Recruitment to Golgi membranes of ADP-ribosylation factor 1 is mediated by the cytoplasmic domain of p23.

Binding to Golgi membranes of ADP ribosylation factor 1 (ARF1) is the first event in the initiation of COPI coat assembly. Based on binding studies, a proteinaceous receptor has been proposed to be critical for this process. We now report that p23, a member of the p24 family of Golgi-resident transmembrane proteins, is involved in ARF1 binding to membranes. Using a cross-link approach based on a photolabile peptide corresponding to the cytoplasmic domain of p23, the GDP form of ARF1 (ARF1-GDP) is shown to interact with p23 whereas ARF1-GTP has no detectable affinity to p23. The p23 binding is shown to localize specifically to a 22 amino acid C-terminal fragment of ARF1. While a monomeric form of a non-photolabile p23 peptide does not significantly inhibit formation of the cross-link product, the corresponding dimeric form does compete efficiently for this interaction. Consistently, the dimeric p23 peptide strongly inhibits ARF1 binding to native Golgi membranes suggesting that an oligomeric form of p23 acts as a receptor for ARF1 before nucleotide exchange takes place.

p24 and p23, the major transmembrane proteins of COPI-coated transport vesicles, form hetero-oligomeric complexes and cycle between the organelles of the early secretory pathway.

COPI-coated vesicles that bud off the Golgi complex contain two major transmembrane proteins, p23 and p24. We have localized the protein at the Golgi complex and at COPI-coated vesicles. Transport from the intermediate compartment (IC) to the Golgi can be blocked at 15 degrees C, and under these conditions p24 accumulates in peripheral punctated structures identified as IC. Release from the temperature block leads to a redistribution of p24 to the Golgi, showing that p24, similar to p23, cycles between the IC and Golgi complex. Immunoprecipitations of p24 from cell lysates and from detergent-solubilized Golgi membranes and COPI-coated vesicles show that p24 and p23 interact with each other to form a complex. Transient transfection of p23 in HeLa cells shows that p23 and p24 colocalize in structures induced by the overexpression of p23. Taken together p24 interacts with p23 and constitutively cycles between the organelles of the early secretory pathway.

Crystallization and preliminary X-ray analysis of tryparedoxin I from Crithidia fasciculata.

The thioredoxin-related protein tryparedoxin I from Crithidia fasciculata has been crystallized using PEG 4000 as a precipitant. The enzyme forms long needle-shaped crystals which diffract to at least 1.7 A. A native data set has been collected at the DESY synchrotron from a flash-frozen crystal at 90 K to 1.7 A resolution. The data set shows that the crystals belong to the orthorhombic space group P212121 and have unit-cell parameters a = 37.94, b = 51. 39, c = 71.46 A. Tryparedoxin I is involved in a trypanothione-dependent peroxide metabolic pathway specific for trypanosomatids and may therefore be a suitable candidate for the design of drugs for the specific treatment of a variety of important tropical diseases caused by these parasites.

A role for ADP ribosylation factor in the control of cargo uptake during COPI-coated vesicle biogenesis.

ARF-mediated hydrolysis of GTP has been demonstrated to regulate coat disassembly of Golgi-derived COPI transport vesicles (Tanigawa, G., Orci, L., Amherdt, M., Ravazzola, M., Helms, J.B. and Rothman, J.E. (1993) J. Cell Biol. 123, 1365-1371). In addition, a requirement for GTP hydrolysis at an early stage of COPI vesicle biogenesis has been established since cargo uptake is impaired in the presence of GTPgammaS (Nickel, W., Malsam, J., Gorgas, K., Ravazzola, M., Jenne, N., Helms, J.B. and Wieland, F.T. (1998) J. Cell Sci. 111, 3081-3090), a non-hydrolyzable analogue of GTP. We now demonstrate that the GTPase involved in the regulation of cargo uptake is ARF, revealing a multi-functional role of this GTPase in COPI-mediated vesicular transport. The molecular mechanism of cargo uptake as well as the functional implications of these findings on the overall process of COPI vesicle biogenesis are discussed.

Catalytic characteristics of tryparedoxin.

Tryparedoxin, a thioredoxin-related protein from Crithidia fasciculata with a molecular mass of 16 kDa catalyses the reduction of a peroxiredoxin-type peroxidase, Cf21, at the expense of trypanothione [Nogoceke, E., Gommel, D. U., Kiess, M., Kalisz, H. M. & Flohé, L. E. (1997) Biol. Chem. Hoppe-Seyler 378, 827-836]. The kinetic analysis of tryparedoxin revealed an enzyme substitution mechanism. The corresponding molecular event was elucidated to be a reversible oxidoreduction of the disulfide bridge in the thioredoxin-related motif WCPPC. The amino-proximal cysteine residue of this active site was more reactive in S-alkylation experiments than the distal residue. The natural substrates of tryparedoxin, trypanothione and Cf21, could only be substituted by glutathione and glutathione disulfide with considerable loss in activity. The pronounced specificity of tryparedoxin is further accentuated by low limiting Km values for Cf21 and trypanothione (2.2 microM and 130 microM, respectively, as compared to 990 microM for gluthathione disulfide and an infinite value for glutathione). Tryparedoxin can therefore be classified as a trypanothione: peroxiredoxin oxidoreductase. The reduction of tryparedoxin by trypanothione appears to be the rate-limiting step in the trypanothione-dependent hydroperoxide reduction because(a) the regeneration of reduced tryparedoxin from the tryparedoxin-trypanothione complex is rate limiting (k[cat] 392 min[-1]), (b) the physiological trypanothione concentrations may not always saturate tryparedoxin, and (c) the rate constants for the net forward reaction of Cf21 are faster than those of the tryparedoxin reaction. The functional characteristics of tryparedoxin explain the limited capacity of trypanosomatids in coping with oxidative stress and qualify the enzyme as a potential target for the design of specific trypanocidal compounds.

A unique cascade of oxidoreductases catalyses trypanothione-mediated peroxide metabolism in Crithidia fasciculata.

Parasitic trypanosomatids comprise causative agents of debilitating or life-threatening tropical diseases. The limited capacity of these parasites to cope with oxidative stress has been discussed as a target area for therapeutic approaches but success has been hampered by a lack of comprehension of their peculiar oxidant defense system depending on the unique redox metabolite trypanothione. Here we report that trypanothione-dependent hydroperoxide metabolism in Crithidia fasciculata is catalysed by two distinct proteins working in concert. One is Cf16, a unique protein which, apart from a WCPPC sequence that resembles the thioredoxin-type WCG(A)PC motif, only shows low similarity to thioredoxin-like proteins of bacteria and invertebrates. The second component is Cf21, which can be classified as a member of the peroxiredoxin family of proteins. The two proteins have been purified to homogeneity and shown to be essential for the trypanothione-dependent removal of hydroperoxides. By means of selective derivatisation of the substrate-reduced proteins the flux of reduction equivalents from trypanothione to Cf16, Cf21 and finally to the hydroperoxide was elucidated. Cf21 proved to be a moderately efficient peroxidase with broad specificity. The rate constants for the reaction of the reduced protein with H2O2, t-butyl hydroperoxide, linoleic acid hydroperoxide and phosphatidylcholine hydroperoxide were 1.0 x 10(5), 1.2 x 10(5), 1.0 x 10(5) and 0.4 x 10(5) M-1S-1, respectively. The apparent rate constant for the regeneration of reduced Cf21 by Cf16 was in the range of 1.5-3.5 x 10(6) M-1S-1. This newly discovered metabolic pathway adds two further candidates to the list of potential targets for trypanocidal drugs.