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Eph receptor tyrosine kinases (RTKs) mediate numerous developmental processes. Their activity is regulated by auto‐phosphorylation on two tyrosines within the juxtamembrane segment (JMS) immediately N‐terminal to the kinase domain (KD). Here, we probe the molecular details of Eph kinase activation through mutational analysis, X‐ray crystallography and NMR spectroscopy on auto‐inhibited and active EphB2 and EphA4 fragments. We show that a Tyr750Ala gain‐of‐function mutation in the KD and JMS phosphorylation independently induce disorder of the JMS and its dissociation from the KD. Our X‐ray analyses demonstrate that this occurs without major conformational changes to the KD and with only partial ordering of the KD activation segment. However, conformational exchange for helix αC in the N‐terminal KD lobe and for the activation segment, coupled with increased inter‐lobe dynamics, is observed upon kinase activation in our NMR analyses. Overall, our results suggest that a change in inter‐lobe dynamics and the sampling of catalytically competent conformations for helix αC and the activation segment rather than a transition to a static active conformation underlies Eph RTK activation.

Serine hydroxymethyltransferase 2 (SHMT2) converts serine plus tetrahydrofolate (THF) into glycine plus methylene-THF and is upregulated at the protein level in lung and other cancers. In order to better understand the role of SHMT2 in cancer a model system of HeLa cells engineered for inducible over-expression or knock-down of SHMT2 was characterized for cell proliferation and changes in metabolites and proteome as a function of SHMT2. Ectopic over-expression of SHMT2 increased cell proliferation in vitro and tumor growth in vivo. Knockdown of SHMT2 expression in vitro caused a state of glycine auxotrophy and accumulation of phosphoribosylaminoimidazolecarboxamide (AICAR), an intermediate of folate/1-carbon-pathway-dependent de novo purine nucleotide synthesis. Decreased glycine in the HeLa cell-based xenograft tumors with knocked down SHMT2 was potentiated by administration of the anti-hyperglycinemia agent benzoate. However, tumor growth was not affected by SHMT2 knockdown with or without benzoate treatment. Benzoate inhibited cell proliferation in vitro, but this was independent of SHMT2 modulation. The abundance of proteins of mitochondrial respiration complexes 1 and 3 was inversely correlated with SHMT2 levels. Proximity biotinylation in vivo (BioID) identified 48 mostly mitochondrial proteins associated with SHMT2 including the mitochondrial enzymes Acyl-CoA thioesterase (ACOT2) and glutamate dehydrogenase (GLUD1) along with more than 20 proteins from mitochondrial respiration complexes 1 and 3. These data provide insights into possible mechanisms through which elevated SHMT2 in cancers may be linked to changes in metabolism and mitochondrial function.

PDZ (Post-synaptic density, 95 kDa, Discs large, Zona Occludens-1) domains are protein interaction domains that bind to the carboxy-terminal amino acids of binding partners, heterodimerize with other PDZ domains, and also bind phosphoinositides. PDZ domain containing proteins are frequently involved in the assembly of multi-protein complexes and clustering of transmembrane proteins. LNX1 (Ligand of Numb, protein X 1) is a RING (Really Interesting New Gene) domain-containing E3 ubiquitin ligase that also includes four PDZ domains suggesting it functions as a scaffold for a multi-protein complex. Here we use a human protein array to identify direct LNX1 PDZ domain binding partners. Screening of 8,000 human proteins with isolated PDZ domains identified 53 potential LNX1 binding partners. We combined this set with LNX1 interacting proteins identified by other methods to assemble a list of 220 LNX1 interacting proteins. Bioinformatic analysis of this protein list was used to select interactions of interest for future studies. Using this approach we identify and confirm six novel LNX1 binding partners: KCNA4, PAK6, PLEKHG5, PKC-alpha1, TYK2 and PBK, and suggest that LNX1 functions as a signalling scaffold.

Receptor tyrosine kinases (RTKs) are transmembrane receptors of great clinical interest due to their role in disease. Historically, therapeutics targeting RTKs have been identified using in vitro kinase assays. Due to frequent development of drug resistance, however, there is a need to identify more diverse compounds that inhibit mutated but not wild-type RTKs. Here, we describe MaMTH-DS (mammalian membrane two-hybrid drug screening), a live-cell platform for high-throughput identification of small molecules targeting functional protein–protein interactions of RTKs. We applied MaMTH-DS to an oncogenic epidermal growth factor receptor (EGFR) mutant resistant to the latest generation of clinically approved tyrosine kinase inhibitors (TKIs). We identified four mutant-specific compounds, including two that would not have been detected by conventional in vitro kinase assays. One of these targets mutant EGFR via a new mechanism of action, distinct from classical TKI inhibition. Our results demonstrate how MaMTH-DS is a powerful complement to traditional drug screening approaches. A high-throughput small-molecule drug screening platform enabled the detection of compounds targeting the functional interactions of receptor tyrosine kinases and identifies four new EGFR triple-mutant inhibitors.

Abstract The Src-like adaptor proteins (SLAP/SLAP2) bind to CBL E3 ubiquitin ligase to downregulate antigen, cytokine, and tyrosine kinase receptor signaling. In contrast to phospho-tyrosine dependent binding of CBL substrates through its tyrosine kinase binding domain (TKBD), CBL TKBD associates with the C-terminal tail of SLAP2 in a phospho-independent manner. To understand the distinct nature of this interaction, a purification protocol for SLAP2 in complex with CBL TKBD was established and the complex crystallized. However, determination of the complex crystal structure was hindered by apparent SLAP2 degradation during the crystallization process, such that only CBL TKBD residues could be modeled initially. Close examination of the CBL TKBD structure revealed a unique dimer interface that included two short segments of electron density of unknown origin. To elucidate which residues of SLAP2 to model in this unassigned density, a co-expression system was generated to test SLAP2 deletion mutants and define the minimal SLAP2 binding region. As well, SLAP2 degradation products were analyzed by mass spectrometry. Model building and map generation features of the Phenix software package were employed, leading to successful modeling of the C-terminal tail of SLAP2 in the unassigned electron density segments. Competing Interest Statement The authors have declared no competing interest.

Abstract CBL is a RING type E3 ubiquitin ligase that functions as a negative regulator of tyrosine kinase signaling and loss of CBL E3 function is implicated in several forms of leukemia. The Src-like adaptor proteins (SLAP/SLAP2) bind to CBL and are required for CBL-dependent downregulation of antigen receptor, cytokine receptor, and receptor tyrosine kinase signaling. Despite the established role of SLAP/SLAP2 in regulating CBL activity, the nature of the interaction and the mechanisms involved are not known. To understand the molecular basis of the interaction between SLAP/SLAP2 and CBL, we solved the crystal structure of CBL tyrosine kinase binding domain (TKBD) in complex with SLAP2. The carboxy-terminal region of SLAP2 adopts an α-helical structure which binds in a cleft between the 4H, EF-hand, and SH2 domains of the TKBD. This SLAP2 binding site is remote from the canonical TKBD phospho-tyrosine peptide binding site but overlaps with a region important for stabilizing CBL in its autoinhibited conformation. In addition, binding of SLAP2 to CBL in vitro activates the ubiquitin ligase function of autoinhibited CBL. Disruption of the CBL/SLAP2 interface through mutagenesis demonstrated a role for this protein-protein interaction in regulation of CBL E3 ligase activity in cells. Our results reveal that SLAP2 binding to a regulatory cleft of the TKBD provides an alternative mechanism for activation of CBL ubiquitin ligase function. Competing Interest Statement The authors have declared no competing interest.

The balance between comprehensively analyzing the proteome and using valuable mass spectrometry time is a genuine challenge in the field of proteomics. Multidimensional fractionation strategies have significantly increased proteome coverage, but often at the cost of increased mass analysis time, despite advances in mass spectrometer acquisition rates. Recently, the Evosep One liquid chromatography system was shown to analyze peptide samples in a high throughput manner without sacrificing in depth proteomics coverage. We demonstrate incorporation of Evosep One technology into our multiplexing workflow for quantitative analysis of tandem mass tag (TMT)-labeled non-small cell lung carcinoma (NSCLC) patient-derived xenografts (PDXs). Using a 30 samples per day Evosep workflow, >12,000 proteins were identified in 48 hours of mass spectrometry time, which is comparable to the number of proteins identified by our conventional concatenated EASY-nLC workflow in 67.5 hours. Shorter Evosep gradient lengths reduced the number of protein identifications by 10%, while decreasing mass analysis time by 50%. Thus, our Evosep workflow enables quantitative analysis of multiplexed samples in less time without conceding depth of proteome coverage.

Serine hydroxymethyltransferase 2 (SHMT2) converts serine plus tetrahydrofolate (THF) into glycine plus methylene-THF and is upregulated at the protein level in lung and other cancers. In order to better understand the role of SHMT2 in cancer a model system of HeLa cells engineered for inducible over-expression or knock-down of SHMT2 was characterized for cell proliferation and changes in metabolites and proteome as a function of SHMT2. Ectopic over-expression of SHMT2 increased cell proliferation in vitro and tumor growth in vivo. Knockdown of SHMT2 expression in vitro caused a state of glycine auxotrophy and accumulation of phosphoribosylaminoimidazolecarboxamide (AICAR), an intermediate of folate/1-carbon-pathway-dependent de novo purine nucleotide synthesis. Decreased glycine in the HeLa cell-based xenograft tumors with knocked down SHMT2 was potentiated by administration of the anti-hyperglycinemia agent benzoate. However, tumor growth was not affected by SHMT2 knockdown with or without benzoate treatment. Benzoate inhibited cell proliferation in vitro, but this was independent of SHMT2 modulation. The abundance of proteins of mitochondrial respiration complexes 1 and 3 was inversely correlated with SHMT2 levels. Proximity biotinylation in vivo (BioID) identified 48 mostly mitochondrial proteins associated with SHMT2 including the mitochondrial enzymes Acyl-CoA thioesterase (ACOT2) and glutamate dehydrogenase (GLUD1) along with more than 20 proteins from mitochondrial respiration complexes 1 and 3. These data provide insights into possible mechanisms through which elevated SHMT2 in cancers may be linked to changes in metabolism and mitochondrial function.