Regulated alternative polyadenylation is an important feature of gene expression, but how gene transcription rate affects this process remains to be investigated. polo is a cell-cycle gene that uses two poly(A) signals in the 3 0 untranslated region (UTR) to produce alternative messenger RNAs that differ in their 3 0 UTR length. Using a mutant Drosophila strain that has a lower transcriptional elongation rate, we show that transcription kinetics can determine alternative poly(A) site selection. The physiological consequences of incorrect polo poly(A) site choice are of vital importance; transgenic flies lacking the distal poly(A) signal cannot produce the longer transcript and die at the pupa stage due to a failure in the proliferation of the precursor cells of the abdomen, the histoblasts. This is due to the low translation efficiency of the shorter transcript produced by proximal poly(A) site usage. Our results show that correct polo poly(A) site selection functions to provide the correct levels of protein expression necessary for histoblast proliferation, and that the kinetics of RNA polymerase II have an important role in the mechanism of alternative polyadenylation.

Background: PEX5 binds newly synthesized peroxisomal proteins in the cytosol and releases them in the organelle matrix. Results: PEX5 binds monomeric catalase and releases it in the presence of PEX14. Conclusion: PEX14 participates in the cargo release step. Significance: Knowing how PEX5 interacts with cargo proteins and which factors disrupt this interaction are crucial for understanding this protein sorting pathway.

Background:The mammalian deubiquitinase that hydrolyzes the ubiquitin-PEX5 thioester conjugate was unknown. Results: USP9X was found to be the most active deubiquitinase acting on ubiquitin-PEX5. Conclusion:We propose that USP9X participates in the PEX5-mediated peroxisomal protein import pathway. Significance: The unbiased biochemical strategy described here will be useful to identify deubiquitinases acting on other substrates.

Background: How the soluble receptor PEX5 delivers its cargoes to the peroxisome remains largely unknown. Results: Cargo translocation occurs after docking of the receptor at the peroxisome and before any ATP-dependent step. Conclusion: Translocation is concomitant with PEX5 insertion into the docking/translocation machinery. Significance: These results support a model in which cargoes are pushed across the peroxisomal membrane by PEX5.

The peroxisomal protein import machinery displays remarkable properties. Be it its capacity to accept already folded proteins as substrates, its complex architecture or its energetics, almost every aspect of this machinery seems unique. The list of unusual properties is still growing as shown by the recent finding that one of its central components, Pex5p, is transiently monoubiquitinated at a cysteine residue. However, the data gathered in recent years also suggest that the peroxisomal import machinery is not that exclusive and similarities with p97/Cdc48-mediated processes and with multisubunit RING-E3 ligases are starting to emerge. Here, we discuss these data trying to distill the principles by which this complex machinery operates.

Essentials Recently, ADAMTS‐13 has been shown to undergo substrate induced conformation activation.Conformational quiescence of ADAMTS‐13 may serve to prevent off‐target proteolysis in plasma.Conformationally active ADAMTS‐13 variants are capable of proteolysing the Aα chain of fibrinogen.This should be considered as ADAMTS‐13 variants are developed as potential therapeutic agents. Click to hear Dr Zheng's presentation on structure function and cofactor-dependent regulation of ADAMTS‐13 SummaryBackgroundRecent work has revealed that ADAMTS‐13 circulates in a ‘closed’ conformation, only fully interacting with von Willebrand factor (VWF) following a conformational change. We hypothesized that this conformational quiescence also maintains the substrate specificity of ADAMTS‐13 and that the ‘open’ conformation of the protease might facilitate proteolytic promiscuity.ObjectivesTo identify a novel substrate for a constitutively active gain of function (GoF) ADAMTS‐13 variant (R568K/F592Y/R660K/Y661F/Y665F).MethodsFibrinogen proteolysis was characterized using SDS PAGE and liquid chromatography‐tandem mass spectrometry (LC‐MS/MS). Fibrin formation was monitored by turbidity measurements and fibrin structure visualized by confocal microscopy.Results ADAMTS‐13 exhibits proteolytic activity against the Aα chain of human fibrinogen, but this is only manifest on its conformational activation. Accordingly, the GoF ADAMTS‐13 variant and truncated variants such as MDTCS exhibit this activity. The cleavage site has been determined by LC‐MS/MS to be Aα chain Lys225‐Met226. Proteolysis of fibrinogen by GoF ADAMTS‐13 impairs fibrin formation in plasma‐based assays, alters clot structure and increases clot permeability. Although GoF ADAMTS‐13 does not appear to proteolyse preformed cross‐linked fibrin, its proteolytic activity against fibrinogen increases the susceptibility of fibrin to tissue‐type plasminogen activator (t‐PA)‐induced lysis by plasmin and increases the fibrin clearance rate more than 8‐fold compared with wild‐type (WT) ADAMTS‐13 (EC50 values of 3.0 ± 1.7 nm and 25.2 ± 9.7 nm, respectively) in in vitro thrombosis models.ConclusionThe ‘closed’ conformation of ADAMTS‐13 restricts its specificity and protects against fibrinogenolysis. Induced substrate promiscuity will be important as ADAMTS‐13 variants are developed as potential therapeutic agents against thrombotic thrombocytopenic purpura (TTP) and other cardiovascular diseases.

SummaryArgininosuccinate synthase 1 (ASS1) is the rate-limiting enzyme for arginine biosynthesis. ASS1 expression is lost in a range of tumor types, including 50% of malignant pleural mesotheliomas. Starving ASS1-deficient cells of arginine with arginine blockers such as ADI-PEG20 can induce selective lethality and has shown great promise in the clinical setting. We have generated a model of ADI-PEG20 resistance in mesothelioma cells. This resistance is mediated through re-expression of ASS1 via demethylation of the ASS1 promoter. Through coordinated transcriptomic and metabolomic profiling, we have shown that ASS1-deficient cells have decreased levels of acetylated polyamine metabolites, together with a compensatory increase in the expression of polyamine biosynthetic enzymes. Upon arginine deprivation, polyamine metabolites are decreased in the ASS1-deficient cells and in plasma isolated from ASS1-deficient mesothelioma patients. We identify a synthetic lethal dependence between ASS1 deficiency and polyamine metabolism, which could potentially be exploited for the treatment of ASS1-negative cancers.

Peroxisomal matrix proteins are synthesized on cytosolic ribosomes and transported by the shuttling receptor PEX5 to the peroxisomal membrane docking/translocation machinery, where they are translocated into the organelle matrix. Under certain experimental conditions this protein import machinery has the remarkable capacity to accept already oligomerized proteins, a property that has heavily influenced current models on the mechanism of peroxisomal protein import. However, whether or not oligomeric proteins are really the best and most frequent clients of this machinery remain unclear. In this work, we present three lines of evidence suggesting that the peroxisomal import machinery displays a preference for monomeric proteins. First, in agreement with previous findings on catalase, we show that PEX5 binds newly synthesized (monomeric) acyl-CoA oxidase 1 (ACOX1) and urate oxidase (UOX), potently inhibiting their oligomerization. Second, in vitro import experiments suggest that monomeric ACOX1 and UOX are better peroxisomal import substrates than the corresponding oligomeric forms. Finally, we provide data strongly suggesting that although ACOX1 lacking a peroxisomal targeting signal can be imported into peroxisomes when co-expressed with ACOX1 containing its targeting signal, this import pathway is inefficient.