A novel heterodimeric antimicrobial peptide from the tree-frog Phyllomedusa distincta.

We present here the purification and the analysis of the structural and functional properties of distinctin, a 5.4 kDa heterodimeric peptide with antimicrobial activity from the tree-frog Phyllomedusa distincta. This peptide was isolated from the crude extract of skin granular glands by different chromatographic steps. Its minimal inhibitory concentration was determined against pathogenic Escherichia coli, Staphylococcus aureus, Enterococcus faecalis and Pseudomonas aeruginosa strains. Amino acid sequencing and mass spectrometric investigations demonstrated that distinctin is constituted of two different polypeptide chains connected by an intermolecular disulphide bridge. Circular dichroism and Fourier-transformed infrared spectroscopy studies showed that this molecule adopts, in water, a structure containing a significant percentage of anti-parallel beta-sheet. A conformational variation was observed under experimental conditions mimicking a membrane-like environment. Database searches did not show sequence similarities with any known antimicrobial peptides. In the light of these results, we can consider distinctin as the first example of a new class of antimicrobial heterodimeric peptides from frog skin.

Modulation of aldose reductase activity through S-thiolation by physiological thiols.

The glutathionyl-modified aldose reductase (GS-ALR2) is unique, among different S-thiolated enzyme forms, in that it displays a lower specific activity than the native enzyme (ALR2). Specific interactions of the bound glutathionyl moiety (GS) with the ALR2 active site, were predicted by a low perturbative molecular modelling approach. The outcoming GS allocation, involving interactions with residues relevant for catalysis and substrate allocation, explains the rationale behind the observed differences in the activity between GS-ALR2 and other thiol-modified enzyme forms. The reversible S-glutathionylation of ALR2 observed in cultured intact bovine lens undergoing an oxidative/non oxidative treatment cycle is discussed in terms of the potential of ALR2/GS-ALR2 inter-conversion as a response to oxidative stress conditions.

Proteomic analysis of human thyroid cell lines reveals reduced nuclear localization of Mn-SOD in poorly differentiated thyroid cancer cells.

Differential protein arrays between nuclear extracts of human thyroid cell lines obtained from tumors with different degree of differentiation were exploited to define molecular alterations occurring during thyroid tumor progression. Nuclear extracts from the well differentiated TPC-1 (from papillary carcinoma) and the poorly differentiated ARO (from anaplastic carcinoma) cells showed an overall similar pattern of protein expression as revealed by two-dimensional gel electrophoresis analysis. However, manganese-superoxide dismutase (Mn-SOD) was clearly identified by mass spectrometry procedures as significantly less expressed in ARO compared to TPC-1 cells. A reduced expression of Mn-SOD in the nuclear compartment was confirmed by Western blot and immunofluorescence analysis. A similar expression pattern of nuclear Mn-SOD was detected by immunohistochemistry in human thyroid tumors, with the lowest or absent detection in anaplastic carcinomas. Moreover, the levels of nuclear Mn-SOD in tumor cells were lower than in the normal thyrocytes. These data indicate that an altered nuclear expression of Mn-SOD parallels, together with changes in other elements of the antioxidant protective system, the loss of differentiation occurring during the progression of thyroid tumors.

Effect of glutaredoxin and protein disulfide isomerase on the glutathione-dependent folding of ribonuclease A.

Protein folding, associated with oxidation and isomerization of disulfide bonds, was studied using reduced and denatured RNase A (rd-RNase A) and mixed disulfide between glutathione and reduced RNase A derivative (GS-RNase A) as starting materials. Folding was initiated by addition of free glutathione (GSH + GSSG) and was monitored by electrospray mass spectrometry (ESMS) time-course analysis and recovery of the native catalytic activity. The ESMS analysis permitted both the identification and quantitation of the population of intermediates present during the refolding process. Refolding of rd-RNase A and GS-RNase A was also performed in the presence of glutaredoxin (Grx) and/or protein disulfide isomerase (PDI). All the analyses indicate a pathway of sequential reactions in the formation of native RNase A. First, the reduced protein reacts with a single glutathione molecule to form a mixed disulfide which then evolves to an intramolecular S-S bond via thiol-disulfide exchange. Only at this stage, the intermediate containing one intramolecular S-S reacts with a further glutathione molecule, reiterating the process. An analogous mechanism occurs in the refolding of GS-RNase A. The structural analysis of the intermediates formed during the refolding of RNase A showed for the first time that Grx is actually able to catalyze both formation and reduction of mixed disulfides involving glutatione. In both refolding processes, starting from either rd-RNase A or GS-RNase A, Grx displays a significant catalysis at the early stages of the process. Addition of PDI led to a net catalysis of the entire process without appearing to alter the refolding pathway. In the presence of both Grx and PDI, the two enzymes showed a synergistic activity either starting from rd-RNase A, as previously reported [Lundström, J., and Holmgren, A. (1995) J. Biol. Chem. 270, 7822-7828], or starting from GS-RNase A. Present data suggest that the synergistic effect can be explained assuming that Grx actually facilitates PDI action by catalyzing formation or reduction of mixed disulfides. The mixed disulfides are then rapidly converted into intramolecular disulfides in the presence of PDI. These steps are repeated sequentially throughout the whole refolding, resulting in an immediate formation of fully oxidized species even at the very beginning of the reaction. Finally, a Grx mutant, C14S Grx, in which one of the active site cysteine residues (Cys14) had been replaced by serine, had a similar effect on the distribution of folding intermediates, compared to the wild-type protein, thus demonstrating that Grx acts by a monothiol mechanism either in the reduction or in the oxidation step.

Slow folding of three-fingered toxins is associated with the accumulation of native disulfide-bonded intermediates.

Snake neurotoxins are short all-beta proteins that display a complex organization of the disulfide bonds: two bonds connect consecutive cysteine residues (C43-C54, C55-C60), and two bonds intersect when bridging (C3-C24, C17-C41) to form a particular structure classified as "disulfide beta-cross". We investigated the oxidative folding of a neurotoxin variant, named alpha62, to define the chemical nature of the three-disulfide intermediates that accumulate during the process in order to describe in detail its folding pathway. These folding intermediates were separated by reverse-phase HPLC, and their disulfide bonds were identified using a combination of tryptic hydrolysis, manual Edman degradation, and mass spectrometry. Two dominant intermediates containing three native disulfide bonds were identified, lacking the C43-C54 and C17-C41 pairing and therefore named des-[43-54] and des-[17-41], respectively. Both species were individually allowed to reoxidize under folding conditions, showing that des-[17-41] was a fast-forming nonproductive intermediate that had to interconvert into the des-[43-54] isomer before forming the native protein. Conversely, the des-[43-54] intermediate appeared to be the immediate precursor of the oxidized neurotoxin. A kinetic model for the folding of neurotoxin alpha62 which fits with the observed time-course accumulation of des-[17-41] and des-[43-54] is proposed. The effect of turn 2, located between residues 17 and 24, on the overall kinetics is discussed in view of this model.