Mechanism of specific target recognition and RNA hydrolysis by ribonucleolytic toxin restrictocin

Restrictocin, a member of the fungal ribotoxin family, specifically cleaves a single phosphodiester bond in the 28S rRNA and potently inhibits eukaryotic protein synthesis. Residues Tyr47, His49, Glu95, Phe96, Pro97, Arg120, and His136 have been predicted to form the active site of restrictocin. In this study, we have individually mutated these amino acids to alanine to probe their role in restrictocin structure and function. The role of Tyr47, His49, Arg120, and His136 was further investigated by making additional mutants. Mutating Arg120 or His136 to alanine or the other amino acids rendered the toxin completely inactive, whereas mutating Glu95 to alanine only partially inactivated the toxin. Mutation of Phe96 and Pro97 to Ala had no effect on the activity of restrictocin. The Tyr47 to alanine mutant was inactive in inhibiting protein synthesis, and had a nonspecific ribonuclease activity on 28S rRNA similar to that shown previously for the His49 to Ala mutant. Unlike the His136 to Ala mutant, the double mutants containing Tyr47 or His49 mutated to alanine along with His136 did not compete with restrictocin to cause a significant reduction in the extent of cleavage of 28S rRNA. In a model of restrictocin and a 29-mer RNA substrate complex, residues Tyr47, His49, Glu95, Arg120, and His136 were found to be near the cleavage site on RNA. It is proposed that in restrictocin Glu95 and His136 are directly involved in catalysis, Arg120 is involved in the stabilization of the enzyme-substrate complex, Tyr47 provides structural stability to the active site, and His49 determines the substrate specificity.

Interaction of human pancreatic ribonuclease with human ribonuclease inhibitor. Generation of inhibitor-resistant cytotoxic variants

Mammalian ribonucleases interact very strongly with the intracellular ribonuclease inhibitor (RI). Eukaryotic cells exposed to mammalian ribonucleases are protected from their cytotoxic action by the intracellular inhibition of ribonucleases by RI. Human pancreatic ribonuclease (HPR) is structurally and functionally very similar to bovine RNase A and interacts with human RI with a high affinity. In the current study, we have investigated the involvement of Lys-7, Gln-11, Asn-71, Asn-88, Gly-89, Ser-90, and Glu-111 in HPR in its interaction with human ribonuclease inhibitor. These contact residues were mutated either individually or in combination to generate mutants K7A, Q11A, N71A, E111A, N88R, G89R, S90R, K7A/E111A, Q11A/E111A, N71A/E111A, K7A/N71A/E111A, Q11A/N71A/E111A, and K7A/Q11A/N71A/E111A. Out of these, eight mutants, K7A, Q11A, N71A, S90R, E111A, Q11A/E111A, N71A/E111A, and K7A/N71A/E111A, showed an ability to evade RI more than the wild type HPR, with the triple mutant K7A/N71A/E111A having the maximum RI resistance. As a result, these variants exhibited higher cytotoxic activity than wild type HPR. The mutation of Gly-89 in HPR produced no change in the sensitivity of HPR for RI, whereas it has been reported that mutating the equivalent residue Gly-88 in RNase A yielded a variant with increased RI resistance and cytotoxicity. Hence, despite its considerable homology with RNase A, HPR shows differences in its interaction with RI. We demonstrate that interaction between human pancreatic ribonuclease and RI can be disrupted by mutating residues that are involved in HPR-RI binding. The inhibitor-resistant cytotoxic HPR mutants should be useful in developing therapeutic molecules.

Structural characterisation of a rhamnan and a fucorhamnan, both present in the lipopolysaccharide of Burkholderia vietnamiensis strain LMG 10926

The polymeric fraction isolated after mild acid hydrolysis of the lipopolysaccharide (LPS) from Burkholderia vietnamiensis strain LMG 10926 contained L-rhamnose (Rha) and D-fucose (Fuc). From NMR studies supported by the results of methylation analysis and Smith degradation, it could be inferred that the material was probably a mixture of two glycans. One component was a linear rhamnan with a trisaccharide repeating unit (1); the other was a branched fucorhamnan with a tetrasaccharide repeating unit (2). The presence of two distinct polymeric fractions in LPS is a common feature for Burkholderia species. [structures: see text]

Lipopolysaccharide from Burkholderia vietnamiensis strain LMG 6999 contains two polymers identical to those present in the reference strain for Burkholderia cepacia serogroup O4

Lipopolysaccharide was isolated from strain LMG 6999 of Burkholderia vietnamiensis. Degradative and NMR spectroscopic studies established the presence of two polymeric fractions based on the following trisaccharide repeating units: I:-->3)-alpha-D-Galp-(1-->3)-beta-D-Galp-(1-->3)-beta-D-GalpNAc- (1-->; II:-->3)-alpha-D-GalpNAc-(1-->3)-beta-D-GalpNAc-(1-->4)- alpha-L-Rhap-(1-->. The same polymers have previously been found together in lipopolysaccharide from the reference strain for Burkholderia cepacia serogroup O4 and, individually, in those from B. cepacia serogroups C (I) and A (II).

High prevalence of p53 gene alterations and protein overexpression in human esophageal cancer: correlation with dietary risk factors in India

Epidemiological studies have demonstrated an association between human esophageal cancer and dietary/nutritional risk factors in developing countries. We examined the expression of p53 protein in 51 cases of esophageal squamous cell carcinomas (ESCCs) and paired normal esophageal tissues by immunohistochemistry. Alterations in the tumor suppressor gene p53 (exons 5-8) were analyzed in 51 cases of ESCC and paired normal tissues by PCR and single-strand conformation polymorphism. p53 protein expression was correlated with major dietary risk factors and common carcinogens identified in India. Immunohistochemical analysis of frozen esophageal tissue sections using anti-p53 monoclonal antibody (D0-1) showed overexpression of the protein in the nuclei of epithelial cells in 39 of 51 (76%) cases. The histopathologically proven normal esophageal tissue sections taken from a distant site from esophageal cancer patients did not show any detectable level of p53 immunoreactivity. The PCR-single-strand conformation polymorphism analysis showed a mobility shift in 37 of 51 (72%) ESCCs. Intake of chilies was positively associated with p53 protein expression (P < 0. 01) in the esophageal cancerous lesions. Our results suggest that p53 alterations are frequent events in esophageal oncogenesis in the Indian population, which has dietary habits that are considerably different from those of the Western population.

Structure of the O-specific polysaccharide from Burkholderia vietnamiensis strain LMG 6998

The putative O-specific polymer containing D-mannose and L-rhamnose was isolated from the lipopolysaccharide obtained from cells walls of Burkholderia vietnamiensis strain LMG 6998. NMR and degradative studies showed that the polymer has a linear trisaccharide repeating-unit of the structure shown. The same polymer carrying an O-acetyl group at position 3 of the 4-substituted mannose residue has previously been found as the O antigen in the related species Burkholderia cepacia serogroup J. [formula: see text]