MAVS Positively Regulates Mitochondrial Integrity and Metabolic Fitness in B Cells

Activated B cells experience metabolic changes that require mitochondrial remodeling, in a process incompletely defined. In this study, we report that mitochondrial antiviral signaling protein (MAVS) is involved in BCR-initiated cellular proliferation and prolonged survival. MAVS is well known as a mitochondrial-tethered signaling adaptor with a central role in viral RNA-sensing pathways that induce type I IFN. The role of MAVS downstream of BCR stimulation was recognized in absence of IFN, indicative of a path for MAVS activation that is independent of viral infection. Mitochondria of BCR-activated MAVS-deficient mouse B cells exhibited a damaged phenotype including disrupted mitochondrial morphology, excess mitophagy, and the temporal progressive blunting of mitochondrial oxidative capacity with mitochondrial hyperpolarization and cell death. Costimulation of MAVS-deficient B cells with anti-CD40, in addition to BCR stimulation, partially corrected the mitochondrial structural defects and functionality. Our data reveal a (to our knowledge) previously unrecognized role of MAVS in controlling the metabolic fitness of B cells, most noticeable in the absence of costimulatory help.

Plasmodium curtails autoimmune nephritis via lasting bone marrow alterations, independent of hemozoin accumulation

The host response against infection with Plasmodium commonly raises self-reactivity as a side effect, and antibody deposition in kidney has been cited as a possible cause of kidney injury during severe malaria. In contrast, animal models show that infection with the parasite confers long-term protection from lethal lupus nephritis initiated by autoantibody deposition in kidney. We have limited knowledge of the factors that make parasite infection more likely to induce kidney damage in humans, or the mechanisms underlying protection from autoimmune nephritis in animal models. Our experiments with the autoimmune-prone FcγR2B[KO] mice have shown that a prior infection with P. yoelii 17XNL protects from end-stage nephritis for a year, even when overall autoreactivity and systemic inflammation are maintained at high levels. In this report we evaluate post-infection alterations, such as hemozoin accumulation and compensatory changes in immune cells, and their potential role in the kidney-specific protective effect by Plasmodium. We ruled out the role of pigment accumulation with the use of a hemozoin-restricted P. berghei ANKA parasite, which induced a self-resolved infection that protected from autoimmune nephritis with the same mechanism as parasitic infections that accumulated normal levels of hemozoin. In contrast, adoptive transfer experiments revealed that bone marrow cells were altered by the infection and could transmit the kidney protective effect to a new host. While changes in the frequency of bone marrow cell populations after infection were variable and unique to a particular parasite strain, we detected a sustained bias in cytokine/chemokine expression that suggested lower fibrotic potential and higher Th1 bias likely affecting multiple cell populations. Sustained changes in bone marrow cell activation profile could have repercussions in immune responses long after the infection was cleared.

CCL17-producing cDC2s are essential in end-stage lupus nephritis and averted by a parasitic infection

Lupus nephritis is a severe organ manifestation in systemic lupus erythematosus leading to kidney failure in a subset of patients. In lupus-prone mice, controlled infection with Plasmodium parasites protects against the progression of autoimmune pathology including lethal glomerulonephritis. Here, we demonstrate that parasite-induced protection was not due to a systemic effect of infection on autoimmunity as previously assumed, but rather to specific alterations in immune cell infiltrates into kidneys and renal draining lymph nodes. Infection of lupus-prone mice with a Plasmodium parasite did not reduce the levels or specificities of autoreactive antibodies, vasculitis, immune complex-induced innate activation, or hypoxia. Instead, infection uniquely reduced kidney-infiltrating CCL17-producing bone marrow-derived type 2 inflammatory dendritic cells (iDC2s). Bone marrow reconstitution experiments revealed that infection with Plasmodium caused alterations in bone marrow cells that hindered the ability of DC2s to infiltrate the kidneys. The essential role for CCL17 in lupus nephritis was confirmed by in vivo depletion with a blocking antibody, which reduced kidney pathology and immune infiltrates, while bypassing the need for parasitic infection. Therefore, infiltration into the kidneys of iDC2s, with the potential to prime local adaptive responses, is an essential regulated event in the transition from manageable glomerulonephritis to lethal tubular injury.

Systemic activation of B and CD4 but not CD8 cells drives early disease in lupus prone mice, while macrophages, CD4 and CD8 cells infiltrate the kidney with the exclusion of B cells

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease characterized by the presence of autoantibodies in serum and multi-organ inflammation. Immune complex deposition in kidney can lead to lupus nephritis, which is a major risk factor for morbidity and mortality in SLE. Using FcγRIIB−/− mice as a lupus animal model, we performed a detailed time course analysis of the extent of the spontaneous leukocyte activation and expansion in spleen and in renal, inguinal, mesenteric lymph nodes. We then correlated the data with the onset of pathology and leukocyte infiltration in the kidney. In the FcγRIIB−/− mouse model, effector-memory CD4 cells and germinal centers appeared first in the spleen and renal lymph nodes, but not in other immune organs such as inguinal or mesenteric lymph nodes. As the disease progresses in these mice, lymphocytes undergo an extensive expansion that involves all immune organs in a systemic manner. At the same moment we can detect the first signs of kidney pathology, measured by histopathology and by the levels of creatinine and albumin in urine. In FcγRIIB−/− mice, the onset of kidney pathology correlates with leukocyte infiltration in kidney, detected both by parenchyma-restricted flow cytometry and histopathology. Macrophages, CD4 and CD8 cells are found both in glomerular and interstitial spaces with the exclusion of B cells, most likely suggesting a role for T cells and/or macrophage specific chemokine signals in lupus nephritic kidney.

A single infection with a malaria parasite protects mice from lethal autoimmune glomerulonephritis

The progression of autoimmune diseases such as Systemic Lupus Erythematous (SLE) can be altered by environmental factors, including parasitic infections. Epidemiological studies have shown a relative absence of SLE in West Africa (areas of endemic malaria), even though SLE is highly prevalent in populations of African descent living outside of Africa. We hypothesize that malaria infections might reduce the incidence of lupus. In the present work, we assess the effect of the non-lethal Plasmodium yoelii 17XNL in a well-characterized animal model of SLE (FcγRIIB−/−). Mice deficient in the inhibitory Fc receptor for IgG (FcγRIIB) develop spontaneous disease driven by the production of anti-nuclear autoantibodies, a broad immune activation, splenomegaly, anemia and lethal glomerulonephritis with a pathology similar to human SLE. We infected FcγRIIB−/− mice at 2 months of age with P. yoelii and followed the progression of autoimmune disease for 5 months. Infection with P. yoelii increased the titer of a broad range of autoantibodies, with concomitant immune-complex deposition and complement fixation in the kidney. While lupus-prone mice succumbed from renal disease detected by protein in urine, malaria-infected lupus-prone mice did not develop proteinuria or lethal disease. Infection with the parasite did not alter tolerance mechanisms leading to lupus but instead prevented end-point tissue pathology. P. yoelii infection seems to inhibit pathogenic leukocyte infiltration without affecting the autoimmune inflammation of glomeruli. These results indicate a possible route for lupus protection that specifically targets renal function, which remains a cause of the substantial morbidity and mortality in SLE patients.

VSV-primed CD8+ T cells suppress autoreactive germinal centers

Pathogenic infections are important environmental modifiers of autoimmune disease as they can alter the immune system in a way that either promotes or reduces autoimmune responses. We tested the effect of infection with vesicular stomatitis virus (VSV) on the FcγR2B−/− (R2) mouse strain that spontaneously develops lupus disease. Infecting FcγR2B−/− mice with live VSV reduces autoantibody levels and inflammatory pathology and prolongs survival. We showed that VSV-mediated protection can be transferred in a CD8 cell adoptive transfer experiment. After further testing the transfer of various CD8 T cell subsets, we have identified CD8+CD122+NKG2D+ cells as mediator in VSV-induced Lupus amelioration. RNA Sequencing data for gene expression profiles among various CD8 subsets, indicate that the protective cells express higher level of genes linked to memory phenotype. Our working hypothesis is that the viral infection may induce the expansion of long lived CD8 populations that regulate the antibody response.

In support of the idea we initiated to identify the target of the VSV-primed CD8 T cells in vivo. We examined the effect of VSV-primed CD8 T cells on the immune phenotypes of recipient R2 mice at the initial stages, e.g. after 1, 2 or 3 weeks of transfer. Interestingly, we found that Tfh and germinal center B cells are significantly reduced after three weeks of VSV-CD8 transfer. Thus, the protective CD8 T cells from VSV-infected mice might target autoreactive germinal center to regulate the antibody response. How VSV-CD8 T cells might control germinal center & why it takes about 3 weeks for controlling is under investigation.

Lupus-suppressing effect of VSV-primed CD8+ T cell subsets

Pathogenic infections are important environmental modifiers of autoimmune disease as they can alter the immune system in a way that either promotes or reduces autoimmune responses. We tested the effect of infection with vesicular stomatitis virus (VSV) on the FcγR2B−/− (R2) mouse strain that spontaneously develops lupus disease. Infecting FcγR2B−/− mice with live VSV reduces autoantibody levels and inflammatory pathology, and prolongs survival. We showed that VSV-mediated protection can be transferred in a CD8 cell adoptive transfer experiment. Legend screening for surface markers on CD8 T cells indicated that CD8 T cells from VSV infected mice express high level of markers linked to exhausted and regulatory phenotypes. After testing the transfer of various CD8 T cell subsets, we have identified CD8+CD122+NKG2D+ cells as mediator in VSV-induced Lupus amelioration. Preliminary analysis of RNA Sequencing data for gene expression profiles among various CD8 subsets, indicate that the protective cells express higher level of genes linked to memory phenotype. Our working hypothesis is that the viral infection may induce the expansion of long lived CD8 populations that regulate the humoral response.

A single infection with a malaria parasite protects mice from lethal autoimmune glomerulonephritis

Infections can alter the prevalence of autoimmune diseases such as SLE. Epidemiological studies have showed a relative absence SLE in West Africa (areas of endemic malaria), even though SLE is highly prevalent in populations of African descent living outside of Africa. We hypothesize that malaria infections might reduce the incidence of autoimmune lupus. In the present work, we assess the effect of the non-lethal Plasmodium yoelii 17XNL in a well-characterized animal model of SLE (FcγRIIB−/−). Mice deficient in the inhibitory Fc receptor for IgG (FcγRIIB) develop spontaneous disease driven by the production of anti-nuclear autoantibodies, a broad immune activation, splenomegaly, anemia and lethal glomerulonephritis with a pathology similar to human SLE. We infected FcγRIIB−/− mice at 2 months of age with P.yoelii and followed the progression of autoimmune disease for 5 months. Infection with P. yoelii increased the titer of a broad range of autoantibodies, with concomitant immune-complex deposition and complement fixation in the kidney. While lupus-prone mice succumbed from renal disease detected by protein in urine and serum urea nitrogen, malaria-infected lupus-prone mice did not develop proteinuria or lethal disease. Infection with the parasite did not alter tolerance mechanisms leading to lupus but instead prevented end-point tissue pathology. P. yoelii infection seems to inhibit pathogenic leukocyte infiltration without affecting the autoimmune inflammation of glomeruli. These results indicate a possible route for lupus protection that specifically targets renal function, which remains a cause of the substantial morbidity and mortality in SLE patients.

Infection with VSV ameliorates autoimmune disease in FcγR2B deficient mice (50.23)

Both genetic and environmental factors, including infection by pathogens, affect the development of autoimmune diseases such as lupus. We are investigating the effect of viral infections on the lupus model of FcγR2B-/- mice. We infected FcγR2B-/- mice a single time at 2 months of age with vesicular stomatitis virus (VSV) and observed the development of autoimmune disease for the following 6 months. Infection with VSV significantly reduced autoantibody titers in serum and improved survival of FcγR2B-/- mice. B cell development in the bone marrow was not affected in VSV infected mice whereas memory B cells, and germinal center, plasma, and IgG producing mature B cell numbers were reduced. VSV infected FcγR2B-/- mice had lower number of follicular helper T cells and CD11c+ cells in the spleen. Increased survival in these mice correlated with a reduction of inflammatory cytokines and chemokines in the serum. The fact that a single round of infection with VSV confers long time protection from lupus in this mouse model indicates that the suppressor mechanism underlying the VSV effect on autoimmunity could be an excellent candidate for therapies seeking long term efficacy.

T cell-specific deletion of the inositol phosphatase SHIP reveals its role in regulating Th1/Th2 and cytotoxic responses

The 5'-phosphoinositol phosphatase SHIP negatively regulates signaling pathways triggered by antigen, cytokine and Fc receptors in both lymphocytes and myeloid cells. Mice with germ-line (null) deletion of SHIP develop a myeloproliferative-like syndrome that causes early lethality. Lymphocyte anomalies have been observed in SHIP-null mice, but it is unclear whether they are due to an intrinsic requirement of SHIP in these cells or a consequence of the severe myeloid pathology. To precisely address the function of SHIP in T cells, we have generated mice with T cell-specific deletion of SHIP. In the absence of SHIP, we found no differences in thymic selection or in the activation state and numbers of regulatory T cells in the periphery. In contrast, SHIP-deficient T cells do not skew efficiently to Th2 in vitro. Mice with T cell-specific deletion of SHIP show poor antibody responses on Alum/NP-CGG immunization and diminished Th2 cytokine production when challenged with Schistosoma mansoni eggs. The failure to skew to Th2 responses may be the consequence of increased basal levels of the Th1-associated transcriptional factor T-bet, resulting from enhanced sensitivity to cytokine-mediated T-bet induction. SHIP-deficient CD8(+) cells show enhanced cytotoxic responses, consistent with elevated T-bet levels in these cells. Overall our experiments indicate that in T cells SHIP negatively regulates cytokine-mediated activation in a way that allows effective Th2 responses and limits T cell cytotoxicity.

Cloning and characterization of nucleolar-specific autoantibodies from a murine model of lupus/scleroderma (129.6)

FcγRIIB knock-out mice in B6 background develop a lupus like autoimmune disease. With the addition of the yaa (Y-chromosome autoimmune accelerator) gene, pathogenecity in these male mice is significantly enhanced and autoantibodies switch from nuclear to nucleolar specificity. To characterize the autoantibodies produced by RIIB−/−yaa mice and the nucleolar antigens that react with these antibodies, we have produced hybridomas by fusing RIIB−/−yaa mouse splenocytes with myeloma cells. Hybridoma H526 produced anti-nucleolar and hybridoma H333 produced anti-nuclear antibodies. H526 antibody binds RNA in an anti-RNA ELISA. Antibodies from H333 are IgG2a-κ whereas H526 are IgG2a-λ. For sequence analysis, variable regions of heavy chain (VH) and light chain (VL) of H526 and H333 antibodies were amplified by PCR from respective cDNAs, cloned, and sequenced. The VH and VL region sequences of both the antibodies showed high homology with anti-nuclear antibodies, chromosome 12 of C57BL/6 mouse genome, and the J558 immunoglobulin variable regions. There were only a few amino acid changes in the variable regions. The functional roles of the amino acid changes and the pathogenecity of the antibodies are under investigation. H526 nucleolar antibodies pull down five proteins from a nuclear extract as detected by Coomassie blue staining. Characterization of these nucleolar antigenic proteins is in progress.

Discrete region of the insulin receptor carboxyl terminus plays key role in insulin action

In the present study, we attempted to determine the importance of a 23-amino-acid sequence within the carboxyl terminus of the human insulin receptor (IR) molecule in modulating insulin action in Chinese hamster ovary cells. Stable expression of a minigene encoding the receptor fragment led to an increase in insulin-induced IR autophosphorylation that was 2.4-fold higher when compared to that of IR-expressing cells transfected with empty vector. Insulin-stimulated downstream signaling was also significantly elevated in cells expressing the minigene. It was found that expression of the minigene had no effect toward insulin-like growth factor I receptor kinase activity and function. These results indicate that the IR carboxyl terminus contains a motif that acts as a physiologic modulator of insulin signaling. J. Cell. Biochem. 78:160-169, 2000. Published 2000 Wiley-Liss, Inc.

Cbl-b regulates the CD28 dependence of T-cell activation

Whereas co-stimulation of the T-cell antigen receptor (TCR) and CD28 triggers T-cell activation, stimulation of the TCR alone may result in an anergic state or T-cell deletion, both possible mechanisms of tolerance induction. Here we show that T cells that are deficient in the adaptor molecule Cbl-b (ref. 3) do not require CD28 engagement for interleukin-2 production, and that the Cbl-b-null mutation (Cbl-b(-/-)) fully restores T-cell-dependent antibody responses in CD28-/- mice. The main TCR signalling pathways, such as tyrosine kinases Zap-70 and Lck, Ras/mitogen-activated kinases, phospholipase Cgamma-1 and Ca2+ mobilization, were not affected in Cbl-b(-/-) T cells. In contrast, the activation of Vav, a guanine nucleotide exchange factor for Rac1/Rho/CDC42, was significantly enhanced. Our findings indicate that Cbl-b may influence the CD28 dependence of T-cell activation by selectively suppressing TCR-mediated Vav activation. Mice deficient in Cbl-b are highly susceptible to experimental autoimmune encephalomyelitis, suggesting that the dysregulation of signalling pathways modulated by Cbl-b may also contribute to human autoimmune diseases such as multiple sclerosis.

Inhibition of the transmembrane protein tyrosine phosphatase lar by 3S-peptide-I enhances insulin receptor phosphorylation in intact cells

3S-peptide-I, a tris-sulfotyrosyl dodecapeptide that corresponds to the major autophosphorylation domain within the insulin receptor beta-subunit, selectively enhances insulin signal transduction by specifically inhibiting dephosphorylation of the insulin receptor catalyzed by protein tyrosine phosphatases (PTPases). Because of the potential role of the transmembrane PTPase LAR in the regulation of insulin signaling, we assessed the effect of 3S-peptide-I on recombinant LAR PTPase activity and in McA-RH7777 rat hepatoma cells overexpressing full-length LAR protein (McA4B/LAR). 3S-peptide-I significantly reduced insulin receptor dephosphorylation by recombinant LAR (p < 0.001) while blocking dephosphorylation of the insulin receptor by approximately 72% in semi-permeabilized McA4B/LAR cells (p < 0.001). Increased LAR expression resulted in 40% reduction in ligand-mediated phosphorylation of the insulin receptor compared with null vector control (p < 0.001). However, treatment of intact McA4B/LAR cells with a fatty acid derivative of 3S-peptide-I (50 microM) led to an enhancement of insulin-stimulated receptor phosphorylation by 89% (p < 0.001). As a result, control and McA4B/LAR cells showed comparable steady-state levels of insulin receptor phosphorylation in the presence of insulin. These findings provide evidence that 3S-peptide-I may improve insulin responsiveness in intact cells by inhibiting LAR, an enzyme whose activity has been implicated in the pathogenesis of insulin resistance.

Altered thymic positive selection and intracellular signals in Cbl-deficient mice

Cbl is the product of the protooncogene c-cbl and is involved in T cell antigen receptor (TCR)-mediated signaling. To understand the role of Cbl for immune system development and function, we generated a Cbl-deficient mouse strain. In Cbl-deficient mice, positive selection of the thymocytes expressing major histocompatibility complex class II-restricted transgenic TCR was significantly enhanced. Two factors may have contributed to the altered thymic selection. First, Cbl deficiency markedly up-regulated the activity of ZAP-70 and mitogen-activated protein kinases. The mitogen-activated protein kinase pathway was shown previously to be involved in thymic positive selection. Second, Cbl-deficient thymocytes expressed CD3 and CD4 molecules at higher levels, which consequently may increase the avidity of TCR/major histocompatibility complex/coreceptor interaction. Thus, Cbl plays a novel role in modulating TCR-mediated multiple signaling pathways and fine-tunes the signaling threshold for thymic selection.

Amyloid precursor protein requires the insulin signaling pathway for neurotrophic activity

Picomolar concentrations of purified amyloid precursor protein (APP) potentiate the neurotrophic activity of suboptimal concentrations of NGF on PC12 cells. To understand the molecular basis for this potentiation, we have characterized the signal transduction pathway used by APP for its neurotrophic activity. APP stimulated the tyrosine phosphorylation of a number of proteins including insulin receptor substrate-1 (IRS-1). Incubation of naive cells with antisense oligonucleotides to IRS-1 mRNA resulted in a dramatic reduction of IRS-1 levels and inhibition of APP stimulated neurite outgrowth. Phosphotidylinositol 3-kinase became associated with IRS-1 and activated upon APP stimulation. Extracellular signal-regulated kinase (ERK 1 and ERK 2) phosphorylation was detected by both immunoblot analysis and immunocytochemistry using antibodies directed to their phosphorylated (and hence, activated) form. There was also an elevation of ERK kinase activity. The potentiation of NGF activity was reflected in a correspondingly synergistic elevation of tyrosine phosphorylated ERK. The pattern of signal transduction targets indicates that APP potentiated the neurotrophic effects of NGF via the activation of the IRS-1 signaling pathway.

Regulation of glucose transporters and hexose uptake in 3T3-L1 adipocytes: glucagon-like peptide-1 and insulin interactions

Glucagon-like peptide-1 (7-36 amide) (GLP-1) is known to increase insulin release when given as a bolus in the fasted and fed state. GLP-1 also increases glucose uptake and lipid synthesis in cultured adipocytes. In this study we investigated the effects of GLP-1 on glucose uptake and on the levels of expression of the facilitative glucose transporters, GLUT1 and GLUT4, in fully differentiated 3T3-L1 adipocytes. Cells were incubated with GLP-1 (10 nM) with or without insulin (10 and 100 nM) for 24 h. Under these conditions, GLP-1 alone caused an increase in basal and acute insulin-stimulated glucose uptake along with an increase in GLUT1 and GLUT4 protein levels. However, there was no change in the expression of GLUT1 and GLUT4 mRNAs. In the absence of GLP-1, prolonged exposure to insulin caused a marked reduction in the levels of GLUT4 mRNA and protein, and an inhibition of glucose uptake after an acute exposure to insulin. This insulin-induced down-regulation of GLUT4 was prevented when GLP-1 was present during the 24-h treatment. In contrast, the acute insulin-stimulated glucose uptake could not be restored by GLP-1. GLP-1 is therefore the first gut hormone shown to be capable of modulating glucose transporter levels in cultured adipocytes.

Specific inhibition of insulin receptor dephosphorylation by a synthetic dodecapeptide containing sulfotyrosyl residues as phosphotyrosyl mimetic

We have synthesized a tris-sulfotyrosyl dodecapeptide (3S-peptide-I) that corresponds to the major autophosphorylation domain within the insulin receptor beta-subunit and showed that it potently inhibited insulin receptor dephosphorylation by protein tyrosine phosphatases (PTPases) in vitro. 3S-peptide-I also inhibited tyrosine dephosphorylation of a synthetic peptide by the recombinant PTPase PTP-1B, indicating that 3S-peptide-I interacts directly with PTPase, causing its inactivation. The peptide had no effect on the activity of serine/threonine phosphatases, PP-1 and PP-2A, or alkaline phosphatase. Furthermore, we found that the introduction of a N-stearyl derivative of 3S-peptide-I in CHO/HIRc cells caused a significant increase in insulin-stimulated phosphorylation of the insulin receptor. In contrast, ligand-stimulated phosphorylation of epidermal growth factor (EGF) receptor in CHO cells overexpressing EGF receptors was not affected by the presence of N-stearyl-3S-peptide-I. These data suggest that by inhibiting dephosphorylation of the insulin receptor in intact cells, 3S-peptide-I may specifically enhance insulin signalling.

A synthetic peptide derived from a COOH-terminal domain of the insulin receptor specifically enhances insulin receptor signaling

The role of the insulin receptor COOH-terminal domain in the regulation of insulin signal transduction was explored with a variety of synthetic peptides. One of the peptides, termed peptide HC, whose structure corresponds to residues 1293-1307 of the insulin proreceptor sequence, enhanced insulin-stimulated autophosphorylation of the insulin receptor in cell-free systems and in semipermeabilized Chinese hamster ovary (CHO) cells that had been transfected with an expression plasmid encoding the human insulin receptor (CHO/HIRc) at concentrations where there was no detectable effect on basal autophosphorylation levels or on receptor dephosphorylation. A lipophilic analogue of peptide HC, stearyl peptide HC, added to intact CHO/HIRc cells enhanced significantly insulin-stimulated insulin receptor autophosphorylation while having no effect on ligand-stimulated receptor phosphorylation in CHO cells overexpressing either the IGF-1 receptor or epidermal growth factor receptor. Addition of stearyl peptide HC to CHO/HIRc cells resulted in a 2.4 +/- 0.3-fold increase in the amount of insulin-stimulated phosphatidylinositol 3-kinase detected in anti-IRS-1 immunoprecipitates and a 2.1 +/- 0.6-fold increase in the levels of tyrosine phosphorylation of mitogen-activated protein kinase in response to insulin. Finally, a derivative of peptide HC coupled to a biotin moiety was prepared and showed to bind with the beta-subunit of the wild-type insulin receptor and a truncated receptor that lacks 43 amino acids from its carboxyl terminus. However, there was little binding, if any, of the peptide with the IGF-1 receptors or the epidermal growth factor receptors. Taken together, our data demonstrate that a pentadecapeptide related to the carboxyl terminus of the insulin receptor binds to the insulin receptor beta-subunit and that this interaction may contribute to the increased receptor's intrinsic activity and signal transduction.

A peptide-based protein-tyrosine phosphatase inhibitor specifically enhances insulin receptor function in intact cells

3S-peptide-I is a synthetic tris-sulfotyrosyl dodecapeptide corresponding to the major site of insulin receptor autophosphorylation that potently inhibits dephosphorylation of the insulin receptor in a cell-free system and in digitonin-permeabilized Chinese hamster ovary (CHO) cells overexpressing the human insulin receptors (CHO/HIRc cells) (Liotta, A. S., Kole, H. K., Fales, H. M., Roth, J., and Bernier, M. (1994) J. Biol. Chem. 269, 22996-23001). In the present study, we found that 3S-peptide-I was not capable of inhibiting dephosphorylation of the epidermal growth factor (EGF) receptors in digitonin-permeabilized CHO cells that overexpress human EGF receptors (CHO/EGF-R cells). Moreover, the addition of a N-stearyl derivative of 3S-peptide-I to intact CHO/HIRc cells caused a concentration-dependent increase in insulin-stimulated phosphorylation of the insulin receptor, with a maximum effect (approximately 2.7-fold) at 50 microM. In contrast, ligand-stimulated EGF receptor phosphorylation in CHO/EGF-R cells was not affected by the presence of stearyl 3S-peptide-I. Furthermore, treatment of CHO/HIRc cells with this N-stearyl peptide led to a significant enhancement of the insulin-induced association of phosphatidylinositol (PI) 3-kinase activity with insulin receptor substrate 1 and the activation of mitogen-activated protein kinase. However, stearyl 3S-peptide-I had no effect on the EGF-stimulated activation of PI-3-kinase and mitogen-activated protein kinase in CHO/EGF-R cells. These data indicate that this tris-sulfotyrosyl dodecapeptide selectively enhances insulin signal transduction by specifically inhibiting dephosphorylation of the insulin receptor in intact cells.

4'-O-[2-(2-fluoromalonyl)]-L-tyrosine: a phosphotyrosyl mimic for the preparation of signal transduction inhibitory peptides

Development of phosphotyrosyl (pTyr) mimetics which are stable to protein-tyrosine phosphatases (PTPs), yet can retain biological potency when incorporated into peptides, is an active area of drug development. Since a majority of pTyr mimetics derive their "phosphofunctionality" from phosphorus-containing moieties, such as phosphonates, evolution of new inhibitors and modes of prodrug derivatization have been restricted to chemistries appropriate for phosphorus-containing moieties. A new, nonphosphorus-containing pTyr mimetic has recently been reported, L-O-(2-malonyl)tyrosine (OMT,5), which can be incorporated into peptides that exhibit good PTP and Src homology 2 (SH2) domain inhibitory potency. For phosphonate-based pTyr mimetics such as phosphonomethyl phenylalanine (Pmp,2) introduction of fluorines alpha to the phosphorus has provided higher affinity pTyr mimetics. This strategy has now been applied to OMT, and herein is reported 4'-O-[2-(2-fluoromalonyl)]-L-tyrosine (FOMT,6) a new fluorine-containing nonphosphorus pTyr mimetic. Incorporation of FOMT into appropriate peptides results in good inhibition of both PTP and SH2 domains. In an assay measuring the inhibition of PTP 1B-mediated dephosphorylation of phosphorylated insulin receptor, the peptide Ac-D-A-D-E-X-L-amide exhibited a 10-fold enhancement in inhibitory potency for X = FOMT (19) (IC(50) = 10 microM) relative to the unfluorinated peptide, X = OMT (18) (IC(50) = 10 microM. Molecular modeling indicated that this increased affinity may be attributable to new hydrogen-bonding interactions between the fluorine and the enzyme catalytic site, and not due to lowering of pKa values. In a competition binding assay using the p85 PI 3-kinase C-terminal SH2 domain GST fusion construct, the inhibitory peptide, Ac-D-X-V-P-M-L-amide, showed no enhancement of inhibitory potency for X = FOMT (22) (IC(50) = 18 microM) relative to the unfluorinated peptide, X = OMT (21) (IC(50) = 14 microM). The use of FOMT would therefore appear to have particular potential for the development of PTP inhibitors.

Phosphonate inhibitors of protein-tyrosine and serine/threonine phosphatases

In all, 15 aryl-containing phosphonates have been synthesized and tested for their effect on protein-tyrosine phosphatase (PTPase) activity. Two compounds, (naphth-2-yl) difluoromethylphosphonic acid (12) and (napthy-1-yl) difluoromethylphosphonic acid (13) have been found to inhibit dephosphorylation of [32P]insulin receptors by PTP-1B, a protein tyrosine phosphatase (PTPase), with IC50 values of 40-50 microM. Compound 12 competitively inhibited insulin-receptor dephosphorylation by PTP-1B. Compound 12 also inhibited PTP-1B-catalysed dephosphorylation of a synthetic tyrosine phosphorylated substrate poly(Glu80-Tyr20) at the same potency, indicating that 12 acted via interaction with the PTPase. Additionally, 12 inhibited insulin-receptor PTPase(s) and epridermal-growth-factor-receptor PTPase(s) present in solubilized membranes from CHO (Chinese-hamster ovary)/HIRc and A431 cells respectively. IC50 values of 40-50 microM were obtained in all cases with compound 12. Of note is the fact that these compounds did not have any effect on insulin-receptor autophosphorylation. Nine out of the 15 compounds potently inhibited serine/threonine phosphatase PP-2A activity without any effect on serine/threonine phosphatase PP-1 when tested at a concentration as high as 675 microM. The most potent compounds acting toward PP-2A had IC50 values of 45-50 microM. These PP-2A inhibitors could be useful tools for studying serine/threonine-phosphatase-mediated signal transduction. Two compounds, 12 and 13, inhibited both tyrosine phosphatase PTP-1B and serine/threonine phosphatase PP-2A with similar potency; IC50 values being 40-50 microM in both cases. Details of the synthesis of compounds 10, 11 and 13 are given in Supplementary Publication SUP 50177 (6 pages), which has been deposited at the British Library Document Supply Centre, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1995) 305, 9.

Thiol-specific biotinylation of the insulin receptor in permeabilized cells enhances receptor function

We examined the reactivity of insulin receptor sulfhydryls to biotinylation in Chinese hamster ovary cells that express high levels of human insulin receptors (CHO/HIRc cells). Following the biotinylation reaction, the insulin receptor was purified by immunoprecipitation, and resolved by SDS-polyacrylamide gel electrophoresis before electrotransfer to membranes. The use of enzyme-linked streptavidin in conjunction with a chemiluminescent technique allowed the detection of thiol-biotinylated receptor beta-subunit, with no modification of the alpha-subunit. In cells expressing large numbers of IGF-1 receptors, the same technique enabled the detection of thiol-biotinylated IGF-1 receptors as well. Thiol-alkylation of intact CHO/HIRc cells with an impermeant reagent did not impair the ability of maleimidodibutyrylbiocytin (MBB) to biotinylate sulfhydryls on the receptor beta-subunit after cell permeabilization with digitonin. In contrast, thiol-alkylation of digitonin-permeabilized cells prevented MBB-induced receptor biotinylation. The basal and insulin-activated insulin receptors exhibited a comparable reactivity to MBB. Furthermore, the use of affinity purification on monomeric avidin-agarose enabled us to learn that the biotinylation reaction was near-quantitative. MBB had no effect on insulin binding nor on receptor autophosphorylation and insulin-dependent receptor kinase activity. However, basal levels of receptor kinase activity were significantly elevated by thiol-biotinylation. Further, in the presence of vanadate, MBB retained the ability to enhance receptor kinase activity in permeabilized cells, consistent with the notion that this increased exogenous substrate phosphorylation was not accounted for by inactivation of protein tyrosine phosphatases. The dephosphorylation of thiol-biotinylated, 32P-labeled insulin receptors by particulate protein tyrosine phosphatases was not affected.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein-tyrosine phosphatase inhibition by a peptide containing the phosphotyrosyl mimetic, L-O-malonyltyrosine

Peptides containing phosphonate based non-hydrolyzable phosphotyrosyl (pTyr) mimetics previously have been shown to be competitive inhibitors of protein-tyrosine phosphatases (PTPs). These agents suffer from low cellular penetration which is partially attributable to ionization of the phosphonate group at physiological pH. We have developed the non-phosphorus containing pTyr mimetic, L-O-malonyltyrosine (L-OMT) and herein demonstrate using a PTP 1B enzyme assay that it is superior to phosphonomethyl phenylalanine (Pmp) as a pTyr mimetic when incorporated into the hexamer peptide Ac-D-A-D-E-X-L-amide (X = D,L-Pmp, IC50 = 200 microM; X = L-OMT, IC50 = 10 microM). Prodrug protection of L-OMT as its carboxylic acid diester could potentially increase cellular penetration, thereby making this a valuable reagent for cellular studies.

Potent inhibition of insulin receptor dephosphorylation by a hexamer peptide containing the phosphotyrosyl mimetic F2Pmp

Phosphonomethyl phenylalanine (Pmp) is a non-hydrolyzable phosphotyrosyl (pTyr) mimetic, which has been incorporated into eleven-mer Pmp-containing peptides that have previously been reported to competitively inhibit the protein-tyrosine phosphatases PTP1 and PTP 1B. We have recently shown that phosphonodifluoromethyl phenylalanine (F2Pmp) is superior to Pmp as a pTyr mimetic in SH2 domain-binding peptides. Herein we find using the hexameric peptide sequence Ac-D-A-D-E-X-L-amide, where X = (D/L)-Pmp or L-F2Pmp, that the half maximal inhibition values of these two peptides against PTP 1B-mediated dephosphorylation of autophosphorylated insulin receptor to be 200 microM and 100 nM, respectively. These data indicate that F2Pmp induces a three orders of magnitude enhancement in affinity relative to Pmp, resulting in an exceptionally potent peptide-based PTP inhibitor. We conclude that F2Pmp may be a generally useful tool in the preparation of selective, high affinity PTP inhibitors.

A synthetic tris-sulfotyrosyl dodecapeptide analogue of the insulin receptor 1146-kinase domain inhibits tyrosine dephosphorylation of the insulin receptor in situ

A synthetic tris-sulfotyrosyl dodecapeptide (TRDIY(S)ETDY(S)Y(S)RK-amide), whose primary sequence is identical to the 1142-1153 sequence of the insulin proreceptor, inhibited insulin receptor dephosphorylation in solubilized membranes, and digitonin-permeabilized cells derived from Chinese hamster ovary (CHO) cells expressing high levels of human insulin receptors (CHO/HIRc). It also inhibited the dephosphorylation of a synthetic tyrosine phosphorylated substrate by recombinant PTP-1B, a protein tyrosine phosphatase (PTPase), indicating that it acted via interaction with PTPase(s). A N-stearyl derivative of the peptide caused an approximately 4.5-fold increase in insulin-stimulated receptor autophosphorylaction in intact CHO/HIRc cells. The peptide displayed specificity toward tyrosine-class phosphatases only, as it had no effect on the activities of the serine/threonine phosphatases PP-1 and PP-2A, or alkaline phosphatase. The tyrosine sulfate ester bonds of the peptide were stable when incubated with PTP-1B (1 h, 30 degrees C). These data suggest that the sulfotyrosyl peptide functions as a nonhydrolyzable phosphotyrosyl peptide analogue capable of direct interaction with PTPase catalytic domain.

Dynamic regulation of intact and C-terminal truncated insulin receptor phosphorylation in permeabilized cells

Using digitonin-permeabilized Chinese hamster ovary (CHO) cells that were transfected with intact human insulin receptors (CHO/HIRc cells), we examined insulin receptor phosphorylation and dephosphorylation using pulse-chase techniques. Insulin activated receptor autophosphorylation on tyrosyl residues to a level severalfold over basal, reaching maximal levels after 2, 5, and 10 min of stimulation at 34, 18, and 6 degrees C, respectively. Phosphopeptide analysis revealed that the triply phosphorylated form of the 1146-kinase domain of the insulin receptor was the major species, which is characteristic of the fully active tyrosine kinase function. The dephosphorylation reaction was time- and temperature-dependent with t1/2 values of 0.67 and 2 min at 18 and 6 degrees C, respectively. Vanadate completely inhibited dephosphorylation. Under similar permeabilization conditions when compared with CHO/HIRc cells, CHO/delta CT cells (CHO cells overexpressing a mutated form of the receptor with a 43 amino acid deletion at the C-terminus) stimulated with insulin exhibited larger increases in receptor autophosphorylation levels and in tyrosine kinase activity toward a synthetic peptide substrate; the rate of CHO/delta CT receptor dephosphorylation was not reduced. There was near-complete absence of insulin receptor substrate 1 (IRS-1) in the cell ghosts after permeabilization. We therefore examined the pattern of tyrosine phosphorylation and dephosphorylation of residual cellular proteins in permeabilized CHO/HIRc cells by Western blot analysis. In addition to the 95-kDa receptor beta-subunit, we detected the phosphorylation of two glycoproteins which included the commonly found 120-kDa protein and a novel 195-kDa protein whose dephosphorylation rate is slower than that of receptor beta-subunit.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization and partial purification of an insulinase from Neurospora crassa

An insulin-binding metal- and thiol-dependent proteinase has been purified 1491-fold from high speed cytosolic fractions of the fungus Neurospora crassa. This enzyme resembles insulin-degrading enzymes (insulinases) present in mammalian cells and in Drosophila melanogaster in the following ways: (i) it degrades radiolabeled insulin with a specificity similar to that of rat muscle insulinase, as demonstrated by HPLC analysis of the degradation products; (ii) it is inhibited by bacitracin, EDTA, 1,10-phenanthroline, and the sulfhydryl-reactive compounds N-ethylmaleimide and p-chloromercuribenzoate, but not by inhibitors of serine proteases or by lysosomal protease inhibitors. Cross-linking with 125I-insulin labels a band of ca. 120 kDa, and several smaller bands which may represent degradation products. The N. crassa insulinase is stimulated by Mn2+ and strongly inhibited by Zn2+; Mn2+ can also reactivate the enzyme after inhibition by EDTA, but Zn2+ is ineffective. The N. crassa protein differs in this regard from mammalian and insect insulinases which are generally activated by both Mn2+ and Zn2+. This finding extends the apparent evolutionary conservation of these metal- and thiol-dependent proteases into the microbial realm.

Insulin-induced stimulation of protein phosphorylation in Neurospora crassa cells

1) Insulin stimulated the phosphorylation of at least 14 discrete proteins in Neurospora crassa cells. Specific proteins were phosphorylated at serine, threonine, and tyrosine residues, as determined by phosphoamino acid analysis of discrete spots on two-dimensional gels. 2) Insulin stimulated the phosphorylation by [gamma-32P]ATP of at least six discrete proteins in solubilized N. crassa membrane preparations at serine and tyrosine residues. 3) A phosphotyrosine-containing protein of 38 kDa, pI 7.0-7.2, reacted by both immunoblotting and immunoprecipitation with antiserum to P2, a peptide from the human insulin receptor that contains an autophosphorylated tyrosine residue. In N. crassa cells, therefore, as in mammalian cells, insulin induces a variety of protein phosphorylations, some of which may be part of an evolutionarily conserved signal transduction pathway.

Regulation of 6-phosphofructo-1-kinase activity in ras-transformed rat-1 fibroblasts

Fructose 2,6-bisphosphate (F-2,6-P2) stimulated glycolysis in cell-free extracts of both normal and ras-transfected rat-1 fibroblasts. The extract of the transformed cell glycolyzed more rapidly in both the absence and the presence of F-2,6-P2 than the extract of the parent fibroblast. Addition of mitochondrial ATPase (F1) or inorganic phosphate (Pi) further stimulated lactate production in both cell lines. F-2,6-P2 stimulated the 6-phosphofructo-1-kinase (PFK-1) activity in extracts of normal and transfected cells. The activity in extracts of transformed cells tested with a fructose 6-phosphate regenerating system was considerably higher than in the extract of normal cells. Stimulation of PFK-1 activity by cAMP of both cell lines was not as pronounced as that by F-2,6-P2. In the absence of F-2,6-P2 the PFK-1 activity was strongly inhibited in the transformed cell by ATP concentrations higher than 1 mM, whereas in the normal cell only a marginal inhibition was noted even at 2 or 3 mM ATP. F-2,6-P2 reversed the inhibition of PFK-1 by ATP. Nicotinamide adenine dinucleotide (NAD) at 100 microM (in the presence of 2 mM ATP and 1 microM F-2,6-P2) stimulated PFK-1 activity only in the transformed cell, whereas nicotinamide adenine dinucleotide phosphate (NADP) inhibited PFK-1 activity (in the presence or absence of 1 microM F-2,6-P2) in extracts of both cell lines. No previous observations of stimulation or inhibition by NAD or NADP on PFK-1 activity appear to have been reported. A threefold increase in the intracellular concentration of F-2,6-P2 was observed after transfection of rat-1 fibroblast by the ras oncogene. We conclude from these data that the PFK-1 activity of ras-transfected rat-1 fibroblasts shows a greater response to certain stimulating and inhibitory regulating factors than that of the parent cell.

Multiple changes induced by cholera toxin contribute to the stimulation of aerobic lactate production in rat-1 fibroblasts

Exposure of rat-1 fibroblasts to cholera toxin increased aerobic lactate production 3- to 8-fold with maximal stimulation observed between 1 and 2 h at a concentration of 1-2 micrograms/ml. Concomitant with this change was a 10- to 40-fold elevation in the intracellular concentration of cAMP. The cell permeable cAMP analogue, N6,2'-O-dibutyryl cAMP and the cyclic nucleotide phosphodiesterase inhibitor RO-20-1724 also increased lactate production and intracellular cAMP levels, although less effectively. Cholera toxin and dibutyryl cAMP induced a 2- to 3-fold elevation of intracellular fructose 2,6-bisphosphate and 2- to 3-fold increases in both 3-O-methylglucose and inorganic phosphate transport. A survey of five additional cell lines revealed striking variabilities in their individual responses to cholera toxin and dibutyryl cAMP. All were observed to be considerably less sensitive to either agent than rat-1 cells. These data suggest that a cooperative effect involving multiple parameters may be responsible for the observed increases in aerobic lactate production in response to cAMP and that these parameters may vary significantly among cell lines.

Purification and properties of a membrane-bound insulin binding protein, a putative receptor, from Neurospora crassa

The protein that is responsible for specific, high-affinity binding of insulin to the surface of Neurospora crassa cells has been purified to homogeneity. The insulin binding activity of solubilized plasma membranes resembled that of intact cells with regard to affinity of binding, specificity for mammalian insulins, and amount of insulin bound per cell. Insulin binding activity was purified from Triton X-100 solubilized membranes in two steps: FPLC on a MonoQ HR5/5 column; and affinity chromatography on insulin-agarose. The pure material migrated as a single band of ca. 66 kDa on SDS gels, pI = 7.4 by isoelectric focusing. The protein bound 5.34 pmol of insulin/micrograms, or 35% of that expected for univalent binding. Cross-linking of 125I-insulin to pure protein or to solubilized membranes revealed a single labeled band of 67-70 kDa on SDS gels. In nonreducing native gels, two labeled bands of ca. 55 and 110 kDa were produced after cross-linking, and two bands of similar molecular weight bound iodinated insulin after transfer to nitrocellulose filters. These may correspond to active monomer and dimer forms. The pure protein possessed no protein kinase activity against itself, or against exogenous substrates (histone H2, casein, or the synthetic peptide Glu80-Tyr20), and possessed no detectable phosphorylated amino acids. It is suggested, however, that this 66-kDa protein is the "receptor" that mediates insulin-induced downstream metabolic effects.

Stimulation of phosphorylation of lipocortin at threonine residues by epidermal growth factor (EGF) and the EGF receptor: addition of protein kinase P with polylysine inhibits this effect

In this paper we show that epidermal growth factor (EGF) stimulates the phosphorylation of lipocortin 1, at threonine as well as at tyrosine residues, by a highly purified preparation of the EGF receptor. The phosphorylation of threonine residues is catalyzed by an enzyme that contaminates the receptor preparations, since crude extracts of A431 plasma membranes contain larger amounts of the threonine kinase than does the receptor preparation. Protein kinase P (2.5 ng) inhibits both threonine and tyrosine phosphorylation of lipocortin 1 while greatly stimulating the autophosphorylation of the EGF receptor. Acetyllipocortin 1 is poorly phosphorylated at tyrosine residues by the EGF receptor kinase, but it becomes readily phosphorylated in the presence of polylysine. The most likely explanation for this observation is that there is an interaction between polylysine and acetyllipocortin that converts the latter into a suitable substrate for the EGF receptor. These and other experiments described in this paper point to a role of surface charges in the susceptibility of substrates to attach by protein kinases.

Specific dephosphorylation of phosphoproteins by protein-serine and -tyrosine kinases

Five protein kinases are shown to serve as specific phosphatases in the absence of ADP. Although the rates of hydrolysis are very slow compared to the forward phosphorylation rates under optimal conditions, they are of the same order as the reverse reaction in the presence of ADP. Because cells contain approximately equal to 3 mM ATP, neither the reverse reaction nor the phosphatase is likely to play a physiological role. beta-casein B phosphorylated by the catalytic subunit of cAMP-dependent protein kinase (protein kinase A) is specifically dephosphorylated by protein kinase A but not by polypeptide-dependent protein kinase (protein kinase P). beta-casein B phosphorylated by protein kinase P is specifically dephosphorylated by protein kinase P but not by protein kinase A. Histone H1 phosphorylated by protein kinase C is dephosphorylated by the same enzyme in the absence of ADP. In all cases tested addition of ADP and F1-ATPase accelerates moderately the rate of dephosphorylation. Native H+-ATPase from yeast plasma membranes is isolated mainly in the phosphorylated form. It is dephosphorylated and rephosphorylated by protein kinase P but not by protein kinase A. Protein-tyrosine kinase of the epidermal growth factor receptor phosphorylates the random synthetic polypeptide poly(Glu80Tyr20). The phosphorylated polymer is specifically dephosphorylated in the absence of ADP by epidermal growth factor receptor preparations but not by insulin receptor preparations. The same polymer phosphorylated by insulin receptor is dephosphorylated by insulin receptor but not by epidermal growth factor receptor preparations. By using a cycle of dephosphorylation-rephosphorylation, it is possible to identify proteins that are phosphorylated by these protein kinases in vivo. Should this method be applicable to additional protein kinases, it should be possible to estimate the quantitative contribution of each protein kinase to a single phosphoprotein.

Effect of protein kinase P on phosphorylations catalyzed by the epidermal growth factor

Protein kinase P (PK-P) activated by histones or certain other basic compounds has been purified previously from yeast [Yanagita, Y., Abdel-Ghany, M., Raden, D., Nelson, N. & Racker, E. (1987) Proc. Natl. Acad. Sci. USA 84, 925-929]. It is shown here that PK-P is present in solubilized membranes of A-431 carcinoma cells where it changes the epidermal growth factor (EGF) receptor kinase activity. Polylysine, a weak PK-P activator, inhibited the autophosphorylation of the EGF receptor both in the absence and presence of EGF. Increased PK-P activity induced by histone 1, a potent activator, gave rise to increased autophosphorylation of the EGF receptor as well as phosphorylation at tyrosine residues of numerous other endogenous membrane components. The stimulation by histone was particularly striking in the presence of EGF. A similar stimulation was achieved with polylysine and EGF on addition of yeast PK-P. However, addition of yeast PK-P in the presence of histone 1 markedly inhibited the EGF-stimulated phosphorylation of endogenous membrane proteins. We conclude from these results that the effect of PK-P on the EGF receptor takes place in three phases: at low levels PK-P inhibits the autophosphorylation, at intermediate levels it stimulates the autophosphorylation as well as the EGF-dependent phosphorylation of numerous other membrane proteins, and at high levels it inhibits the phosphorylation of these proteins.

Mycoversilin, a new antifungal antibiotic. III. Mechanism of action on a filamentous fungus Trichophyton rubrum

Mycoversilin is active against filamentous fungi, being specifically inhibitory to Trichophyton rubrum, minimum inhibitory concentration being 15 micrograms/ml. Mycoversilin inhibits sporulation to the extent of 28.5% even at the growth inhibitory concentration whereas inhibition of spore germination requires higher concentration. It has no effect on radial growth. Further it shows no action either on the release of UV absorbing materials or on the respiration of T. rubrum. However, the antibiotic inhibits in vivo synthesis of protein fairly strongly, DNA moderately and RNA slightly at the minimum inhibitory concentration. Cell-free protein synthesis is also strongly inhibited, the site of action being the inhibition of leucyl-tRNA formation by the antibiotic which has no action on leucine activation.