Evolution of mitochondrial TAT translocases illustrates the loss of bacterial protein transport machines in mitochondria

BACKGROUND

Bacteria and mitochondria contain translocases that function to transport proteins across or insert proteins into their inner and outer membranes. Extant mitochondria retain some bacterial-derived translocases but have lost others. While BamA and YidC were integrated into general mitochondrial protein transport pathways (as Sam50 and Oxa1), the inner membrane TAT translocase, which uniquely transports folded proteins across the membrane, was retained sporadically across the eukaryote tree.

RESULTS

We have identified mitochondrial TAT machinery in diverse eukaryotic lineages and define three different types of eukaryote-encoded TatABC-derived machineries (TatAC, TatBC and TatC-only). Here, we investigate TatAC and TatC-only machineries, which have not been studied previously. We show that mitochondria-encoded TatAC of the jakobid Andalucia godoyi represent the minimal functional pathway capable of substituting for the Escherichia coli TatABC complex and can transport at least one substrate. However, selected TatC-only machineries, from multiple eukaryotic lineages, were not capable of supporting the translocation of this substrate across the bacterial membrane. Despite the multiple losses of the TatC gene from the mitochondrial genome, the gene was never transferred to the cell nucleus. Although the major constraint preventing nuclear transfer of mitochondrial TatC is likely its high hydrophobicity, we show that in chloroplasts, such transfer of TatC was made possible due to modifications of the first transmembrane domain.

CONCLUSIONS

At its origin, mitochondria inherited three inner membrane translocases Sec, TAT and Oxa1 (YidC) from its bacterial ancestor. Our work shows for the first time that mitochondrial TAT has likely retained its unique function of transporting folded proteins at least in those few eukaryotes with TatA and TatC subunits encoded in the mitochondrial genome. However, mitochondria, in contrast to chloroplasts, abandoned the machinery multiple times in evolution. The overall lower hydrophobicity of the Oxa1 protein was likely the main reason why this translocase was nearly universally retained in mitochondrial biogenesis pathways.

T cell activation-inducing epitopes of the house dust mite allergen Der p I. Proliferation and lymphokine production patterns by Der p I-specific CD4+ T cell clones

Cloned human CD4+ T cell lines specific for the house dust mite Dermatophagoides pteronyssinus were used to map minimal T cell activation-inducing epitopes on the Group I allergen in D. pteronyssinus extracts (Der p I) molecule. Most of these Der p I-specific T cell clones expressed different TCR V alpha and V beta gene products. Using recombinant deletion proteins, three T cell epitopes were identified on the Der p I molecule; p45-67 and p117-143 were recognized by HLA-DR7-restricted T cells, whereas p94-104 was recognized in the context of HLA-DR2, DRw11 (DR5), and -DR8 molecules. This degenerate class II MHC restriction appears to be due to shared Phe and Asp residues at positions 67 and 70, respectively, in the third variable domain of the HLA-DR beta chain. All three T cell epitopes induced Th2-like cytokine production profiles by the Der p I-specific T cell clones, which were characterized by the production of very high levels of IL-4 and IL-5, as compared with those secreted by tetanus toxin-specific T cell clones derived from the same patients, but no or low amounts of IL-2 and IFN-gamma. This Th2-like production profile was, however, not an intrinsic property of the Der p I-specific T cells, but was dependent upon their mode of activation. Stimulation with Con A also induced very low or no measurable levels of IL-2 and IFN-gamma, whereas activation with TPA and the calcium ionophore A23187 resulted in the production of high levels of IL-4, IL-5, IL-2, and IFN-gamma. These results indicate that Der p I-specific T cell clones are not defective in their capacity to produce high levels of Th1 cytokines.

T cell activation-inducing epitopes of the house dust mite allergen Der p I. Proliferation and lymphokine production patterns by Der p I-specific CD4+ T cell clones

Cloned human CD4+ T cell lines specific for the house dust mite Dermatophagoides pteronyssinus were used to map minimal T cell activation-inducing epitopes on the Group I allergen in D. pteronyssinus extracts (Der p I) molecule. Most of these Der p I-specific T cell clones expressed different TCR V alpha and V beta gene products. Using recombinant deletion proteins, three T cell epitopes were identified on the Der p I molecule; p45-67 and p117-143 were recognized by HLA-DR7-restricted T cells, whereas p94-104 was recognized in the context of HLA-DR2, DRw11 (DR5), and -DR8 molecules. This degenerate class II MHC restriction appears to be due to shared Phe and Asp residues at positions 67 and 70, respectively, in the third variable domain of the HLA-DR beta chain. All three T cell epitopes induced Th2-like cytokine production profiles by the Der p I-specific T cell clones, which were characterized by the production of very high levels of IL-4 and IL-5, as compared with those secreted by tetanus toxin-specific T cell clones derived from the same patients, but no or low amounts of IL-2 and IFN-gamma. This Th2-like production profile was, however, not an intrinsic property of the Der p I-specific T cells, but was dependent upon their mode of activation. Stimulation with Con A also induced very low or no measurable levels of IL-2 and IFN-gamma, whereas activation with TPA and the calcium ionophore A23187 resulted in the production of high levels of IL-4, IL-5, IL-2, and IFN-gamma. These results indicate that Der p I-specific T cell clones are not defective in their capacity to produce high levels of Th1 cytokines.

Interleukin-7 induces the proliferation of normal human B-cell precursors

In the present study, we investigated the effects of human recombinant interleukin-7 (IL-7) on the proliferation of enriched hematopoietic cells isolated from human adult and fetal bone marrow (BM). In cultures of CD34+ cells, IL-7 was found to induce dose-dependent incorporation of 3H-thymidine (3H-TdR), but had no demonstrable effect on the development of myeloid colony-forming cells. Numbers of B-cell precursors (BCP), initially present within CD34+ populations and which included a CD34+CD20+ subset, were significantly increased when CD34+ BM cells were cultured in the presence of IL-7. This effect was most striking on CD20+ BCP, and resulted at least partly from higher numbers of cycling cells as indicated by Hoechst 33342 fluorescence (Calbiochem, Behring Diagnostics, La Jolla, CA). These results indicate that IL-7 promotes the growth of BCP within the CD34+ compartment. In line with the B-lineage affiliation of CD34+ target cells, committed BCP (CD10+ CD19+ surface IgM-) isolated from BM were also found to proliferate in response to IL-7. Interestingly, this effect of IL-7 was strongly potentiated by the addition of IL-3. Taken together, and in accordance with previous observations on murine cells, our data indicate that IL-7 acts as a growth factor during the ontogeny of human B lymphocytes.

Interleukin-6 interacts with interleukin-4 and other hematopoietic growth factors to selectively enhance the growth of megakaryocytic, erythroid, myeloid, and multipotential progenitor cells

The growth-promoting activities of interleukin-6 (IL-6) in combination with different factors were assessed in bone marrow (BM) cultures prepared from normal mice and from mice treated with 5-fluorouracil (5-FU). Effects on hematopoietic colony formation with respect to number, size, and cellular composition were evaluated. In agreement with previous reports, IL-6 acts synergistically with IL-3 to stimulate increased numbers of granulocyte/macrophage (GM) and multilineage colonies in day-2 and day-4 post-5-FU BM cultures. Furthermore, day 4 but not day 2 post-5-FU BM showed enhanced GM colony formation when stimulated with IL-6 plus interleukin-4 (IL-4) or granulocyte colony-stimulating factor (G-CSF). In contrast, IL-6 did not increase the number of colonies supported by M-CSF or GM-CSF. Nevertheless IL-6 interacted with all factors, including M-CSF and GM-CSF, to stimulate an increase in colony size. Many of these myeloid colonies attained a diameter of greater than or equal to 0.5 mm, suggesting they derive from high proliferative potential cells (HPP-CFC). The response of normal and day-8 post-5-FU BM containing high numbers of more mature progenitors was also assessed. We found IL-6 enhanced colony formation by lineage-restricted megakaryocytic and erythroid progenitors in the presence of IL-3 and IL-4 plus erythropoietin (Epo), respectively. The sum of these results shows that IL-6 interacts with a variety of factors to regulate the growth of progenitor cells at different stages of lineage commitment and maturation.

Interleukin 5 enhances interleukin 4-induced IgE production by normal human B cells. The role of soluble CD23 antigen

Interleukin 4 (IL 4)-induced IgE production by peripheral blood lymphocytes and tonsil cells from normal donors was enhanced in a dose-dependent fashion by IL 5. IL 5 tested alone was not effective. The synergistic effects of IL 5 were most pronounced at suboptimal IL 4 concentrations, whereas at saturating IL 4 concentrations (200-300 U/ml), IL 5 had no effect. Interferon-gamma (IFN-gamma) and F(ab')2 fragments of monoclonal antibody 25 directed against the CD23 antigen, that blocked IL 4-induced IgE synthesis, also inhibited the production of IgE in the presence of combinations of IL 4 and IL 5, indicating that IL 5 potentiates the activation pathway through which IL 4 induces IgE production. In contrast, IL 4 (50 U/ml) blocked IL 5-induced IgA synthesis. IL 5 was ineffective in inducing the release of soluble CD23 (sCD23), but in the presence of IL 4 an enhanced release of sCD23 was observed, provided IL 4 was present at suboptimal concentrations. IFN-gamma completely blocked sCD23 release induced by IL 4 and IL 5. These results demonstrate that there is a strong quantitative correlation between sCD23 release and induction of IgE synthesis. sCD23 fraction-correlation between sCD23 release and induction of IgE synthesis. sCD23 fractionated from the Epstein-Barr virus-transformed B cell line RPMI 8866 was ineffective in inducing IgE production. However, sCD23 acted synergistically with suboptimal concentrations of IL 4. sCD23 did not modulate the IgE response at saturating concentrations of IL 4. Collectively, these data indicate that sCD23 plays an important regulatory role in the modulation of IL 4-induced IgE synthesis mediated by IFN-gamma and IL 5.

Human prothymosin alpha: amino acid sequence and immunologic properties

Prothymosin alpha has been purified from human thymus and its amino acid sequence determined, except for a 15 amino acid segment including 10 glutamyl residues near the middle of the molecule. Like prothymosin alpha from rat thymus [A. A. Haritos, R. Blacher, S. Stein, J. Caldarella, and B. L. Horecker (1985) Proc. Natl. Acad. Sci. USA 82, 343-346], human prothymosin contains the thymosin alpha 1 sequence at its NH2-terminus. It contains a total of 109-110 residues compared to 111-112 for rat prothymosin alpha, with deletions corresponding to positions Gln39 and Lys108 of the rat polypeptide. Human prothymosin alpha also differs from rat prothymosin alpha at positions corresponding to residues 87, 92, and 102 of the latter, with substitutions of alanine for proline, alanine for valine, and aspartic acid for glutamic acid, respectively. Human prothymosin is significantly less active than rat prothymosin in protecting mice against infection with Candida albicans and in stimulating release in vivo of migration inhibitory factor. Thus, the differences in amino acid sequences, present mainly the COOH-terminal half of the polypeptides, may determine species specificity in biological properties.

Identification of the alpha subunit half-cystine specifically labeled by an affinity reagent for the acetylcholine receptor binding site

Nicotinic acetylcholine receptors contain a readily reducible disulfide bond at the periphery of the acetylcholine binding site. Following reduction of this disulfide, the binding site is susceptible to affinity labeling by electrophilic reagents with quaternary ammonium moieties. We reduced purified receptor from Torpedo californica electric tissue and affinity alkylated it with 4-(N-maleimido)benzyltri[3H]methylammonium iodide. The label was incorporated solely into the alpha subunit of the receptor. Isolated, labeled alpha subunit was cleaved with CNBr, and the fragments were separated by reverse-phase high-performance liquid chromatography. A uniquely labeled CNBr fragment was isolated, and its partial sequence was determined by automated Edman degradation. This CNBr fragment was cleaved at tryptophan residues, the subfragments were separated, and the labeled subfragments were partially sequenced. From our protein sequence information, we identify the labeled CNBr fragment as residues 179 to 207 of the sequence of alpha predicted from the cDNA sequence (Noda, M., Takahashi, H., Tanabe, T., Toyosato, M., Furutani, Y., Hirose, T., Asai, M., Inayama, S., Miyata, T., and Numa, S. (1982) Nature (Lond.) 299, 793-797). From the cycle of the Edman degradation in which radioactive residues are released, we conclude that Cys 192 and, possibly in addition, Cys 193 are the residues specifically labeled by 4-(N-maleimido)benzyltri[3H]methylammonium iodide. They are, therefore, close to the acetylcholine binding site.

Use of high-performance liquid chromatography for preparing samples for microsequencing

A method has been developed for the microsequencing of protein at subnanomole levels. The protein is carboxymethylated and freed of salts and reagents by reversed-phase chromatography prior to automated Edman degradation on a gas-phase sequencer. The carboxymethylated protein can also be fragmented chemically or enzymatically for further sequence analysis. The analytical techniques used to monitor the progress of the reactions all have picomole level sensitivity.

Purification to homogeneity and partial characterization of interleukin 2 from a human T-cell leukemia

A method utilizing reversed-phase high-performance liquid chromatography has been developed for the purification to homogeneity of interleukin 2 (IL-2) isolated from a human T-cell leukemia. A final purification of 500,000-fold was obtained with a specific activity of pure IL-2 of 10(9) units/mg. The amino acid analysis of natural IL-2 is strikingly similar to the composition deduced from sequence analysis of a cDNA coding for human IL-2. Protein sequence analysis of CNBr-derived peptides yields data consistent with the sequence proposed from cloned cDNA. The availability of homogeneous IL-2 will allow accurate biological studies of its activity free from the contamination of the numerous lymphokine species that are known to be co-produced with IL-2 during the induction procedure.

Characterization of small amounts of peptides and proteins

The use of microchemical methods for the characterization of both natural and recombinant proteins of biomedical importance is discussed. The methods include gel electrophoresis, high-performance liquid chromatography, protein fragmentation, amino acid analysis and automated Edman degradation. Each procedure is applicable at the picomole level.

Purification and properties of a type beta transforming growth factor from bovine kidney

Type beta transforming growth factor (TGF-beta) has been purified 200 000-fold from bovine kidneys. This peptide is characterized by its ability to induce anchorage-dependent normal rat kidney cells to grow in soft agar in the presence of epidermal growth factor (EGF); TGF-beta is not mitogenic for cells grown in monolayer culture. Purified TGF-beta does not compete with EGF for binding to membrane receptors. The concentration of TGF-beta required to elicit a half-maximal response for formation of colonies greater than 3100 micron2 in the soft agar assay is 2-3 pM (55 pg/mL) when assayed in the presence of 0.8 nM EGF (5 ng/mL). The four-step purification procedure which includes chromatography of acid--ethanol tissue extracts on polyacrylamide sizing gels, cation exchange, and two steps of high-pressure liquid chromatography results in a 10% overall yield of colony-forming activity with a recovery of 3-4 micrograms/kg. Amino acid analysis of purified TGF-beta shows 16 half-cystine residues per mole. Analysis of the purified polypeptide by electrophoresis on sodium dodecyl sulfate-polyacrylamide gels indicates that TGF-beta is composed of two closely related polypeptide chains cross-linked by disulfide bonds. In the absence of beta-mercaptoethanol, the colony-forming activity is associated with a single silver-staining band of molecular weight 25 000; in the presence of beta-mercaptoethanol, the TGF-beta is converted to an inactive species that migrates as a single band of molecular weight 12 500-13 000. Sequence analysis indicates that at least the first 15 N-terminal amino acids of the two TGF-beta subunits are identical.

Binding assay for opioid peptides with neuroblastoma x glioma hybrid cells: specificity of the receptor site

A sensitive and rapid radioreceptor assay has been developed to monitor opioid peptides in column effluents. It is based on competitive binding to NG 108-15 cells using [3H-Tyr]Leu-enkephalin as the displaced ligand. The specificity of binding to the cells of naturally occurring opioid peptides and synthetic analogs has been shown to be similar to that found with synaptic plasma membranes.