates (the closest phylogenetic group to metazoans) possess numbers of tyrosine kinases that are comparable to those in complex metazoans. Here, we have characterized tyrosine kinases from the filasterean Capsaspora owczarzaki, a unicellular protist representing the sister group to choanoflagellates and metazoans. Two Src-like tyrosine kinases have been identified in C. owczarzaki (CoSrc1 and CoSrc2), both of which have the arrangement of SH3, SH2, and catalytic domains seen in mammalian Src kinases. In Capsaspora cells, CoSrc1 and CoSrc2 localize to punctate structures in filopodia that may represent primordial focal adhesions. We have cloned, expressed, and purified both enzymes. CoSrc1 and CoSrc2 are active tyrosine kinases. Mammalian Src kinases are normally regulated in a reciprocal fashion by autophosphorylation in the activation loop (which increases activity) and by Csk-mediated phosphorylation of the C-terminal tail (which inhibits activity). Similar to mammalian Src kinases, the enzymatic activities of CoSrc1 and CoSrc2 are increased by autophosphorylation in the activation loop. We have identified a Csk-like kinase (CoCsk) in the genome of C. owczarzaki. We cloned, expressed, and purified CoCsk and found that it has no measurable tyrosine kinase activity. Furthermore, CoCsk does not phosphorylate or regulate CoSrc1 or CoSrc2 in cells or in vitro, and CoSrc1 and CoSrc2 are active in Capsaspora cell lysates. Thus, the function of Csk as a negative regulator of Src family kinases appears to have arisen with the emergence of metazoans.

Silencing of p130cas in ovarian carcinoma: a novel mechanism for tumor cell death

BACKGROUND

We investigated the clinical and biological significance of p130cas, an important cell signaling molecule, in ovarian carcinoma.

METHODS

Expression of p130cas in ovarian tumors, as assessed by immunohistochemistry, was associated with tumor characteristics and patient survival. The effects of p130cas gene silencing with small interfering RNAs incorporated into neutral nanoliposomes (siRNA-DOPC), alone and in combination with docetaxel, on in vivo tumor growth and on tumor cell proliferation (proliferating cell nuclear antigen) and apoptosis (terminal deoxynucleotidyl transferase dUTP nick-end labeling) were examined in mice bearing orthotopic taxane-sensitive (HeyA8 and SKOV3ip1) or taxane-resistant (HeyA8-MDR) ovarian tumors (n = 10 per group). To determine the specific mechanisms by which p130cas gene silencing abrogates tumor growth, we measured cell viability (MTT assay), apoptosis (fluorescence-activated cell sorting), autophagy (immunoblotting, fluorescence, and transmission electron microscopy), and cell signaling (immunoblotting) in vitro. All statistical tests were two-sided.

RESULTS

Of 91 ovarian cancer specimens, 70 (76%) had high p130cas expression; and 21 (24%) had low p130cas expression. High p130cas expression was associated with advanced tumor stage (P < .001) and higher residual disease (>1 cm) following primary cytoreduction surgery (P = .007) and inversely associated with overall survival and progression-free survival (median overall survival: high p130cas expression vs low expression, 2.14 vs 9.1 years, difference = 6.96 years, 95% confidence interval = 1.69 to 9.48 years, P < .001; median progression-free survival: high p130cas expression vs low expression, 1.04 vs 2.13 years, difference = 1.09 years, 95% confidence interval = 0.47 to 2.60 years, P = .01). In mice bearing orthotopically implanted HeyA8 or SKOV3ip1 ovarian tumors, treatment with p130cas siRNA-DOPC in combination with docetaxel chemotherapy resulted in the greatest reduction in tumor growth compared with control siRNA therapy (92%-95% reduction in tumor growth; P < .001 for all). Compared with control siRNA therapy, p130cas siRNA-DOPC reduced SKOV3ip1 cell proliferation (31% reduction, P < .001) and increased apoptosis (143% increase, P < .001) in vivo. Increased tumor cell apoptosis may have persisted despite pan-caspase inhibition by the induction of autophagy and related signaling pathways.

CONCLUSIONS

Increased p130cas expression is associated with poor clinical outcome in human ovarian carcinoma, and p130cas gene silencing decreases tumor growth through stimulation of apoptotic and autophagic cell death.

Phosphorylation mechanism and structure of serine-arginine protein kinases

The splicing of mRNA requires a group of essential factors known as SR proteins, which participate in the maturation of the spliceosome. These proteins contain one or two RNA recognition motifs and a C-terminal domain rich in Arg-Ser repeats (RS domain). SR proteins are phosphorylated at numerous serines in the RS domain by the SR-specific protein kinase (SRPK) family of protein kinases. RS domain phosphorylation is necessary for entry of SR proteins into the nucleus, and may also play important roles in alternative splicing, mRNA export, and other processing events. Although SR proteins are polyphosphorylated in vivo, the mechanism underlying this complex reaction has only been recently elucidated. Human alternative splicing factor [serine/arginine-rich splicing factor 1 (SRSF1)], a prototype for the SR protein family, is regiospecifically phosphorylated by SRPK1, a post-translational modification that controls cytoplasmic-nuclear localization. SRPK1 binds SRSF1 with unusually high affinity, and rapidly modifies about 10-12 serines in the N-terminal region of the RS domain (RS1), using a mechanism that incorporates sequential, C-terminal to N-terminal phosphorylation and several processive steps. SRPK1 employs a highly dynamic feeding mechanism for RS domain phosphorylation in which the N-terminal portion of RS1 is initially bound to a docking groove in the large lobe of the kinase domain. Upon subsequent rounds of phosphorylation, this N-terminal segment translocates into the active site, and a β-strand in RNA recognition motif 2 unfolds and occupies the docking groove. These studies indicate that efficient regiospecific phosphorylation of SRSF1 is the result of a contoured binding cavity in SRPK1, a lengthy Arg-Ser repetitive segment in the RS domain, and a highly directional processing mechanism.

Intratumoral localization of aromatase and interaction between stromal and parenchymal cells in the non-small cell lung carcinoma microenvironment

Estrogens produced as a result of intratumoral aromatization has been recently shown to play important roles in proliferation of human non-small cell lung carcinomas (NSCLC), but the details have remained largely unknown. Therefore, in this study, we evaluated the possible roles of intratumoral aromatase in NSCLCs as follows: (a) evaluation of intratumoral localization of aromatase mRNA/protein in six lung adenocarcinoma cases using laser capture microdissection combined with quantitative reverse transcriptase-PCR and immunohistochemistry; (b) examination of the possible effects of isolated stromal cells from lung carcinoma tissues on aromatase mRNA transcript expression in lung carcinoma cell lines (A549 and LK87) through a coculture system; and (c) screening of cytokines derived from stromal LK001S and LK002S cells using cytokine antibody arrays and subsequent evaluation of effects of these cytokines on aromatase expression in A549 and LK87. Both aromatase mRNA and protein were mainly detected in intratumoral carcinoma cells but not in stromal cells. Aromatase expression of A549 and LK87 was upregulated in the presence of LK001S or LK002S cells. Several cytokines such as interleukin-6 (IL-6), oncostatin M, and tumor necrosis factor-alpha, all known as inducible factors of aromatase gene, were detected in conditioned media of LK001S and LK002S cells. Treatment of both oncostatin M and IL-6 induced aromatase gene expression in A549 an LK87, respectively. These results all indicated that intratumoral microenvironments, especially carcinoma-stromal cell interactions, play a pivotal role in the regulation of intratumoral estrogen synthesis through aromatase expression in human lung adenocarcinomas.

The SH2 domain protein Shep1 regulates the in vivo signaling function of the scaffolding protein Cas

The members of the p130Cas (Cas) family are important scaffolding proteins that orchestrate cell adhesion, migration and invasiveness downstream of integrin adhesion receptors and receptor tyrosine kinases by recruiting enzymes and structural molecules. Shep1, BCAR3/AND-34 and NSP1 define a recently identified family of SH2 domain-containing proteins that constitutively bind Cas proteins through a Cdc25-type nucleotide exchange factor-like domain. To gain insight into the functional interplay between Shep1 and Cas in vivo, we have inactivated the Shep1 gene in the mouse through Cre-mediated deletion of the exon encoding the SH2 domain. Analysis of Cas tyrosine phosphorylation in the brains of newborn mice, where Shep1 is highly expressed, revealed a strong decrease in Cas substrate domain phosphorylation in knockout compared to wild-type brains. Src family kinases bind to Cas via their SH3 and SH2 domains, which contributes to their activation, and phosphorylate multiple tyrosines in the Cas substrate domain. These tyrosine-phosphorylated motifs represent docking sites for the Crk adaptor, linking Cas to the downstream Rac1 and Rap1 GTPases to regulate cell adhesion and actin cytoskeleton organization. Accordingly, we detected lower Cas-Crk association and lower phosphorylation of the Src activation loop in Shep1 knockout brains compared to wild-type. Conversely, Shep1 transfection in COS cells increases Cas tyrosine phosphorylation. The SH2 domain is likely critical for the effects of Shep1 on Cas and Src signaling because the knockout mice express Shep1 fragments that lack the amino-terminal region including the SH2 domain, presumably due to aberrant translation from internal ATG codons. These fragments retain the ability to increase Cas levels in transfected cells, similar to full-length Shep1. However, they do not affect Cas phosphorylation on their own or in the presence of co-transfected full-length Shep1. They also do not show dominant negative effects on the activity of full-length Shep1 in vivo because the heterozygous mice, which express the fragments, have a normal life span. This is in contrast to the homozygous knockout mice, most of which die soon after birth. These data demonstrate that Shep1 plays a critical role in the in vivo regulation of Src activity and Cas downstream signaling through Crk, and suggest that the SH2 domain of Shep1 is critical for these effects.

Mechanism of dephosphorylation of the SR protein ASF/SF2 by protein phosphatase 1

SR proteins are essential splicing factors whose function is controlled by multi-site phosphorylation of a C-terminal domain rich in arginine-serine repeats (RS domain). The protein kinase SRPK1 has been shown to polyphosphorylate the N-terminal portion of the RS domain (RS1) of the SR protein ASF/SF2, a modification that promotes nuclear entry of this splicing factor and engagement in splicing function. Later, dephosphorylation is required for maturation of the spliceosome and other RNA processing steps. While phosphates are attached to RS1 in a sequential manner by SRPK1, little is known about how they are removed. To investigate factors that control dephosphorylation, we monitored region-specific mapping of phosphorylation sites in ASF/SF2 as a function of the protein phosphatase PP1. We showed that 10 phosphates added to the RS1 segment by SRPK1 are removed in a preferred N-to-C manner, directly opposing the C-to-N phosphorylation by SRPK1. Two N-terminal RNA recognition motifs in ASF/SF2 control access to the RS domain and guide the directional mechanism. Binding of RNA to the RNA recognition motifs protects against dephosphorylation, suggesting that engagement of the SR protein with exonic splicing enhancers can regulate phosphoryl content in the RS domain. In addition to regulation by N-terminal domains, phosphorylation of the C-terminal portion of the RS domain (RS2) by the nuclear protein kinase Clk/Sty inhibits RS1 dephosphorylation and disrupts the directional mechanism. The data indicate that both RNA-protein interactions and phosphorylation in flanking sequences induce conformations of ASF/SF2 that increase the lifetime of phosphates in the RS domain.

Cas utilizes Nck2 to activate Cdc42 and regulate cell polarization during cell migration in response to wound healing

Integrin-mediated activation of Cdc42 is essential for cell polarization, whereas the integrin adaptor protein Cas is required for cell migration during wound healing. After phosphorylation on tyrosine residues, Cas recruits the adaptor proteins Crk and Nck to execute integrin-mediated signals. However, the mechanisms leading to Cdc42 activation and its relationship with Cas, Crk and Nck have not been elucidated clearly. In the present study, we demonstrate that Cas utilizes Nck2 to activate Cdc42 and induce cell polarization in response to wounding. By contrast, Cas recruits CrkII to activate Rac1 and promote the extension of cell protrusions needed for cell motility. These results indicate that Cas utilizes Nck2 and CrkII in a coordinated set of distinct pathways leading to cell migration.

Myristoylation and membrane binding regulate c-Src stability and kinase activity

Myristoylation is critical for membrane association of Src kinases, but a role for myristate in regulating other aspects of Src biology has not been explored. In the c-Abl tyrosine kinase, myristate binds within a hydrophobic pocket at the base of the kinase domain and latches the protein into an autoinhibitory conformation. A similar pocket has been predicted to exist in c-Src, raising the possibility that Src might also be regulated by myristoylation. Here we show that in contrast to the case for c-Abl, myristoylation exerts a positive effect on c-Src kinase activity. We also demonstrate that myristoylation and membrane binding regulate c-Src ubiquitination and degradation. Nonmyristoylated c-Src exhibited reduced kinase activity but had enhanced stability compared to myristoylated c-Src. We then mutated critical residues in the predicted myristate binding pocket of c-Src. Mutation of L360 and/or E486 had no effect on c-Src membrane binding or localization. However, constructs containing a T456A mutation were partially released from the membrane, suggesting that mutagenesis could induce c-Src to undergo an artificial myristoyl switch. All of the pocket mutants exhibited decreased kinase activity. We concluded that myristoylation and the pocket residues regulate c-Src, but in a manner very different from that for c-Abl.

SRC induces podoplanin expression to promote cell migration

Nontransformed cells can force tumor cells to assume a normal morphology and phenotype by the process of contact normalization. Transformed cells must escape this process to become invasive and malignant. However, mechanisms underlying contact normalization have not been elucidated. Here, we have identified genes that are affected by contact normalization of Src-transformed cells. Tumor cells must migrate to become invasive and malignant. Src must phosphorylate the adaptor protein Cas (Crk-associated substrate) to promote tumor cell motility. We report here that Src utilizes Cas to induce podoplanin (Pdpn) expression to promote tumor cell migration. Pdpn is a membrane-bound extracellular glycoprotein that associates with endogenous ligands to promote tumor cell migration leading to cancer invasion and metastasis. In fact, Pdpn expression accounted for a major part of the increased migration seen in Src-transformed cells. Moreover, nontransformed cells suppressed Pdpn expression in adjacent Src-transformed cells. Of >39,000 genes, Pdpn was one of only 23 genes found to be induced by transforming Src activity and suppressed by contact normalization of Src-transformed cells. In addition, we found 16 genes suppressed by Src and induced by contact normalization. These genes encode growth factor receptors, adaptor proteins, and products that have not yet been annotated and may play important roles in tumor cell growth and migration.

Reengineering the signaling properties of a Src family kinase

Src family kinases (SFKs) are modular signaling proteins possessing SH3, SH2, and tyrosine kinase domains. The SH3 and SH2 domains of SFKs have dual roles: they regulate the activity of the kinases, and they also target SFKs to their cellular substrates. We generated a series of novel SFKs by replacing the SH2 and SH3 domains of Hck with the syntrophin PDZ domain. In some constructs, the negative regulatory tyrosine in the C-terminal tail was also replaced with a PDZ ligand sequence. When expressed in mammalian cells, the substrate specificity of the PDZ-kinases was directed to a different group of proteins than wild-type Hck. The PDZ-kinases phosphorylate neuronal nitric oxide synthase (nNOS), a known binding partner of the syntrophin PDZ domain. We also introduced a PDZ ligand at the C-terminus of the adaptor protein Cas. PDZ-Hck kinases phosphorylate the engineered Cas protein in Cas(-/-) cells and restore the migration defect of these cells. A PDZ-kinase was also functional in rewiring MAPK signaling via an engineered ErbB2 construct containing a PDZ ligand sequence. Several of the PDZ-kinases show autoregulatory properties similar to natural SFKs. Thus, the PDZ-ligand interaction is able to functionally replace the normal SH2-pY527 interaction that regulates SFKs. Our data highlight the modularity and evolvability of signaling proteins.

Synthesis of functional signaling domains by combinatorial polymerization of phosphorylation motifs

The adaptor protein Cas contains a core substrate domain with multiple YXXP motifs that are phosphorylated by Src and other tyrosine kinases. Here, we used a synthetic strategy to determine the importance of the arrangement, spacing, and identity of the YXXP motifs. By polymerizing short DNA sequences encoding two phosphorylation motifs, we created a panel of Cas mutants in which the entire substrate domain was replaced by synthetic domains containing random numbers and arrangements of the motifs. Most of these synthetic Cas variants were recognized and phosphorylated by Src in vitro and in intact mammalian cells. The random polymer mutants also restored migration activity to Cas knockout cells; even artificial proteins containing a single motif retained some biological function. Our results suggest that the arrangement of Cas motifs is not critical for signaling. This method could be used to identify the minimal functional units in other signaling proteins.

AND-34/BCAR3 regulates adhesion-dependent p130Cas serine phosphorylation and breast cancer cell growth pattern

NSP protein family members associate with p130Cas, a focal adhesion adapter protein best known as a Src substrate that integrates adhesion-related signaling. Over-expression of AND-34/BCAR3/NSP2 (BCAR3), but not NSP1 or NSP3, induces anti-estrogen resistance in human breast cancer cell lines. BCAR3 over-expression in epithelial MCF-7 cells augments levels of a phosphorylated p130Cas species that migrates more slowly on SDS-PAGE while NSP1 and NSP3 induce modest or no phosphorylation, respectively. Conversely, reduction in BCAR3 expression in mesenchymal MDA-231 cells by inducible shRNA results in loss of such p130Cas phosphorylation. Replacement of NSP3's serine/proline-rich domain with that of AND-34/BCAR3 instills the ability to induce p130Cas phosphorylation. Phospho-amino acid analysis demonstrates that BCAR3 induces p130Cas serine phosphorylation. Mass spectrometry identified phosphorylation at p130Cas serines 139, 437 and 639. p130Cas serine phosphorylation accumulates for several hours after adhesion of MDA-231 cells to fibronectin and is dependent upon BCAR3 expression. BCAR3 knockdown alters p130Cas localization and converts MDA-231 growth to an epithelioid pattern characterized by striking cohesiveness and lack of cellular projections at colony borders. These studies demonstrate that BCAR3 regulates p130Cas serine phosphorylation that is adhesion-dependent, temporally distinct from previously well-characterized rapid Fak and Src kinase-mediated p130Cas tyrosine phosphorylation and that correlates with invasive phenotype.

Crk and CrkL adaptor proteins: networks for physiological and pathological signaling

The Crk adaptor proteins (Crk and CrkL) constitute an integral part of a network of essential signal transduction pathways in humans and other organisms that act as major convergence points in tyrosine kinase signaling. Crk proteins integrate signals from a wide variety of sources, including growth factors, extracellular matrix molecules, bacterial pathogens, and apoptotic cells. Mounting evidence indicates that dysregulation of Crk proteins is associated with human diseases, including cancer and susceptibility to pathogen infections. Recent structural work has identified new and unusual insights into the regulation of Crk proteins, providing a rationale for how Crk can sense diverse signals and produce a myriad of biological responses.

The substrate domain of BCAR1 is essential for anti-estrogen-resistant proliferation of human breast cancer cells

To unravel the mechanisms underlying failure of endocrine therapy of breast cancer, we have previously executed a functional genetic screen and identified the adaptor protein BCAR1 to be causative for tamoxifen resistance. As a consequence of the manifold of interactions with other proteins, we characterized the contribution of individual protein domains of BCAR1 to anti-estrogen-resistant proliferation of human breast cancer cells. We took advantage of the observation that the closely related family member HEF1 was unable to support long-term anti-estrogen-resistant cell proliferation. Chimerical proteins containing defined domains of BCAR1 and HEF1 were evaluated for anti-estrogen-resistant growth. Exchange of the SH3 and C-terminal domains did not modify the capacity to support cell proliferation. Full support of anti-estrogen resistant proliferation was observed for chimerical molecules containing the central part of BCAR1. The bi-partite SRC-binding site or the Serine-rich domain did not explain the differential capacity of BCAR1. These findings indicate that the differences between BCAR1 and HEF1 with respect to support of anti-estrogen resistance reside in the substrate domain which contains multiple sites for tyrosine phosphorylation. The crucial interactions required for anti-estrogen resistance occur within the substrate domain of BCAR1. Further deciphering of these interactions may resolve the growth regulatory mechanism and provide an explanation for the observation that primary tumors with high levels of BCAR1 are likely to fail on tamoxifen therapy. This information may also help to devise alternative personalized treatment strategies with improved outcome for breast cancer patients.

Multisite protein phosphorylation--from molecular mechanisms to kinetic models

Multisite phosphorylation is an important mechanism for fine-tuned regulation of protein function. Mathematical models developed over recent years have contributed to elucidation of the functional consequences of a variety of molecular mechanisms involved in processing of the phosphorylation sites. Here we review the results of such models, together with salient experimental findings on multisite protein phosphorylation. We discuss how molecular mechanisms that can be distinguished with respect to the order and processivity of phosphorylation, as well as other factors, regulate changes in the sensitivity and kinetics of the response, the synchronization of molecular events, signalling specificity, and other functional implications.

Paxillin-Y118 phosphorylation contributes to the control of Src-induced anchorage-independent growth by FAK and adhesion

Background

Focal adhesion kinase (FAK) and Src are protein tyrosine kinases that physically and functionally interact to facilitate cancer progression by regulating oncogenic processes such as cell motility, survival, proliferation, invasiveness, and angiogenesis.

Method

To understand how FAK affects oncogenesis through the phosphorylation of cellular substrates of Src, we analyzed the phosphorylation profile of a panel of Src substrates in parental and v-Src-expressing FAK+/+ and FAK-/- mouse embryo fibroblasts, under conditions of anchorage-dependent (adherent) and -independent (suspension) growth.

Results

Total Src-induced cellular tyrosine phosphorylation as well as the number of phosphotyrosyl substrates was higher in suspension versus adherent cultures. Although the total level of Src-induced cellular phosphorylation was similar in FAK+/+ and FAK-/- backgrounds, the phosphorylation of some substrates was influenced by FAK depending on adherence state. Specifically, in the absence of FAK, Src induced higher phosphorylation of p190RhoGAP, paxillin (poY118) and Crk irrespective of adhesion state, PKC-δ (poY311), connexin-43 (poY265) and Sam68 only under adherent conditions, and p56Dok-2 (poY351) and p120catenin (poY228) only under suspension conditions. In contrast, FAK enhanced the Src-induced phosphorylation of vinculin (poY100 and poY1065) and p130CAS (poY410) irrespective of adherence state, p56Dok-2 (poY351) and p120catenin (poY228) only under adherent conditions, and connexin-43 (poY265), cortactin (poY421) and paxillin (poY31) only under suspension conditions. The Src-induced phosphorylation of Eps8, PLC-γ1 and Shc (poY239/poY240) were not affected by either FAK or adherence status. The enhanced anchorage-independent growth of FAK-/-[v-Src] cells was selectively decreased by expression of paxillin^Y118F, but not by WT-paxillin, p120catenin^Y228F or Shc^Y239/240F, identifying for the first time a role for paxillin^poY118 in Src-induced anchorage-independent growth. Knockdown of FAK by siRNA in the human colon cancer lines HT-25 and RKO, resulted in increased paxillin^poY118 levels under suspension conditions as well as increased anchorage-independent growth, supporting the notion that FAK attenuates anchorage-independent growth by suppressing adhesion-dependent phosphorylation of paxillin^Y118.

Conclusion

These data suggest that phosphorylation of Src substrates is a dynamic process, influenced temporally and spatially by factors such as FAK and adhesion.

The evolutionarily conserved arrangement of domains in SRC family kinases is important for substrate recognition

The SH3-SH2-kinase domain arrangement in nonreceptor tyrosine kinases has been conserved throughout evolution. For Src family kinases, the relative positions of the domains are important for enzyme regulation; they permit the assembly of Src kinases into autoinhibited conformations. The SH3 and SH2 domains of Src family kinases have an additional role in determining the substrate specificity of the kinase. We addressed the question of whether the domain arrangement of Src family kinases has a role in substrate specificity by producing mutants with alternative arrangements. Our results suggest that changes in the positions of domains can lead to specific changes in the phosphorylation of Sam68 and Cas by Src. Phosphorylation of Cas by several mutants triggers downstream signaling leading to cell migration. The placement of the SH2 domain with respect to the catalytic domain of Src appears to be especially important for proper substrate recognition, while the placement of the SH3 domain is more flexible. The results suggest that the involvement of the SH3 and SH2 domains in substrate recognition is one reason for the strict conservation of the SH3-SH2-kinase architecture.

Coordinate suppression of Sdpr and Fhl1 expression in tumors of the breast, kidney, and prostate

The Src tyrosine kinase associates with the focal adhesion adaptor protein Cas (Crk-associated substrate) to suppress the expression of potential tumor suppressor genes. For example, Src utilizes Cas to suppress the expression of the LIM-only protein Fhl1 (four and a half LIM domains 1), in order to promote non-anchored tumor-cell growth and migration. Here, we report that the promoter region of the Fhl1 gene was methylated more in Src-transformed cells than non-transformed cells. In addition, global expression analysis indicates that Fhl1 induced expression of serum deprivation response factor (Sdpr) in Src-transformed cells. Moreover, Fhl1 and Sdpr was expressed in approximately 87% and 40% of samples obtained from non-transformed breast, 100% of samples obtained from non-transformed kidney, and over 60% of samples obtained from non-transformed prostate. In contrast, Fhl1 and Sdpr was detected in approximately 40% and 7% of matched samples from mammary carcinoma, less than 11% of matched samples from kidney carcinoma, and in less than 22% of matched samples from prostate carcinoma. These data indicate that Fhl1 and Sdpr expression was significantly reduced in tumors of the breast (P < 0.02 and P < 0.001), kidney (P < 0.01), and prostate (P < 0.05). In addition, although Src can activate mitogen-activated protein kinase (MAPK) to promote tumor-cell growth, our data indicate that Src did not rely on MAPK activity to suppress the expression of Fhl1 and Sdpr in transformed cells. Thus, Src induced methylation of the promoter region of the Fhl1 gene; Src suppressed Fhl1 and Sdpr expression independent of mitogen-activated protein kinase (MAPK) activity; Fhl1 induced the expression of Sdpr in Src-transformed cells; and Fhl1 and Sdpr expression was suppressed in tumors of the breast, kidney, and prostate.

Signaling properties of a non-metazoan Src kinase and the evolutionary history of Src negative regulation

Choanoflagellates, unicellular organisms that are closely related to metazoans, possess cell adhesion and signaling proteins previously thought to be unique to animals, suggesting that these components may have played roles in the evolution of metazoan multicellularity. We have cloned, expressed, and purified the nonreceptor tyrosine kinase MbSrc1 from the choanoflagellate Monosiga brevicollis. The kinase has the same domain arrangement as mammalian Src kinases, and we find that the individual Src homology 3 (SH3), SH2, and catalytic domains have similar functions to their mammalian counterparts. In contrast to mammalian c-Src, the SH2 and catalytic domains of MbSrc1 do not appear to be functionally coupled. We cloned and expressed the M. brevicollis homolog of c-Src C-terminal kinase (MbCsk) and showed that it phosphorylates the C terminus of MbSrc1, yet this phosphorylation does not inhibit MbSrc to the same degree seen in the mammalian Src/Csk pair. Thus, Src autoinhibition likely evolved more recently within the metazoan lineage, and it may have played a role in the establishment of intercellular signaling in metazoans.

Cooperative activation of Src family kinases by SH3 and SH2 ligands

Src family nonreceptor tyrosine kinases are kept in a repressed state by intramolecular interactions involving the SH3 and SH2 domains of the enzymes. Ligands for these domains can displace the intramolecular associations and activate the kinases. Here, we carried out in vitro activation experiments with purified, down-regulated hematopoietic cell kinase (Hck), a Src family kinase. We show that SH3 and SH2 ligands act cooperatively to activate Src family kinases: the presence of one ligand lowers the concentration of the second ligand necessary for activation. To confirm the findings in intact cells, we studied Cas, a Src substrate that possesses SH2 and SH3 ligands. In contrast to wild-type Cas, mutant forms of Cas lacking the SH3 or SH2 ligands were unable to stimulate Src autophosphorylation when expressed in Cas-deficient fibroblasts. Cells expressing the Cas mutants also showed decreased amounts of activated Src at focal adhesions. The results suggest that proteins containing ligands for both SH3 and SH2 domains can produce a synergistic activation of Src family kinases.

Processive phosphorylation: mechanism and biological importance

Recent proteomic data indicate that a majority of the phosphorylated proteins in a eucaryotic cell contain multiple sites of phosphorylation. In many signaling events, a single kinase phosphorylates multiple sites on a target protein. Processive phosphorylation occurs when a protein kinase binds once to a substrate and phosphorylates all of the available sites before dissociating. In this review, we discuss examples of processive phosphorylation by serine/threonine kinases and tyrosine kinases. We describe current experimental approaches for distinguishing processive from non-processive phosphorylation. Finally, we contrast the biological situations that are suited to regulation by processive and non-processive phosphorylation.

Ordered multi-site phosphorylation of the splicing factor ASF/SF2 by SRPK1

The human alternative splicing factor ASF/SF2, an SR (serine-arginine-rich) protein involved in mRNA splicing control, is activated by the multisite phosphorylation of its C-terminal RS domain, a segment containing numerous arginine-serine dipeptide repeats. The protein kinase responsible for this modification, SR-specific protein kinase 1 (SRPK1), catalyzes the selective phosphorylation of approximately a dozen serines in only the N-terminal portion of the RS domain (RS1). To gain insights into the nature of selective phosphate incorporation in ASF/SF2, region-specific phosphorylation in the RS domain was monitored as a function of reaction progress. Arg-to-Lys mutations were made at several positions to produce unique protease cleavage sites that separate the RS domain into identifiable N- and C-terminal phosphopeptides upon treatment with lysyl endoproteinase. These studies reveal that SRPK1 docks near the C-terminus of the RS1 segment and then moves in an N-terminal direction along the RS domain. Multiple quadruple Ser-to-Ala and deletion mutations did not disrupt the phosphorylation of other sites regardless of position, suggesting that the active site of SRPK1 docks in a flexible manner at the center of the RS domain. Taken together, these data suggest that SRPK1 uses a unique 'grab-and-pull' mechanism to control the regiospecific phosphorylation of its protein substrate.

SRC utilizes Cas to block gap junctional communication mediated by connexin43

The Src tyrosine kinase phosphorylates Cas (Crk-associated substrate) to confer anchorage independence and invasive growth potential to transformed cells. Gap junctional communication is often lower between aggressive tumor cells compared with normal or benign precursors. The gap junction protein connexin43 (Cx43) is a tumor suppressor that can inhibit tumor cell growth. Src can phosphorylate Cx43 to block gap junctional communication between transformed cells. However, mechanisms by which this event actually closes intercellular channels have not been clearly defined. Here, we report that Src and Cas associate with each other at intercellular junctions. In addition, Cas is required for Src to reduce dye transfer and electrical coupling between cells expressing Cx43. Thus, Src utilizes Cas to inhibit gap junctional communication mediated by Cx43. This finding introduces a novel role of the Cas focal adhesion linker protein in the gap junction complex. This observation may help explain how gap junctional communication can be suppressed between malignant and metastatic tumor cells.

Versatile regulation of multisite protein phosphorylation by the order of phosphate processing and protein-protein interactions

Multisite protein phosphorylation is a common regulatory mechanism in cell signaling, and dramatically increases the possibilities for protein-protein interactions, conformational regulation, and phosphorylation pathways. However, there is at present no comprehensive picture of how these factors shape the response of a protein's phosphorylation state to changes in kinase and phosphatase activities. Here we provide a mathematical theory for the regulation of multisite protein phosphorylation based on the mechanistic description of elementary binding and catalytic steps. Explicit solutions for the steady-state response curves and characteristic (de)phosphorylation times have been obtained in special cases. The order of phosphate processing and the characteristics of protein-protein interactions turn out to be of overriding importance for both sensitivity and speed of response. Random phosphate processing gives rise to shallow response curves, favoring intermediate phosphorylation states of the target, and rapid kinetics. Sequential processing is characterized by steeper response curves and slower kinetics. We show systematically how qualitative differences in target phosphorylation--including graded, switch-like and bistable responses--are determined by the relative concentrations of enzyme and target as well as the enzyme-target affinities. In addition to collective effects of several phosphorylation sites, our analysis predicts that distinct phosphorylation patterns can be finely tuned by a single kinase. Taken together, this study suggests a versatile regulation of protein activation by the combined effect of structural, kinetic and thermodynamic aspects of multisite phosphorylation.