The human immunodeficiency virus-1 (HIV-1) transactivator Tat occurs extracellularly and exerts immunosuppressive effects. Interestingly, Tat inhibits dipeptidyl peptidase IV (DP IV) activity of the T cellactivation marker CD26. The short N-terminal nonapeptideTat(l-9), MDPVDPNIE, also inhibits DP IV activity and suppresses DNA synthesis of tetanus toxoid-stimulated peripheral blood mononuclear cells (PBMC). Here, we present the influence of amino acid exchanges in the first three positions of Tat(l-9). For instance, the replacement of D2 of Tat(l-9) by G or K generated peptides, which inhibit DP IV-catalyzed IL-2(1-12) cleavage nearly threefold stronger. Similar effects were observed on the suppression of DNA synthesis of Tetanus toxoid-stimulated PBMC. This correlation suggests that Tat(l-9)-deduced peptides mediate antiproliferative effects at least in part via specific DP IV interactions and supports the hypothesis that CD26 plays a key role in the regulation of lymphocyte growth.

Based on the recently published structure of prolyl oligopeptidase (POP) a model of the C-terminal part of dipeptidyl peptidase IV (DPP IV) which contains the active site has been developed. The structure of the model of DPP IV shows considerable similarity to the structure of POP particularly in the active site. A hydrophobic pocket (Tyr666, Tyr670, Tyr 631, Val556) forms the S1-binding site for recognition of proline. Tyr547 may stabilise the oxyanion formed in the tetrahedral intermediates by a strong hydrogen bond. The positively charged N-terminus of ligands of DPP IV is recognised by forming a salt bridge with the acidic side chain Glu668. A second hydrophobic pocket (S2′ to S5′) may represent an important binding site for HIV-1 Tat-protein derivatives, chemokines and others.

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Models of the tertiary structures of cathepsins K, S, H, and F were constructed by using homology protein modelling methods and refinements by interactive graphics and energy minimisation. The predicted structures yield information regarding their substrate binding sites and indicate the residues surrounding these sites. The ligandbinding sites were characterised and compared with each other by means of calculated molecular electrostatic surface potentials. This will allow designing and development of new ligands specific for these cathepsins in future investigations.

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