The bacterial enteropathogen Salmonella typhimurium employs a specialized type III secretion system to inject toxins into host cells, which trigger signaling cascades leading to cell death in macrophages, secretion of pro-inflammatory cytokines, or rearrangements of the host cell cytoskeleton and thus to bacterial invasion. Two of the injected toxins, SopE and the 69% identical protein SopE2, are highly efficient guanine nucleotide exchange factors for the RhoGTPase Cdc42 of the host cell. However, it has been a puzzle why S. typhimurium might employ two toxins with redundant function. We hypothesized that SopE and SopE2 might have different specificities for certain host cellular RhoGTPases. In vitro guanine nucleotide exchange assays and surface plasmon resonance measurements revealed that SopE is an efficient guanine nucleotide exchange factor for Cdc42 and Rac1, whereas SopE2 was interacting efficiently only with Cdc42, but not with Rac1. Affinity precipitation of Cdc42⅐GTP and Rac1⅐GTP from lysates and characteristic cytoskeletal rearrangements of infected tissue culture cells confirmed that SopE is highly efficient at activating Cdc42 and Rac1 in vivo, whereas SopE2 was efficiently activating Cdc42, but not Rac1. We conclude that the translocated effector proteins SopE and SopE2 allow S. typhimurium to specifically activate different sets of RhoGTPase signaling cascades.

The granule exocytosis cytotoxicity pathway is the major molecular mechanism for cytotoxic T lymphocyte (CTL) and natural killer (NK) cytotoxicity, but the question of how these cytotoxic lymphocytes avoid self-destruction after secreting perforin has remained unresolved. We show that CTL and NK cells die within a few hours if they are triggered to degranulate in the presence of nontoxic thiol cathepsin protease inhibitors. The potent activity of the impermeant, highly cathepsin B–specific membrane inhibitors CA074 and NS-196 strongly implicates extracellular cathepsin B. CTL suicide in the presence of cathepsin inhibitors requires the granule exocytosis cytotoxicity pathway, as it is normal with CTLs from gld mice, but does not occur in CTLs from perforin knockout mice. Flow cytometry shows that CTLs express low to undetectable levels of cathepsin B on their surface before degranulation, with a substantial rapid increase after T cell receptor triggering. Surface cathepsin B eluted from live CTL after degranulation by calcium chelation is the single chain processed form of active cathepsin B. Degranulated CTLs are surface biotinylated by the cathepsin B–specific affinity reagent NS-196, which exclusively labels immunoreactive cathepsin B. These experiments support a model in which granule-derived surface cathepsin B provides self-protection for degranulating cytotoxic lymphocytes.

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N-terminally truncated forms of chicken egg white cystatin and its cyanogen bromide fragments were isolated and assayed for inhibition of papain. Truncated forms beginning with Gly-9 and Ala-10 had a 5000-fold lower affinity for papain than the two isoelectric forms (pi=6.5 and 5.6) of the full-length inhibitor (K~--6 pM and 7 pM) or a truncated form beginning with Leu-7 (/~ = 6 pM), indicating the outstanding importance of one or two residues preceding conserved Gly-9 for binding. A weak inhibition of papain (K~ = 900 nM) was exhibited by the intermediate cyanogen bromide fragment (residues 30-89) containing the chicken cystatin QLVSG variation of the QWAG segment which is conserved in almost all members of the cystatin superfamily. The obtained affinity data provide independent evidence for the validity of the proposed docking model of a chicken cystatin-papain complex [( ) EMBO J. 7, 2593[( -2599.

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Ubiquitous calpains (-and m-calpain) have been repeatedly implicated in apoptosis, but the underlying mechanism(s) remain(s) to be elucidated. We examined ionomycin-induced cell death in LCLC 103H cells, derived from a human large cell lung carcinoma. We detected hallmarks of apoptosis such as membrane blebbing, nuclear condensation, DNA ladder formation, caspase activation, and poly-(ADP-ribose)polymerase cleavage. Apoptosis was prevented by preincubation of the cells with the calpain inhibitor acetyl-calpastatin 27-peptide and the caspase inhibitor Z-DEVD-fmk, implicating both the calpains and caspases in the apoptotic process. The apoptotic events correlated in a calpastatin-inhibitable manner with Bid and Bcl-2 decrease and with activation of caspases-9, -3, and -7. In vitro both ubiquitous calpains cleaved recombinant Bcl-2, Bid, and Bcl-x L at single sites truncating their N-terminal regions. Binding studies revealed diminished interactions of calpain-truncated Bcl-2 and Bid with immobilized intact Bcl-2 family proteins. Moreover, calpain-cleaved Bcl-2 and Bid induced cytochrome c release from isolated mitochondria. We conclude that ionomycin-induced calpain activation promotes decrease of Bcl-2 proteins thereby triggering the intrinsic apoptotic pathway.

Cystatin, the protein inhibitor of cysteine proteinases from chicken egg white was purified by a new method. The two major forms with pi 6.5 (Peak I) and 5.6 (Peak II) were separated. Molecular masses of both forms are approx. 12 700 Da as determined by gel chromatography; Form A from Peak I has a molecular mass of 12 191 Da as calculated from its aminoacid sequence. The complete amino-acid sequence of Form A was determined by automated solid-phase Edman degradation of the whole inhibitor and its cyanogen bromide fragments. It contains 108 amino-acid residues. Form B from Peak II represents an elongation of Form A by 8 amino-acid residues at the N-terminus. Cystatin contains four cysteine residues, presumably forming two disulphide bridges. Comparison of the amino-acid sequences and near ultraviolet circular dichroism spectra of stefin, the cysteine proteinase inhibitor from human granulocytes, and cystatin shows that the two proteins are entirely different. According to the primary structures, probably neither proteinase inhibitor is involved in a thiol-disulphide exchange mechanism in the interaction with its target enzyme. Cysteinproteinasen-Inhibitoren, III. Aminosäuresequenz von Cystatin aus HühnereiweißZusammenfassung: Aus Hühnereiweiß wurde mit einer neuen Methode der CysteinproteinasenInhibitor Cystatin isoliert. Zwei Hauptformen mit isoelektrischen Punkten von 6.5 (Peak I) und 5.6 (Peak II) konnten getrennt werden. Die durch Gelchromatographie bestimmte apparente Molekularmasse beträgt für beide Formen rund 12 700 Da; aus der Aminosäuresequenz wurde für die Form A aus Peak I eine Molekularmasse von 12 191 Da berechnet. Durch automatischen Festphasen-Edman-Abbau des ganzen Inhibitors und seiner Bromcyanfragmente wurde die vollständige Aminosäuresequenz der Form A ermittelt (108 Aminosäurereste). Form B aus Peak II ist gegenüber der Form A um 8 Aminosäurereste am NTerminus verlängert. Cystatin enthält 4 Cysteinreste, die wahrscheinlich über Disulfidbrücken verknüpft sind.