TCTP regulates genotoxic stress and tumorigenicity via intercellular vesicular signaling.

Oncogenic intercellular signaling is regulated by extracellular vesicles (EVs), but the underlying mechanisms remain mostly unclear. Since TCTP (translationally controlled tumor protein) is an EV component, we investigated whether it has a role in genotoxic stress signaling and malignant transformation. By generating a Tctp-inducible knockout mouse model (Tctp-/f-), we report that Tctp is required for genotoxic stress-induced apoptosis signaling via small EVs (sEVs). Human breast cancer cells knocked-down for TCTP show impaired spontaneous EV secretion, thereby reducing sEV-dependent malignant growth. Since Trp53-/- mice are prone to tumor formation, we derived tumor cells from Trp53-/-;Tctp-/f- double mutant mice and describe a drastic decrease in tumori-genicity with concomitant decrease in sEV secretion and content. Remarkably, Trp53-/-;Tctp-/f- mice show highly prolonged survival. Treatment of Trp53-/- mice with sertraline, which inhibits TCTP function, increases their survival. Mechanistically, TCTP binds DDX3, recruiting RNAs, including miRNAs, to sEVs. Our findings establish TCTP as an essential protagonist in the regulation of sEV-signaling in the context of apoptosis and tumorigenicity.

Polypyrimidine-Tract-Binding Protein Isoforms Differentially Regulate the Hepatitis C Virus Internal Ribosome Entry Site.

Translation initiation of the hepatitis C virus (HCV) mRNA depends on an internal ribosome entry site (IRES) that encompasses most of the 5'UTR and includes nucleotides of the core coding region. This study shows that the polypyrimidine-tract-binding protein (PTB), an RNA-binding protein with four RNA recognition motifs (RRMs), binds to the HCV 5'UTR, stimulating its IRES activity. There are three isoforms of PTB: PTB1, PTB2, and PTB4. Our results show that PTB1 and PTB4, but not PTB2, stimulate HCV IRES activity in HuH-7 and HEK293T cells. In HuH-7 cells, PTB1 promotes HCV IRES-mediated initiation more strongly than PTB4. Mutations in PTB1, PTB4, RRM1/RRM2, or RRM3/RRM4, which disrupt the RRM's ability to bind RNA, abrogated the protein's capacity to stimulate HCV IRES activity in HuH-7 cells. In HEK293T cells, PTB1 and PTB4 stimulate HCV IRES activity to similar levels. In HEK293T cells, mutations in RRM1/RRM2 did not impact PTB1's ability to promote HCV IRES activity; and mutations in PTB1 RRM3/RRM4 domains reduced, but did not abolish, the protein's capacity to stimulate HCV IRES activity. In HEK293T cells, mutations in PTB4 RRM1/RRM2 abrogated the protein's ability to promote HCV IRES activity, and mutations in RRM3/RRM4 have no impact on PTB4 ability to enhance HCV IRES activity. Therefore, PTB1 and PTB4 differentially stimulate the IRES activity in a cell type-specific manner. We conclude that PTB1 and PTB4, but not PTB2, act as IRES transacting factors of the HCV IRES.

HIV-1 gRNA, a biological substrate, uncovers the potency of DDX3X biochemical activity.

DEAD-box helicases play central roles in the metabolism of many RNAs and ribonucleoproteins by assisting their synthesis, folding, function and even their degradation or disassembly. They have been implicated in various phenomena, and it is often difficult to rationalize their molecular roles from in vivo studies. Once purified in vitro, most of them only exhibit a marginal activity and poor specificity. The current model is that they gain specificity and activity through interaction of their intrinsically disordered domains with specific RNA or proteins. DDX3 is a DEAD-box cellular helicase that has been involved in several steps of the HIV viral cycle, including transcription, RNA export to the cytoplasm and translation. In this study, we investigated DDX3 biochemical properties in the context of a biological substrate. DDX3 was overexpressed, purified and its enzymatic activities as well as its RNA binding properties were characterized using both model substrates and a biological substrate, HIV-1 gRNA. Biochemical characterization of DDX3 in the context of a biological substrate identifies HIV-1 gRNA as a rare example of specific substrate and unravels the extent of DDX3 ATPase activity. Analysis of DDX3 binding capacity indicates an unexpected dissociation between its binding capacity and its biochemical activity. We further demonstrate that interaction of DDX3 with HIV-1 gRNA relies both on specific RNA determinants and on the disordered N- and C-terminal regions of the protein. These findings shed a new light regarding the potentiality of DDX3 biochemical activity supporting its multiple cellular functions.

Distinct roles for the IIId2 sub-domain in pestivirus and picornavirus internal ribosome entry sites.

Viral internal ribosomes entry site (IRES) elements coordinate the recruitment of the host translation machinery to direct the initiation of viral protein synthesis. Within hepatitis C virus (HCV)-like IRES elements, the sub-domain IIId(1) is crucial for recruiting the 40S ribosomal subunit. However, some HCV-like IRES elements possess an additional sub-domain, termed IIId2, whose function remains unclear. Herein, we show that IIId2 sub-domains from divergent viruses have different functions. The IIId2 sub-domain present in Seneca valley virus (SVV), a picornavirus, is dispensable for IRES activity, while the IIId2 sub-domains of two pestiviruses, classical swine fever virus (CSFV) and border disease virus (BDV), are required for 80S ribosomes assembly and IRES activity. Unlike in SVV, the deletion of IIId2 from the CSFV and BDV IRES elements impairs initiation of translation by inhibiting the assembly of 80S ribosomes. Consequently, this negatively affects the replication of CSFV and BDV. Finally, we show that the SVV IIId2 sub-domain is required for efficient viral RNA synthesis and growth of SVV, but not for IRES function. This study sheds light on the molecular evolution of viruses by clearly demonstrating that conserved RNA structures, within distantly related RNA viruses, have acquired different roles in the virus life cycles.

Two ribosome recruitment sites direct multiple translation events within HIV1 Gag open reading frame.

In the late phase of the HIV virus cycle, the unspliced genomic RNA is exported to the cytoplasm for the necessary translation of the Gag and Gag-pol polyproteins. Three distinct translation initiation mechanisms ensuring Gag production have been described with little rationale for their multiplicity. The Gag-IRES has the singularity to be located within Gag ORF and to directly interact with ribosomal 40S. Aiming at elucidating the specificity and the relevance of this interaction, we probed HIV-1 Gag-IRES structure and developed an innovative integrative modelling strategy to take into account all the gathered information. We propose a novel Gag-IRES secondary structure strongly supported by all experimental data. We further demonstrate the presence of two regions within Gag-IRES that independently and directly interact with the ribosome. Importantly, these binding sites are functionally relevant to Gag translation both in vitro and ex vivo. This work provides insight into the Gag-IRES molecular mechanism and gives compelling evidence for its physiological importance. It allows us to propose original hypotheses about the IRES physiological role and conservation among primate lentiviruses.

LOOP IIId of the HCV IRES is essential for the structural rearrangement of the 40S-HCV IRES complex.

As obligatory intracellular parasites, viruses rely on cellular machines to complete their life cycle, and most importantly they recruit the host ribosomes to translate their mRNA. The Hepatitis C viral mRNA initiates translation by directly binding the 40S ribosomal subunit in such a way that the initiation codon is correctly positioned in the P site of the ribosome. Such a property is likely to be central for many viruses, therefore the description of host-pathogen interaction at the molecular level is instrumental to provide new therapeutic targets. In this study, we monitored the 40S ribosomal subunit and the viral RNA structural rearrangement induced upon the formation of the binary complex. We further took advantage of an IRES viral mutant mRNA deficient for translation to identify the interactions necessary to promote translation. Using a combination of structure probing in solution and molecular modeling we establish a whole atom model which appears to be very similar to the one obtained recently by cryoEM. Our model brings new information on the complex, and most importantly reveals some structural rearrangement within the ribosome. This study suggests that the formation of a 'kissing complex' between the viral RNA and the 18S ribosomal RNA locks the 40S ribosomal subunit in a conformation proficient for translation.

40S recruitment in the absence of eIF4G/4A by EMCV IRES refines the model for translation initiation on the archetype of Type II IRESs.

Initiation of translation on Type II IRESs, such as those of EMCV and FMDV viruses, has been well documented in the recent years. For EMCV, the current model argues for a mechanism in which the key interaction necessary for the pre-initiation complex recruitment is eIF4G binding to the central J-K domains of EMCV-IRES. Here we demonstrate that, in contrast with the current model, the molecular mechanism of EMCV-IRES involves direct recruitment of the 40S subunit. Importantly, we identified a specific structural element that prevents the correct positioning of the initiation codon in the close vicinity of the ribosomal P site. This work clarifies how this interaction could not be anticipated by earlier studies and allows us to propose a new model for initiation complex assembly on EMCV-IRES. The role attributed to eIF4G/4A can thus be refined as stabilizing/promoting the conformational changes that are necessary for IRES function, thus resembling the role conventionally assigned to ITAFs. This raises the interesting possibility that IRESs are primarily ribosome binders, some of which having partly lost the ability to fold into the active structure without the help of proteins.

Combined approaches for drug design points the way to novel proline racemase inhibitor candidates to fight Chagas' disease.

Chagas' disease is caused by Trypanosoma cruzi, a protozoan transmitted to humans by blood-feeding insects, blood transfusion or congenitally. Previous research led us to discover a parasite proline racemase (TcPRAC) and to establish its validity as a target for the design of new chemotherapies against the disease, including its chronic form. A known inhibitor of proline racemases, 2-pyrrolecarboxylic acid (PYC), is water-insoluble. We synthesized soluble pyrazole derivatives, but they proved weak or inactive TcPRAC inhibitors. TcPRAC catalytic site is too small and constrained when bound to PYC to allow efficient search for new inhibitors by virtual screening. Forty-nine intermediate conformations between the opened enzyme structure and the closed liganded one were built by calculating a transition path with a method we developed. A wider range of chemical compounds could dock in the partially opened intermediate active site models in silico. Four models were selected for known substrates and weak inhibitors could dock in them and were used to screen chemical libraries. Two identified soluble compounds, (E)-4-oxopent-2-enoic acid (OxoPA) and its derivative (E)-5-bromo-4-oxopent-2-enoic acid (Br-OxoPA), are irreversible competitive inhibitors that presented stronger activity than PYC on TcPRAC. We show here that increasing doses of OxoPA and Br-OxoPA hamper T. cruzi intracellular differentiation and fate in mammalian host cells. Our data confirm that through to their binding mode, these molecules are interesting and promising as lead compounds for the development of chemotherapies against diseases where active proline racemases play essential roles.

Ribosomal Initiation Complex Assembly within the Wild-Strain of Coxsackievirus B3 and Live-Attenuated Sabin3-like IRESes during the Initiation of Translation.

Coxsackievirus B3 (CVB3) is an enterovirus of the family of Picornaviridae. The Group B coxsackieviruses include six serotypes (B1 to B6) that cause a variety of human diseases, including myocarditis, meningitis, and diabetes. Among the group B, the B3 strain is mostly studied for its cardiovirulence and its ability to cause acute and persistent infections. Translation initiation of CVB3 RNA has been shown to be mediated by a highly ordered structure of the 5'-untranslated region (5'UTR), which harbors an internal ribosome entry site (IRES). Translation initiation is a complex process in which initiator tRNA, 40S and 60S ribosomal subunits are assembled by eukaryotic initiation factors (eIFs) into an 80S ribosome at the initiation codon of the mRNA. We have previously addressed the question of whether the attenuating mutations of domain V of the poliovirus IRES were specific for a given genomic context or whether they could be transposed and extrapolated to a genomic related virus, i.e., CVB3 wild-type strain. In this context, we have described that Sabin3-like mutation (U473→C) introduced in CVB3 genome led to a defective mutant with a serious reduction in translation efficiency. In this study, we analyzed the efficiency of formation of ribosomal initiation complexes 48S and 80S through 10%-30% and 10%-50% sucrose gradients using rabbit reticulocyte lysates (RRLs) and stage-specific translation inhibitors: 5'-Guanylyl-imidodiphosphate (GMP-PNP) and Cycloheximide (CHX), respectively. We demonstrated that the interaction of 48S and 80S ribosomal complexes within the mutant CVB3 RNA was abolished compared with the wild-type RNA by ribosome assembly analysis. Taken together, it is possible that the mutant RNA was unable to interact with some trans-acting factors critical for enhanced IRES function.

In vitro studies reveal that different modes of initiation on HIV-1 mRNA have different levels of requirement for eukaryotic initiation factor 4F.

Expression of the two isoforms p55 and p40 of HIV-1 Gag proteins relies on distinct translation initiation mechanisms, a cap-dependent initiation and two internal ribosome entry sites (IRESs). The regulation of these processes is complex and remains poorly understood. This study was focused on the influence of the 5'-UTR and on the requirement for the eukaryotic initiation factor (eIF)4F complex components. By using an in vitro system, we showed substantial involvement of the 5'-UTR in the control of p55 expression. This highly structured 5'-UTR requires the eIF4F complex, especially RNA helicase eIF4A, which mediates initiation at the authentic AUG codon. In addition, the 5'-UTR regulates expression in an RNA concentration-dependent manner, with a high concentration of RNA triggering specific reduction of full-length Gag p55 production. HIV-1 genomic RNA also has the ability to use a strong IRES element located in the gag coding region. We show that this mechanism is particularly efficient, and that activity of this IRES is only poorly dependent on RNA helicase eIF4A when the viral 5'-UTR is removed. HIV-1 genomic mRNA exhibits in vitro translational features that allow the expression of Gag p55 protein by different mechanisms that involve different requirements for eIF4E, eIF4G, and eIF4A. This suggests that HIV-1 could adapt to its mode of translation according to the availability of the initiation factors in the infected cell.

Structural investigation of HIV-1 genomic RNA dimerization process reveals a role for the Major Splice-site Donor stem loop.

The 5'UnTranslated Region (5'UTR) of HIV-1 genomic RNA, which precedes the Gag coding sequence, fulfills several roles during the lentivirus life cycle. This 335 nucleotides leader contains many stable structures that are crucial for the regulation of genetic expression at the level of transcription, splicing, and translation. In the late steps of the virus cycle, i.e. virions formation, the genomic RNA serves as propagated genome and its encapsidation in new particles relies on its ability to form non-covalent dimers. Dimerization is proposed to be initiated by the intermolecular pairing of a self-complementary sequence located in the apical loop of the DIS hairpin (Dimer Initiation Sequence). The regulation of this phenomenon and the extraordinary stability of the dimers imply that structural elements other than this kissing complex remain to be identified. Here, we show that swapping the Gag open reading frame (ORF) by reporter genes interferes with dimers formation efficiency. Importantly, the nature of the ORF alters specific structures of the 5'UTR. By using a systematic "SHAPE" approach, we pointed out that sequences within the Major Splice Site are involved in the dimerization process. Furthermore, by the use of an antisense oligonucleotide specific for the MSD associated to a SHAPE analysis of the 5'UTR structure, we demonstrated that interfering with the MSD results both in an impaired dimerization and in modifications of the 5'UTR structure. All together these data support a recently proposed model in which intramolecular base pairings are important determinants for the dimerization process. We further conclude that much care should be taken when comparing translation activity of reporter constructs with the viral situation.

A conserved structure within the HIV gag open reading frame that controls translation initiation directly recruits the 40S subunit and eIF3.

Translation initiation on HIV genomic RNA relies on both cap and Internal Ribosome Entry Site (IRES) dependant mechanisms that are regulated throughout the cell cycle. During a unique phenomenon, the virus recruits initiation complexes through RNA structures located within Gag coding sequence, downstream of the initiation codon. We analyzed initiation complexes paused on the HIV-2 gag IRES and revealed that they contain all the canonical initiation factors except eIF4E and eIF1. We report that eIF3 and the small ribosomal subunit bind HIV RNA within gag open reading frame. We thus propose a novel two step model whereby the initial event is the formation of a ternary eIF3/40S/IRES complex. In a second step, dependent on most of the canonical initiation factors, the complex is rearranged to transfer the ribosome on the initiation codons. The absolute requirement of this large structure for HIV translation defines a new function for a coding region. Moreover, the level of information compaction within this viral genome reveals an additional level of evolutionary constraint on the coding sequence. The conservation of this IRES and its properties in rapidly evolving viruses suggest an important role in the virus life cycle and highlight an attractive new therapeutic target.

Genetic engineering of Trypanosoma (Dutonella) vivax and in vitro differentiation under axenic conditions.

Trypanosoma vivax is one of the most common parasites responsible for animal trypanosomosis, and although this disease is widespread in Africa and Latin America, very few studies have been conducted on the parasite's biology. This is in part due to the fact that no reproducible experimental methods had been developed to maintain the different evolutive forms of this trypanosome under laboratory conditions. Appropriate protocols were developed in the 1990s for the axenic maintenance of three major animal Trypanosoma species: T. b. brucei, T. congolense and T. vivax. These pioneer studies rapidly led to the successful genetic manipulation of T. b. brucei and T. congolense. Advances were made in the understanding of these parasites' biology and virulence, and new drug targets were identified. By contrast, challenging in vitro conditions have been developed for T. vivax in the past, and this per se has contributed to defer both its genetic manipulation and subsequent gene function studies. Here we report on the optimization of non-infective T. vivax epimastigote axenic cultures and on the process of parasite in vitro differentiation into metacyclic infective forms. We have also constructed the first T. vivax specific expression vector that drives constitutive expression of the luciferase reporter gene. This vector was then used to establish and optimize epimastigote transfection. We then developed highly reproducible conditions that can be used to obtain and select stably transfected mutants that continue metacyclogenesis and are infectious in immunocompetent rodents.

The different pathways of HIV genomic RNA translation.

Lentiviruses, the prototype of which is HIV-1, can initiate translation either by the classical cap-dependent mechanism or by internal recruitment of the ribosome through RNA domains called IRESs (internal ribosome entry sites). Depending on the virus considered, the mechanism of IRES-dependent translation differs widely. It can occur by direct binding of the 40S subunit to the mRNA, necessitating a subset or most of the canonical initiation factors and/or ITAF (IRES trans-acting factors). Nonetheless, a common feature of IRESs is that ribosomal recruitment relies, at least in part, on IRES structural determinants. Lentiviral genomic RNAs present an additional level of complexity, as, in addition to the 5'-UTR (untranslated region) IRES, the presence of a new type of IRES, embedded within Gag coding region was described recently. This IRES, conserved in all three lentiviruses examined, presents conserved structural motifs that are crucial for its activity, thus reinforcing the link between RNA structure and function. However, there are still important gaps in our understanding of the molecular mechanism underlying IRES-dependent translation initiation of HIV, including the determination of the initiation factors required, the dynamics of initiation complex assembly and the dynamics of the RNA structure during initiation complex formation. Finally, the ability of HIV genomic RNA to initiate translation through different pathways questions the possible mechanisms of regulation and their correlation to the viral paradigm, i.e. translation versus encapsidation of its genomic RNA.

Trypanosoma vivax infections: pushing ahead with mouse models for the study of Nagana. I. Parasitological, hematological and pathological parameters.

African trypanosomiasis is a severe parasitic disease that affects both humans and livestock. Several different species may cause animal trypanosomosis and although Trypanosoma vivax (sub-genus Duttonella) is currently responsible for the vast majority of debilitating cases causing great economic hardship in West Africa and South America, little is known about its biology and interaction with its hosts. Relatively speaking, T. vivax has been more than neglected despite an urgent need to develop efficient control strategies. Some pioneering rodent models were developed to circumvent the difficulties of working with livestock, but disappointedly were for the most part discontinued decades ago. To gain more insight into the biology of T. vivax, its interactions with the host and consequently its pathogenesis, we have developed a number of reproducible murine models using a parasite isolate that is infectious for rodents. Firstly, we analyzed the parasitical characteristics of the infection using inbred and outbred mouse strains to compare the impact of host genetic background on the infection and on survival rates. Hematological studies showed that the infection gave rise to severe anemia, and histopathological investigations in various organs showed multifocal inflammatory infiltrates associated with extramedullary hematopoiesis in the liver, and cerebral edema. The models developed are consistent with field observations and pave the way for subsequent in-depth studies into the pathogenesis of T. vivax - trypanosomosis.

Trypanosoma vivax infections: pushing ahead with mouse models for the study of Nagana. II. Immunobiological dysfunctions.

Trypanosoma vivax is the main species involved in trypanosomosis, but very little is known about the immunobiology of the infective process caused by this parasite. Recently we undertook to further characterize the main parasitological, haematological and pathological characteristics of mouse models of T. vivax infection and noted severe anemia and thrombocytopenia coincident with rising parasitemia. To gain more insight into the organism's immunobiology, we studied lymphocyte populations in central (bone marrow) and peripherical (spleen and blood) tissues following mouse infection with T. vivax and showed that the immune system apparatus is affected both quantitatively and qualitatively. More precisely, after an initial increase that primarily involves CD4(+) T cells and macrophages, the number of splenic B cells decreases in a step-wise manner. Our results show that while infection triggers the activation and proliferation of Hematopoietic Stem Cells, Granulocyte-Monocyte, Common Myeloid and Megacaryocyte Erythrocyte progenitors decrease in number in the course of the infection. An in-depth analysis of B-cell progenitors also indicated that maturation of pro-B into pre-B precursors seems to be compromised. This interferes with the mature B cell dynamics and renewal in the periphery. Altogether, our results show that T. vivax induces profound immunological alterations in myeloid and lymphoid progenitors which may prevent adequate control of T. vivax trypanosomosis.

A new type of IRES within gag coding region recruits three initiation complexes on HIV-2 genomic RNA.

Genomic RNA of primate lentiviruses serves both as an mRNA that encodes Gag and Gag-Pol polyproteins and as a propagated genome. Translation of this RNA is initiated by standard cap dependant mechanism or by internal entry of the ribosome. Two regions of the genomic RNA are able to attract initiation complexes, the 5' untranslated region and the gag coding region itself. Relying on probing data and a phylogenetic study, we have modelled the secondary structure of HIV-1, HIV-2 and SIV(Mac) coding region. This approach brings to light conserved secondary-structure elements that were shown by mutations to be required for internal entry of the ribosome. No structural homologies with other described viral or cellular IRES can be identified and lentiviral IRESes show many peculiar properties. Most notably, the IRES present in HIV-2 gag coding region is endowed with the unique ability to recruit up to three initiation complexes on a single RNA molecule. The structural and functional properties of gag coding sequence define a new type of IRES. Although its precise role is unknown, the conservation of the IRES among fast evolving lentiviruses suggests an important physiological role.

RNA interference in Trypanosoma brucei: role of the n-terminal RGG domain and the polyribosome association of argonaute.

Argonaute proteins (AGOs) are central to RNA interference (RNAi) and related silencing pathways. At the core of the RNAi pathway in the ancient parasitic eukaryote Trypanosoma brucei is a single Argonaute protein, TbAGO1, with an established role in the destruction of potentially harmful retroposon transcripts. One notable feature of TbAGO1 is that a fraction sediments with polyribosomes, and this association is facilitated by an arginine/glycine-rich domain (RGG domain) at the N terminus of the protein. Here we report that reducing the size of the RGG domain and, in particular, mutating all arginine residues severely reduced the association of TbAGO1 with polyribosomes and RNAi-induced cleavage of mRNA. However, these mutations did not change the cellular localization of Argonaute and did not affect the accumulation of single-stranded siRNAs, an essential step in the activation of the RNA-induced silencing complex. We further show that mRNA on polyribosomes can be targeted for degradation, although this alliance is not a pre-requisite. Finally, sequestering tubulin mRNAs from translation with antisense morpholino oligonucleotides reduced the RNAi response indicating that mRNAs not engaged in translation may be less accessible to the RNAi machinery. We conclude that the association of the RNAi machinery and target mRNA on polyribosomes promotes an efficient RNAi response. This mechanism may represent an ancient adaptation to ensure that retroposon transcripts are efficiently destroyed, if they become associated with the translational apparatus.

Proline racemases: insights into Trypanosoma cruzi peptides containing D-proline.

Trypanosoma cruzi proline racemases (TcPRAC) are homodimeric enzymes that interconvert the L and D-enantiomers of proline. At least two paralogous copies of proline racemase (PR) genes are present per parasite haploid genome and they are differentially expressed during T. cruzi development. Non-infective epimastigote forms that overexpress PR genes differentiate more readily into metacyclic infective forms that are more invasive to host cells, indicating that PR participates in mechanisms of virulence acquisition. Using a combination of biochemical and enzymatic methods, we show here that, in addition to free D-amino acids, non-infective epimastigote and infective metacyclic parasite extracts possess peptides composed notably of D-proline. The relative contribution of TcPRAC to D-proline availability and its further assembly into peptides was estimated through the use of wild-type parasites and parasites over-expressing TcPRAC genes. Our data suggest that D-proline-bearing peptides, similarly to the mucopeptide layer of bacterial cell walls, may be of benefit to T. cruzi by providing resistance against host proteolytic mechanisms.