Knowledge of the tertiary structure of the proteinase from human immunodeficiency virus HIV-1 is important to the design of inhibitors that might possess antiviral activity and thus be useful in the treatment of AIDS. The conserved Asp-Thr/Ser-Gly sequence in retroviral proteinases suggests that they exist as dimers similar to the ancestor proposed for the pepsins. Although this has been confirmed by X-ray analyses of Rous sarcoma virus and HIV-1 proteinases, these structures have overall folds that are similar to each other only where they are also similar to the pepsins. We now report a further X-ray analysis of a recombinant HIV-1 proteinase at 2.7 A resolution. The polypeptide chain adopts a fold in which the N- and C-terminal strands are organized together in a four-stranded beta-sheet. A helix precedes the single C-terminal strand, as in the Rous sarcoma virus proteinase and also in a synthetic HIV-1 proteinase, in which the cysteines have been replaced by alpha-aminobuytric acid. The structure reported here provides an explanation for the amino acid invariance amongst retroviral proteinases, but differs from that reported earlier in some residues that are candidates for substrate interactions at P3, and in the mode of intramolecular cleavage during processing of the polyprotein.

Incorporation of 4.5 nmol fluorescein isothiocyanate/mg rabbit sarcoplasmic reticulum, or of 7.4 nmol/ mg purified ATPase, was sufficient to inhibit the activity completely. These results are not consistent with the suggestion (Pick, U. and Karlish, S.J.D. (1980) Biochim. Biophys. Acta 626, 255-261) that 2 mol ATPase were inhibited by each mole of reagent incorporated. A single labelled peptide was purified from the inhibited ATPase and it was shown that Lys 3/190, 10 residues from the N-terminus of tryptic fragment B, was the reactive lysine residue. This site is close to a potential nucleotide-binding fold in the ATPase sequence. A similar peptide showing only 2 conservative replacements was isolated from the sarcoplasmic reticulum of the lobster. (Ca 2 + + Mg 2 + )A TPase inhibition Reactive lysine residue Fluorescein isothiocyanateIntegral membrane protein Nucleotide-binding fold (Sarcoplasmic reticulum)

The divalent cation Zn¥ is concentrated within nerve terminals and packaged into synaptic vesicles in the mammalian central nervous system (Frederickson, 1989). Zinc ions can also be released into the synaptic cleft following nerve terminal stimulation (Assaf & Chung, 1984;Howell et al. 1984) resulting in multiple effects on neuronal excitability by inhibiting or potentiating current flow through ligandgated and voltage-operated ion channels (Smart et al. 1994;Harrison & Gibbons, 1994). In particular, Zn¥ modulates inhibitory ã_aminobutyric acid type A (GABAA) and C (GABAC) receptors and also strychnine-sensitive glycine receptors (GlyRs) by binding to distinct site(s) on these receptor complexes. While some advances have recently been made in identifying determinants of Zn¥ binding sites on GABAA and GABAC receptor subunits (Wang et al. 1995;Wooltorton et al. 1997;Horenstein & Akabas, 1998;Fisher & McDonald, 1998), the corresponding site(s) on GlyRs remain unexplored. A complicating factor has been that Zn¥ exerts a biphasic effect on GlyRs (Bloomenthal et al. 1994;Laube et al. 1995). At low concentrations (0•5-10 ìÒ) Zn¥ potentiates glycine-activated chloride currents, while at higher concentrations (> 100 ìÒ) Zn¥ inhibits the responses to glycine. Initial interpretations of this behaviour favoured Zn¥-induced changes in agonist binding affinity (Laube et al. 1995); however, a recent study (Lynch et al. 1998) has suggested that Zn¥ potentiation of GlyR function probably

Edited by Ivan Sadowski Keywords:Signal transducer and activator of transcription 3 STAT3 PEMSA M67 SH2 X-ray crystallography a b s t r a c tThe STAT3 transcription factor plays a central role in a wide range of cancer types where it is overexpressed. Previously, phosphorylation of this protein was thought to be a prerequisite for direct binding to DNA. However, we have now shown complete binding of a purified unphosphorylated STAT3 (uSTAT3) core directly to M67 DNA, the high affinity STAT3 target DNA sequence, by a protein electrophoretic mobility shift assay (PEMSA). Binding to M67 DNA was inhibited by addition of increasing concentrations of a phosphotyrosyl peptide. X-ray crystallography demonstrates one mode of binding that is similar to that known for the STAT3 core phosphorylated at Y705. Structured summary of protein interactions: pSTAT3btc and pSTAT3btc bind by molecular sieving (View interaction)

The genes which encode glycosomal glyceraldehyde‐phosphate dehydrogenase (gGAPDH) of Trypanosoma cruzi are arranged as a tandemly repeated pair on a single chromosome and are identical at the level of nucleotide sequence. They are separated by an intergenic region which contains a 317 base pair sequence with the properties of a retroposon. The genes express a 1.5 kb mRNA and a 38 kd protein. The amino acid sequence contains features characteristic of glycosomal enzymes such as peptide insertions and a C‐terminal extension. However, T. cruzi gGAPDH lacks one of the positively charged ‘hotspot’ motifs which have been proposed as topogenic signals for import into the glycosome, a unique microbody‐like organelle. Molecular modelling of the T. cruzi and T. brucei enzymes suggests that neither structure would fulfil the requirements of the ‘hotspot’ glycosomal import model.

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