Identification of two pathways mediating protein targeting from ER to lipid droplets.

Pathways localizing proteins to their sites of action are essential for eukaryotic cell organization and function. Although mechanisms of protein targeting to many organelles have been defined, how proteins, such as metabolic enzymes, target from the endoplasmic reticulum (ER) to cellular lipid droplets (LDs) is poorly understood. Here we identify two distinct pathways for ER-to-LD protein targeting: early targeting at LD formation sites during formation, and late targeting to mature LDs after their formation. Using systematic, unbiased approaches in Drosophila cells, we identified specific membrane-fusion machinery, including regulators, a tether and SNARE proteins, that are required for the late targeting pathway. Components of this fusion machinery localize to LD-ER interfaces and organize at ER exit sites. We identified multiple cargoes for early and late ER-to-LD targeting pathways. Our findings provide a model for how proteins target to LDs from the ER either during LD formation or by protein-catalysed formation of membrane bridges.

Mrx6 regulates mitochondrial DNA copy number in Saccharomyces cerevisiae by engaging the evolutionarily conserved Lon protease Pim1.

Mitochondrial function depends crucially on the maintenance of multiple mitochondrial DNA (mtDNA) copies. Surprisingly, the cellular mechanisms regulating mtDNA copy number remain poorly understood. Through a systematic high-throughput approach in Saccharomyces cerevisiae, we determined mtDNA-to-nuclear DNA ratios in 5148 strains lacking nonessential genes. The screen revealed MRX6, a largely uncharacterized gene, whose deletion resulted in a marked increase in mtDNA levels, while maintaining wild type-like mitochondrial structure and cell size. Quantitative superresolution imaging revealed that deletion of MRX6 alters both the size and the spatial distribution of mtDNA nucleoids. We demonstrate that Mrx6 partially colocalizes with mtDNA within mitochondria and interacts with the conserved Lon protease Pim1 in a complex that also includes Mam33 and the Mrx6-related protein Pet20. Acute depletion of Pim1 phenocopied the high mtDNA levels observed in Δmrx6 cells. No further increase in mtDNA copy number was observed upon depletion of Pim1 in Δmrx6 cells, revealing an epistatic relationship between Pim1 and Mrx6. Human and bacterial Lon proteases regulate DNA replication by degrading replication initiation factors, suggesting a model in which Pim1 acts similarly with the Mrx6 complex, providing a scaffold linking it to mtDNA.

Polyanions provide selective control of APC/C interactions with the activator subunit.

Transient interactions between the anaphase-promoting complex/cyclosome (APC/C) and its activator subunit Cdc20 or Cdh1 generate oscillations in ubiquitylation activity necessary to maintain the order of cell cycle events. Activator binds the APC/C with high affinity and exhibits negligible dissociation kinetics in vitro, and it is not clear how the rapid turnover of APC/C-activator complexes is achieved in vivo. Here, we describe a mechanism that controls APC/C-activator interactions based on the availability of substrates. We find that APC/C-activator dissociation is stimulated by abundant cellular polyanions such as nucleic acids and polyphosphate. Polyanions also interfere with substrate ubiquitylation. However, engagement with high-affinity substrate blocks the inhibitory effects of polyanions on activator binding and APC/C activity. We propose that this mechanism amplifies the effects of substrate affinity on APC/C function, stimulating processive ubiquitylation of high-affinity substrates and suppressing ubiquitylation of low-affinity substrates.

The pseudosubstrate inhibitor Acm1 inhibits the anaphase-promoting complex/cyclosome by combining high-affinity activator binding with disruption of Doc1/Apc10 function.

The anaphase-promoting complex/cyclosome (APC/C) is a large, multisubunit ubiquitin ligase involved in regulation of cell division. APC/C substrate specificity arises from binding of short degron motifs in its substrates to transient activator subunits, Cdc20 and Cdh1. The destruction box (D-box) is the most common APC/C degron and plays a crucial role in substrate degradation by linking the activator to the Doc1/Apc10 subunit of core APC/C to stabilize the active holoenzyme and promote processive ubiquitylation. Degrons are also employed as pseudosubstrate motifs by APC/C inhibitors, and pseudosubstrates must bind their cognate activators tightly to outcompete substrate binding while blocking their own ubiquitylation. Here we examined how APC/C activity is suppressed by the small pseudosubstrate inhibitor Acm1 from budding yeast (Saccharomyces cerevisiae). Mutation of a conserved D-box converted Acm1 into an efficient ABBA (cyclin A, BubR1, Bub1, Acm1) motif-dependent APC/CCdh1 substrate in vivo, suggesting that this D-box somehow inhibits APC/C. We then identified a short conserved sequence at the C terminus of the Acm1 D-box that was necessary and sufficient for APC/C inhibition. In several APC/C substrates, the corresponding D-box region proved to be important for their degradation despite poor sequence conservation, redefining the D-box as a 12-amino acid motif. Biochemical analysis suggested that the Acm1 D-box extension inhibits reaction processivity by perturbing the normal interaction with Doc1/Apc10. Our results reveal a simple, elegant mode of pseudosubstrate inhibition that combines high-affinity activator binding with specific disruption of Doc1/Apc10 function in processive ubiquitylation.

Genetic analysis reveals functions of atypical polyubiquitin chains.

Although polyubiquitin chains linked through all lysines of ubiquitin exist, specific functions are well-established only for lysine-48 and lysine-63 linkages in Saccharomyces cerevisiae. To uncover pathways regulated by distinct linkages, genetic interactions between a gene deletion library and a panel of lysine-to-arginine ubiquitin mutants were systematically identified. The K11R mutant had strong genetic interactions with threonine biosynthetic genes. Consistently, we found that K11R mutants import threonine poorly. The K11R mutant also exhibited a strong genetic interaction with a subunit of the anaphase-promoting complex (APC), suggesting a role in cell cycle regulation. K11-linkages are important for vertebrate APC function, but this was not previously described in yeast. We show that the yeast APC also modifies substrates with K11-linkages in vitro, and that those chains contribute to normal APC-substrate turnover in vivo. This study reveals comprehensive genetic interactomes of polyubiquitin chains and characterizes the role of K11-chains in two biological pathways.

Quantitative framework for ordered degradation of APC/C substrates.

BACKGROUND: During cell-cycle progression, substrates of a single master regulatory enzyme can be modified in a specific order. Here, we used experimental and computational approaches to dissect the quantitative mechanisms underlying the ordered degradation of the substrates of the ubiquitin ligase APC/C(Cdc20), a key regulator of chromosome segregation in mitosis. RESULTS: We show experimentally that the rate of catalysis varies with different substrates of APC/C(Cdc20). Using a computational model based on multi-step ubiquitination, we then show how changes in the interaction between a single substrate and APC/C(Cdc20) can alter the timing of degradation onset relative to APC/C(Cdc20) activation, while ensuring a fast degradation rate. Degradation timing and dynamics depend on substrate affinity for the enzyme as well as the catalytic rate at which the substrate is modified. When two substrates share the same pool of APC/C(Cdc20), their relative enzyme affinities and rates of catalysis influence the partitioning of APC/C(Cdc20) among substrates, resulting in substrate competition. Depending on how APC/C(Cdc20) is partitioned among its substrates, competition can have minor or major effects on the degradation of certain substrates. We show experimentally that increased expression of the early APC/C(Cdc20) substrate Clb5 does not delay the degradation of the later substrate securin, arguing against a role for competition with Clb5 in establishing securin degradation timing. CONCLUSIONS: The degradation timing of APC/C(Cdc20) substrates depends on the multi-step nature of ubiquitination, differences in substrate-APC/C(Cdc20) interactions, and competition among substrates. Our studies provide a conceptual framework for understanding how ordered modification can be established among substrates of the same regulatory enzyme, and facilitate our understanding of how precise temporal control is achieved by a small number of master regulators to ensure a successful cell division cycle.

Genetically engineered microvesicles carrying suicide mRNA/protein inhibit schwannoma tumor growth.

Microvesicles (MVs) play an important role in intercellular communication by carrying mRNAs, microRNAs (miRNAs), non-coding RNAs, proteins, and DNA from cell to cell. To our knowledge, this is the first report of delivery of a therapeutic mRNA/protein via MVs for treatment of cancer. We first generated genetically engineered MVs by expressing high levels of the suicide gene mRNA and protein-cytosine deaminase (CD) fused to uracil phosphoribosyltransferase (UPRT) in MV donor cells. MVs were isolated from these cells and used to treat pre-established nerve sheath tumors (schwannomas) in an orthotopic mouse model. We demonstrated that MV-mediated delivery of CD-UPRT mRNA/protein by direct injection into schwannomas led to regression of these tumors upon systemic treatment with the prodrug (5-fluorocytosine (5-FC)), which is converted within tumor cells to 5-fluorouracil (5-FU)-an anticancer agent. Taken together, these studies suggest that MVs can serve as novel cell-derived "liposomes" to effectively deliver therapeutic mRNA/proteins to treatment of diseases.

miR-1289 and "Zipcode"-like Sequence Enrich mRNAs in Microvesicles.

Despite intensive studies, the molecular mechanisms by which the genetic materials are uploaded into microvesicles (MVs) are still unknown. This is the first study describing a zipcode-like 25 nucleotide (nt) sequence in the 3'-untranslated region (3'UTR) of mRNAs, with variants of this sequence present in many mRNAs enriched in MVs, as compared to their glioblastoma cells of origin. When this sequence was incorporated into the 3'UTR of a reporter message and expressed in a different cell type, it led to enrichment of the reporter mRNA in MVs. Critical features of this sequence are both a CUGCC core presented on a stem-loop structure and a miRNA-binding site, with increased levels of the corresponding miRNA in cells further increasing levels of mRNAs in MVs.

miRNA-7 attenuation in Schwannoma tumors stimulates growth by upregulating three oncogenic signaling pathways.

Micro RNAs (miRNA) negatively regulate protein-coding genes at the posttranscriptional level and are critical in tumorigenesis. Schwannomas develop from proliferation of dedifferentiated Schwann cells, which normally wrap nerve fibers to help support and insulate nerves. In this study, we carried out high-throughput miRNA expression profiling of human vestibular schwannomas by using an array representing 407 known miRNAs to explore the role of miRNAs in tumor growth. Twelve miRNAs were found to be significantly deregulated in tumor samples as compared with control nerve tissue, defining a schwannoma-typical signature. Among these miRNAs, we focused on miR-7, which was one of the most downregulated in these tumors and has several known oncogene targets, including mRNAs for epidermal growth factor receptor (EGFR) and p21-activated kinase 1 (Pak1). We found that overexpression of miR-7 inhibited schwannoma cell growth both in culture and in xenograft tumor models in vivo, which correlated with downregulation of these signaling pathways. Furthermore, we identified a novel direct target of miR-7, the mRNA for associated cdc42 kinase 1 (Ack1), with the expression levels of miR-7 and Ack1 being inversely correlated in human schwannoma samples. These results represent the first miRNA profiling of schwannomas and the first report of a tumor suppressor function for miR-7 in these tumors that is mediated by targeting the EGFR, Pak1, and Ack1 oncogenes. Our findings suggest miR-7 as a potential therapeutic molecule for schwannoma treatment, and they prompt clinical evaluation of drugs that can inhibit the EGFR, Pak1, and Ack1 signaling pathways to treat this tumor type.

A novel imaging-compatible sciatic nerve schwannoma model.

Benign schwannomas are common tumors of the cranial and peripheral nerves, causing pain and loss of function. The development of effective therapy for these tumors has been hampered by the lack of relevant experimental in vivo models for convenient testing. Here, we describe a novel schwannoma model in which an immortalized human schwannoma cell line, HEI-193, established from an neurofibromatosis type 2 patient, has been stably transduced with fluorescent protein and luciferase reporters and implanted within the sciatic nerve of nude mice. These cells reliably formed a tumor within several weeks which had pathologic characteristics of schwannoma tumors. This model system will be useful for investigation of schwannoma biology and for preclinical assessment of therapeutic agents.