The α-(1,3)-glucan synthase gene agsE impacts the secretome of Aspergillus niger.

Aspergillus niger is widely used as a cell factory for the industrial production of enzymes. Previously, it was shown that deletion of α-1-3 glucan synthase genes results in smaller micro-colonies in liquid cultures of Aspergillus nidulans. Also, it has been shown that small wild-type Aspergillus niger micro-colonies secrete more protein than large mirco-colonies. We here assessed whether deletion of the agsC or agsE α-1-3 glucan synthase genes results in smaller A. niger micro-colonies and whether this is accompanied by a change in protein secretion. Biomass formation was not affected in the deletion strains but pH of the culture medium had changed from 5.2 in the case of the wild-type to 4.6 and 6.4 for ΔagsC and ΔagsE, respectively. The diameter of the ΔagsC micro-colonies was not affected in liquid cultures. In contrast, diameter of the ΔagsE micro-colonies was reduced from 3304 ± 338 µm to 1229 ± 113 µm. Moreover, the ΔagsE secretome was affected with 54 and 36 unique proteins with a predicted signal peptide in the culture medium of MA234.1 and the ΔagsE, respectively. Results show that these strains have complementary cellulase activity and thus may have complementary activity on plant biomass degradation. Together, α-1-3 glucan synthesis (in)directly impacts protein secretion in A. niger.

Heterogeneity in Spore Aggregation and Germination Results in Different Sized, Cooperative Microcolonies in an Aspergillus niger Culture.

The fungus Aspergillus niger is among the most abundant fungi in the world and is widely used as a cell factory for protein and metabolite production. This fungus forms asexual spores called conidia that are used for dispersal. Notably, part of the spores and germlings aggregate in an aqueous environment. The aggregated conidia/germlings give rise to large microcolonies, while the nonaggregated spores/germlings result in small microcolonies. Here, it is shown that small microcolonies release a larger variety and quantity of secreted proteins compared to large microcolonies. Yet, the secretome of large microcolonies has complementary cellulase activity with that of the small microcolonies. Also, large microcolonies are more resistant to heat and oxidative stress compared to small microcolonies, which is partly explained by the presence of nongerminated spores in the core of the large microcolonies. Together, it is proposed that heterogeneity in germination and aggregation has evolved to form a population of different sized A. niger microcolonies, thereby increasing stress survival and producing a meta-secretome more optimally suited to degrade complex substrates. IMPORTANCE Aspergillus niger can form microcolonies of different size due to partial aggregation of spores and germlings. So far, this heterogeneity was considered a negative trait by the industry. We here, however, show that heterogeneity in size within a population of microcolonies is beneficial for food degradation and stress survival. This functional heterogeneity is not only of interest for the industry to make blends of enzymes (e.g., for biofuel or bioplastic production) but could also play a role in nature for effective nutrient cycling and survival of the fungus.

Quantifying Positional Isomers (QPI) by Top-Down Mass Spectrometry.

Proteomics has exposed a plethora of posttranslational modifications, but demonstrating functional relevance requires new approaches. Top-down proteomics of intact proteins has the potential to fully characterize protein modifications in terms of amount, site(s), and the order in which they are deposited on the protein; information that so far has been elusive to extract by shotgun proteomics. Data acquisition and analysis of intact multimodified proteins have however been a major challenge, in particular for positional isomers that carry the same number of modifications at different sites. Solutions were previously proposed to extract this information from fragmentation spectra, but these have so far mainly been limited to peptides and have entailed a large degree of manual interpretation. Here, we apply high-resolution Orbitrap fusion top-down analyses in combination with bioinformatics approaches to attempt to characterize multiple modified proteins and quantify positional isomers. Automated covalent fragment ion type definition, detection of mass precision and accuracy, and extensive use of replicate spectra increase sequence coverage and drive down false fragment assignments from 10% to 1.5%. Such improved performance in fragment assignment is key to localize and quantify modifications from fragment spectra. The method is tested by investigating positional isomers of Ubiquitin mixed in known concentrations, which results in quantification of high ratios at very low standard errors of the mean (<5%), as well as with synthetic phosphorylated peptides. Application to multiphosphorylated Bora provides an estimation of the so far unknown stoichiometry of the known set of phosphosites and uncovers new sites from hyperphosphorylated Bora.

Feedback-Driven Assembly of the Axon Initial Segment.

The axon initial segment (AIS) is a unique neuronal compartment that plays a crucial role in the generation of action potential and neuronal polarity. The assembly of the AIS requires membrane, scaffolding, and cytoskeletal proteins, including Ankyrin-G and TRIM46. How these components cooperate in AIS formation is currently poorly understood. Here, we show that Ankyrin-G acts as a scaffold interacting with End-Binding (EB) proteins and membrane proteins such as Neurofascin-186 to recruit TRIM46-positive microtubules to the plasma membrane. Using in vitro reconstitution and cellular assays, we demonstrate that TRIM46 forms parallel microtubule bundles and stabilizes them by acting as a rescue factor. TRIM46-labeled microtubules drive retrograde transport of Neurofascin-186 to the proximal axon, where Ankyrin-G prevents its endocytosis, resulting in stable accumulation of Neurofascin-186 at the AIS. Neurofascin-186 enrichment in turn reinforces membrane anchoring of Ankyrin-G and subsequent recruitment of TRIM46-decorated microtubules. Our study reveals feedback-based mechanisms driving AIS assembly.

MAP7 family proteins regulate kinesin-1 recruitment and activation.

Kinesin-1 is responsible for microtubule-based transport of numerous cellular cargoes. Here, we explored the regulation of kinesin-1 by MAP7 proteins. We found that all four mammalian MAP7 family members bind to kinesin-1. In HeLa cells, MAP7, MAP7D1, and MAP7D3 act redundantly to enable kinesin-1-dependent transport and microtubule recruitment of the truncated kinesin-1 KIF5B-560, which contains the stalk but not the cargo-binding and autoregulatory regions. In vitro, purified MAP7 and MAP7D3 increase microtubule landing rate and processivity of kinesin-1 through transient association with the motor. MAP7 proteins promote binding of kinesin-1 to microtubules both directly, through the N-terminal microtubule-binding domain and unstructured linker region, and indirectly, through an allosteric effect exerted by the kinesin-binding C-terminal domain. Compared with MAP7, MAP7D3 has a higher affinity for kinesin-1 and a lower affinity for microtubules and, unlike MAP7, can be cotransported with the motor. We propose that MAP7 proteins are microtubule-tethered kinesin-1 activators, with which the motor transiently interacts as it moves along microtubules.

A System-wide Approach to Monitor Responses to Synergistic BRAF and EGFR Inhibition in Colorectal Cancer Cells.

Intrinsic and/or acquired resistance represents one of the great challenges in targeted cancer therapy. A deeper understanding of the molecular biology of cancer has resulted in more efficient strategies, where one or multiple drugs are adopted in novel therapies to tackle resistance. This beneficial effect of using combination treatments has also been observed in colorectal cancer patients harboring the BRAF(V600E) mutation, whereby dual inhibition of BRAF(V600E) and EGFR increases antitumor activity. Notwithstanding this success, it is not clear whether this combination treatment is the only or most effective treatment to block intrinsic resistance to BRAF inhibitors. Here, we investigate molecular responses upon single and multi-target treatments, over time, using BRAF(V600E) mutant colorectal cancer cells as a model system. Through integration of transcriptomic, proteomic and phosphoproteomics data we obtain a comprehensive overview, revealing both known and novel responses. We primarily observe widespread up-regulation of receptor tyrosine kinases and metabolic pathways upon BRAF inhibition. These findings point to mechanisms by which the drug-treated cells switch energy sources and enter a quiescent-like state as a defensive response, while additionally compensating for the MAPK pathway inhibition.

Regulation of KIF1A-Driven Dense Core Vesicle Transport: Ca2+/CaM Controls DCV Binding and Liprin-α/TANC2 Recruits DCVs to Postsynaptic Sites.

Tight regulation of neuronal transport allows for cargo binding and release at specific cellular locations. The mechanisms by which motor proteins are loaded on vesicles and how cargoes are captured at appropriate sites remain unclear. To better understand how KIF1A-driven dense core vesicle (DCV) transport is regulated, we identified the KIF1A interactome and focused on three binding partners, the calcium binding protein calmodulin (CaM) and two synaptic scaffolding proteins: liprin-α and TANC2. We showed that calcium, acting via CaM, enhances KIF1A binding to DCVs and increases vesicle motility. In contrast, liprin-α and TANC2 are not part of the KIF1A-cargo complex but capture DCVs at dendritic spines. Furthermore, we found that specific TANC2 mutations-reported in patients with different neuropsychiatric disorders-abolish the interaction with KIF1A. We propose a model in which Ca2+/CaM regulates cargo binding and liprin-α and TANC2 recruit KIF1A-transported vesicles.

H4K20me2 distinguishes pre-replicative from post-replicative chromatin to appropriately direct DNA repair pathway choice by 53BP1-RIF1-MAD2L2.

The main pathways for the repair of DNA double strand breaks (DSBs) are non-homologous end-joining (NHEJ) and homologous recombination directed repair (HDR). These operate mutually exclusive and are activated by 53BP1 and BRCA1, respectively. As HDR can only succeed in the presence of an intact copy of replicated DNA, cells employ several mechanisms to inactivate HDR in the G1 phase of cell cycle. As cells enter S-phase, these inhibitory mechanisms are released and HDR becomes active. However, during DNA replication, NHEJ and HDR pathways are both functional and non-replicated and replicated DNA regions co-exist, with the risk of aberrant HDR activity at DSBs in non-replicated DNA. It has become clear that DNA repair pathway choice depends on inhibition of DNA end-resection by 53BP1 and its downstream factors RIF1 and MAD2L2. However, it is unknown how MAD2L2 accumulates at DSBs to participate in DNA repair pathway control and how the NHEJ and HDR repair pathways are appropriately activated at DSBs with respect to the replication status of the DNA, such that NHEJ acts at DSBs in pre-replicative DNA and HDR acts on DSBs in post-replicative DNA. Here we show that MAD2L2 is recruited to DSBs in H4K20 dimethylated chromatin by forming a protein complex with 53BP1 and RIF1 and that MAD2L2, similar to 53BP1 and RIF1, suppresses DSB accumulation of BRCA1. Furthermore, we show that the replication status of the DNA locally ensures the engagement of the correct DNA repair pathway, through epigenetics. In non-replicated DNA, saturating levels of the 53BP1 binding site, di-methylated lysine 20 of histone 4 (H4K20me2), lead to robust 53BP1-RIF1-MAD2L2 recruitment at DSBs, with consequent exclusion of BRCA1. Conversely, replication-associated 2-fold dilution of H4K20me2 promotes the release of the 53BP1-RIF1-MAD2L2 complex and favours the access of BRCA1. Thus, the differential H4K20 methylation status between pre-replicative and post-replicative DNA represents an intrinsic mechanism that locally ensures appropriate recruitment of the 53BP1-RIF1-MAD2L2 complex at DNA DSBs, to engage the correct DNA repair pathway.

Erratum: Microtubule minus-end regulation at spindle poles by an ASPM-katanin complex.

This corrects the article DOI: 10.1038/ncb3511.

Genetic wiring maps of single-cell protein states reveal an off-switch for GPCR signalling.

As key executers of biological functions, the activity and abundance of proteins are subjected to extensive regulation. Deciphering the genetic architecture underlying this regulation is critical for understanding cellular signalling events and responses to environmental cues. Using random mutagenesis in haploid human cells, we apply a sensitive approach to directly couple genomic mutations to protein measurements in individual cells. Here we use this to examine a suite of cellular processes, such as transcriptional induction, regulation of protein abundance and splicing, signalling cascades (mitogen-activated protein kinase (MAPK), G-protein-coupled receptor (GPCR), protein kinase B (AKT), interferon, and Wingless and Int-related protein (WNT) pathways) and epigenetic modifications (histone crotonylation and methylation). This scalable, sequencing-based procedure elucidates the genetic landscapes that control protein states, identifying genes that cause very narrow phenotypic effects and genes that lead to broad phenotypic consequences. The resulting genetic wiring map identifies the E3-ligase substrate adaptor KCTD5 (ref. 1) as a negative regulator of the AKT pathway, a key signalling cascade frequently deregulated in cancer. KCTD5-deficient cells show elevated levels of phospho-AKT at S473 that could not be attributed to effects on canonical pathway components. To reveal the genetic requirements for this phenotype, we iteratively analysed the regulatory network linked to AKT activity in the knockout background. This genetic modifier screen exposes suppressors of the KCTD5 phenotype and mechanistically demonstrates that KCTD5 acts as an off-switch for GPCR signalling by triggering proteolysis of Gßγ heterodimers dissociated from the Gα subunit. Although biological networks have previously been constructed on the basis of gene expression, protein-protein associations, or genetic interaction profiles, we foresee that the approach described here will enable the generation of a comprehensive genetic wiring map for human cells on the basis of quantitative protein states.

Microtubule minus-end regulation at spindle poles by an ASPM-katanin complex.

ASPM (known as Asp in fly and ASPM-1 in worm) is a microcephaly-associated protein family that regulates spindle architecture, but the underlying mechanism is poorly understood. Here, we show that ASPM forms a complex with another protein linked to microcephaly, the microtubule-severing ATPase katanin. ASPM and katanin localize to spindle poles in a mutually dependent manner and regulate spindle flux. X-ray crystallography revealed that the heterodimer formed by the N- and C-terminal domains of the katanin subunits p60 and p80, respectively, binds conserved motifs in ASPM. Reconstitution experiments demonstrated that ASPM autonomously tracks growing microtubule minus ends and inhibits their growth, while katanin decorates and bends both ends of dynamic microtubules and potentiates the minus-end blocking activity of ASPM. ASPM also binds along microtubules, recruits katanin and promotes katanin-mediated severing of dynamic microtubules. We propose that the ASPM-katanin complex controls microtubule disassembly at spindle poles and that misregulation of this process can lead to microcephaly.

Quantitative Map of Proteome Dynamics during Neuronal Differentiation.

Neuronal differentiation is a multistep process that shapes and re-shapes neurons by progressing through several typical stages, including axon outgrowth, dendritogenesis, and synapse formation. To systematically profile proteome dynamics throughout neuronal differentiation, we took cultured rat hippocampal neurons at different developmental stages and monitored changes in protein abundance using a combination of stable isotope labeling and high-resolution liquid chromatography-tandem mass spectrometry (LC-MS/MS). Almost one third of all 4,500 proteins quantified underwent a more than 2-fold expression change during neuronal differentiation, indicating extensive remodeling of the neuron proteome. To highlight the strength of our resource, we studied the neural-cell-adhesion molecule 1 (NCAM1) and found that it stimulates dendritic arbor development by promoting actin filament growth at the dendritic growth cone. We anticipate that our quantitative map of neuronal proteome dynamics is a rich resource for further analyses of the many identified proteins in various neurodevelopmental processes.

Robust, Sensitive, and Automated Phosphopeptide Enrichment Optimized for Low Sample Amounts Applied to Primary Hippocampal Neurons.

Because of the low stoichiometry of protein phosphorylation, targeted enrichment prior to LC-MS/MS analysis is still essential. The trend in phosphoproteome analysis is shifting toward an increasing number of biological replicates per experiment, ideally starting from very low sample amounts, placing new demands on enrichment protocols to make them less labor-intensive, more sensitive, and less prone to variability. Here we assessed an automated enrichment protocol using Fe(III)-IMAC cartridges on an AssayMAP Bravo platform to meet these demands. The automated Fe(III)-IMAC-based enrichment workflow proved to be more effective when compared to a TiO2-based enrichment using the same platform and a manual Ti(IV)-IMAC-based enrichment workflow. As initial samples, a dilution series of both human HeLa cell and primary rat hippocampal neuron lysates was used, going down to 0.1 µg of peptide starting material. The optimized workflow proved to be efficient, sensitive, and reproducible, identifying, localizing, and quantifying thousands of phosphosites from just micrograms of starting material. To further test the automated workflow in genuine biological applications, we monitored EGF-induced signaling in hippocampal neurons, starting with only 200 000 primary cells, resulting in ∼50 µg of protein material. This revealed a comprehensive phosphoproteome, showing regulation of multiple members of the MAPK pathway and reduced phosphorylation status of two glutamate receptors involved in synaptic plasticity.

Membrane-Depolarizing Channel Blockers Induce Selective Glioma Cell Death by Impairing Nutrient Transport and Unfolded Protein/Amino Acid Responses.

Glioma-initiating cells (GIC) are considered the underlying cause of recurrences of aggressive glioblastomas, replenishing the tumor population and undermining the efficacy of conventional chemotherapy. Here we report the discovery that inhibiting T-type voltage-gated Ca2+ and KCa channels can effectively induce selective cell death of GIC and increase host survival in an orthotopic mouse model of human glioma. At present, the precise cellular pathways affected by the drugs affecting these channels are unknown. However, using cell-based assays and integrated proteomics, phosphoproteomics, and transcriptomics analyses, we identified the downstream signaling events these drugs affect. Changes in plasma membrane depolarization and elevated intracellular Na+, which compromised Na+-dependent nutrient transport, were documented. Deficits in nutrient deficit acted in turn to trigger the unfolded protein response and the amino acid response, leading ultimately to nutrient starvation and GIC cell death. Our results suggest new therapeutic targets to attack aggressive gliomas. Cancer Res; 77(7); 1741-52. ©2017 AACR.

Opposite Electron-Transfer Dissociation and Higher-Energy Collisional Dissociation Fragmentation Characteristics of Proteolytic K/R(X)n and (X)nK/R Peptides Provide Benefits for Peptide Sequencing in Proteomics and Phosphoproteomics.

A key step in shotgun proteomics is the digestion of proteins into peptides amenable for mass spectrometry. Tryptic peptides can be readily sequenced and identified by collision-induced dissociation (CID) or higher-energy collisional dissociation (HCD) because the fragmentation rules are well-understood. Here, we investigate LysargiNase, a perfect trypsin mirror protease, because it cleaves equally specific at arginine and lysine residues, albeit at the N-terminal end. LysargiNase peptides are therefore practically tryptic-like in length and sequence except that following ESI, the two protons are now both positioned at the N-terminus. Here, we compare side-by-side the chromatographic separation properties, gas-phase fragmentation characteristics, and (phospho)proteome sequence coverage of tryptic (i.e., (X)nK/R) and LysargiNase (i.e., K/R(X)n) peptides using primarily electron-transfer dissociation (ETD) and, for comparison, HCD. We find that tryptic and LysargiNase peptides fragment nearly as mirror images. For LysargiNase predominantly N-terminal peptide ions (c-ions (ETD) and b-ions (HCD)) are formed, whereas for trypsin, C-terminal fragment ions dominate (z-ions (ETD) and y-ions (HCD)) in a homologous mixture of complementary ions. Especially during ETD, LysargiNase peptides fragment into low-complexity but information-rich sequence ladders. Trypsin and LysargiNase chart distinct parts of the proteome, and therefore, the combined use of these enzymes will benefit a more in-depth and reliable analysis of (phospho)proteomes.

Association of Cell Adhesion Molecules Contactin-6 and Latrophilin-1 Regulates Neuronal Apoptosis.

In view of important neurobiological functions of the cell adhesion molecule contactin-6 (Cntn6) that have emerged from studies on null-mutant mice and autism spectrum disorders patients, we set out to examine pathways underlying functions of Cntn6 using a proteomics approach. We identified the cell adhesion GPCR latrophilin-1 (Lphn1, a.k.a. CIRL1/CL, ADGRL1) as a binding partner for Cntn6 forming together a heteromeric cis-complex. Lphn1 expression in cultured neurons caused reduction in neurite outgrowth and increase in apoptosis, which was rescued by coexpression of Cntn6. In cultured neurons derived from Cntn6-/- mice, Lphn1 knockdown reduced apoptosis, suggesting that the observed apoptosis was Lphn1-dependent. In line with these data, the number of apoptotic cells was increased in the cortex of Cntn6-/- mice compared to wild-type littermate controls. These results show that Cntn6 can modulate the activity of Lphn1 by direct binding and suggests that Cntn6 may prevent apoptosis thereby impinging on neurodevelopment.

Direct screening for chromatin status on DNA barcodes in yeast delineates the regulome of H3K79 methylation by Dot1.

Given the frequent misregulation of chromatin in cancer, it is important to understand the cellular mechanisms that regulate chromatin structure. However, systematic screening for epigenetic regulators is challenging and often relies on laborious assays or indirect reporter read-outs. Here we describe a strategy, Epi-ID, to directly assess chromatin status in thousands of mutants. In Epi-ID, chromatin status on DNA barcodes is interrogated by chromatin immunoprecipitation followed by deep sequencing, allowing for quantitative comparison of many mutants in parallel. Screening of a barcoded yeast knock-out collection for regulators of histone H3K79 methylation by Dot1 identified all known regulators as well as novel players and processes. These include histone deposition, homologous recombination, and adenosine kinase, which influences the methionine cycle. Gcn5, the acetyltransferase within the SAGA complex, was found to regulate histone methylation and H2B ubiquitination. The concept of Epi-ID is widely applicable and can be readily applied to other chromatin features.

Quantitative Proteomics Illuminates a Functional Interaction between mDia2 and the Proteasome.

Formin mDia2 is a cytoskeleton-regulatory protein that switches reversibly between a closed, autoinhibited and an open, active conformation. Although the open conformation of mDia2 induces actin assembly thereby controlling many cellular processes, mDia2 possesses also actin-independent and conformation-insensitive scaffolding roles related to microtubules and p53, respectively. Thus, we hypothesize that mDia2 may have other unappreciated functions and regulatory modes. Here we identify and validate proteasome and Ubiquitin as mDia2-interacting partners using stable isotope labeling with amino acids in cell culture-based quantitative proteomics and biochemistry, respectively. Although mDia2 is ubiquitinated, binds ubiquitinated proteins and free Ubiquitin, it is not a proteasome substrate. Surprisingly, knockdown of mDia2 increases the activity of the proteasome in vitro, whereas mDia2 overexpression has opposite effects only when it adopts the open conformation and cannot induce actin assembly. Consistently, a combination of candidate and unbiased proteome-wide analyses indicates that mDia2 regulates the cellular levels of proteasome substrate ß-catenin and a number of ubiquitinated actin-regulatory proteins. Hence, these findings add more complexity to the mDia2 activity cycle by showing that the open conformation may control actin dynamics also through actin-independent regulation of the proteasome.

Molecular Pathway of Microtubule Organization at the Golgi Apparatus.

The Golgi apparatus controls the formation of non-centrosomal microtubule arrays important for Golgi organization, polarized transport, cell motility, and cell differentiation. Here, we show that CAMSAP2 stabilizes and attaches microtubule minus ends to the Golgi through a complex of AKAP450 and myomegalin. CLASPs stabilize CAMSAP2-decorated microtubules but are not required for their Golgi tethering. AKAP450 is also essential for Golgi microtubule nucleation, and myomegalin and CDK5RAP2 but not CAMSAP2 contribute to this function. In the absence of centrosomes, AKAP450- and CAMSAP2-dependent pathways of microtubule minus-end organization become dominant, and the presence of at least one of them is needed to maintain microtubule density. Strikingly, a compact Golgi can be assembled in the absence of both centrosomal and Golgi microtubules. However, CAMSAP2- and AKAP450-dependent Golgi microtubules facilitate Golgi reorientation and cell invasion in a 3D matrix. We propose that Golgi-anchored microtubules are important for polarized cell movement but not for coalescence of Golgi membranes.

BRAF(V600E) Kinase Domain Duplication Identified in Therapy-Refractory Melanoma Patient-Derived Xenografts.

The therapeutic landscape of melanoma is improving rapidly. Targeted inhibitors show promising results, but drug resistance often limits durable clinical responses. There is a need for in vivo systems that allow for mechanistic drug resistance studies and (combinatorial) treatment optimization. Therefore, we established a large collection of patient-derived xenografts (PDXs), derived from BRAF(V600E), NRAS(Q61), or BRAF(WT)/NRAS(WT) melanoma metastases prior to treatment with BRAF inhibitor and after resistance had occurred. Taking advantage of PDXs as a limitless source, we screened tumor lysates for resistance mechanisms. We identified a BRAF(V600E) protein harboring a kinase domain duplication (BRAF(V600E/DK)) in ∼10% of the cases, both in PDXs and in an independent patient cohort. While BRAF(V600E/DK) depletion restored sensitivity to BRAF inhibition, a pan-RAF dimerization inhibitor effectively eliminated BRAF(V600E/DK)-expressing cells. These results illustrate the utility of this PDX platform and warrant clinical validation of BRAF dimerization inhibitors for this group of melanoma patients.