Possible mechanism of structural transformations induced by StAsp-PSI in lipid membranes.

In the present work we have analyzed the effect of StAsp-PSI (plant-specific insert of potato aspartic protease) on the structural and thermotropic properties of the major phospholipid types of bacterial and animal cells. Results obtained suggest that StAsp-PSI induces a destabilization of the membrane bilayers, depending on the time of interaction between the protein and the bilayers, rather than on its concentration. This temporal delay would be consistent with a lateral diffusion of StAsp-PSI monomers to assemble into aggregates to form pores. Like with the results previously reported for the StAsp-PSI circular dichroism, data obtained here from IR spectroscopy show that there are slight changes in the StAsp-PSI secondary structure in the presence of lipid membranes; suggesting that these changes could be related with the StAsp-PSI self-association. Results obtained from steady-state fluorescence anisotropy and differential scanning calorimetry assays suggest that StAsp-PSI interacts with both uncharged and negatively charged phospholipids, modulates the phase polymorphic behavior of model membranes and partitions and buries differentially in the membrane depending on the presence of negatively charged phospholipids.

Cholesterol and membrane phospholipid compositions modulate the leakage capacity of the swaposin domain from a potato aspartic protease (StAsp-PSI).

Potato aspartic proteases (StAPs) and their swaposin domain (StAsp-PSI) are proteins with cytotoxic activity which involves plasma membrane destabilization. The ability of these proteins to produce cell death varies with the cellular type. Therefore, StAPs and StAsp-PSI selective cytotoxicity could be attributed to the different membrane lipid compositions of target cells. In this work we investigate the possible mechanism by which StAPs and StAsp-PSI produce selective membrane destabilization. Results obtained from leakage assays show that StAsp-PSI is a potent inducer of the leakage of LUVs containing anionic phospholipids, especially those containing phosphatidylglycerol. Based in these results, we suggest that the cytotoxic activity of StAsp-PSI on pathogenic microorganisms could be mediated by the attraction between the exposed positive domains of StAsp-PSI and the negatively charged microorganism membrane. On the other hand, our circular dichroism spectroscopic measurements and analysis by size exclusion chromatography and followed by electrophoresis, indicate that hydrophobic environment is necessary to StAsp-PSI oligomerization and both StAsp-PSI disulfide bounds and membrane with negative charged phospholipids are required by StAsp-PSI to produce membrane destabilization and then induce cell death in tumors and microorganism cell targets. Additionally, we demonstrate that the presence of cholesterol into the LUV membranes strongly diminishes the capacity of StAsp-PSI to produce leakage. This result suggests that the lack of hemolytic and cytotoxic activities on human lymphocytes of StAsp-PSI/StAPs may be partly due by the presence of cholesterol in these cell membrane types.

Isolation and characterization of an aspartic protease from Salpichroa origanifolia fruits.

This report describes the purification of an aspartic protease (salpichroin) from ripe fruits of Salpichroa origanifolia (Solanaceae) starting with precipitation using organic solvents and anionexchange chromatography with 32.1% recovery and 13.4-fold purification. SDS-PAGE and zymograms of this enzyme showed a single band corresponding to an apparent molecular mass of approximately 32 kDa. The biochemical and kinetic characterization of the pure enzyme showed an acidic behavior with an optimal pH value around 3.0-4.5 with hemoglobin and 5.5-6.0 with casein. Salpichroin activity was inhibited by pepstatin but not by phenylmethylsulfonyl fluoride, E-64, EDTA or 1,10-phenanthroline, thus suggesting an aspartic protease behavior. Salpichroin hydrolyzed natural substrates, such as casein and hemoglobin, with high specific activity. Kinetic studies conducted with the synthetic peptide H-Pro- Thr-Glu-Phe-p-(NO2)-Phe-Arg-Leu-OH showed lower affinity (Km 494 µM) than other representative aspartic proteases. By investigating the cleavage of oxidized insulin ß-chain to establish the hydrolytic specificity of salpichroin, we found six cleavage sites on the substrate of peptide bonds similar to those of chymosin. MALDI-TOF/TOF-MS of the tryptic ingel digest of salpichroin showed that the isolated protease shared homologous sequences with other plant proteases of the A1 aspartic protease family. This is the first report concerning the isolation and biochemical characterization of an aspartic protease isolated from Salpichroa origanifolia fruits.

Evaluation of in vitro cytotoxic activity of mono-PEGylated StAP3 (Solanum tuberosum aspartic protease 3) forms.

StAP3 is a plant aspartic protease with cytotoxic activity toward a broad spectrum of pathogens, including potato and human pathogen microorganisms, and cancer cells, but not against human T cells, human red blood cells or plant cells. For this reason, StAP3 could be a promising and potential drug candidate for future therapies. In this work, the improvement of the performance of StAP3 was achieved by means of a modification with PEG. The separation of a mono-PEGylated StAP3 fraction was easily performed by gel filtration chromatography. The mono-PEGylated StAP3 fraction was studied in terms of in vitro antimicrobial activity, exhibiting higher antimicrobial activity against Fusarium solani spores and Bacillus cereus, but slightly lower activity against Escherichia coli than native protein. Such increase in antifungal activity has not been reported previously for a PEGylated plant protein. In addition, PEGylation did not affect the selective cytotoxicity of StAP3, since no hemolytic activity was observed.