To investigate the possible biological function of the lateral "strand dimer" observed in crystal structures of a D1 domain extracellular fragment from N-cadherin, we have undertaken site-directed mutagenesis studies of this molecule. Mutation of most residues important in the strand dimer interface abolish the ability of N-cadherin to mediate cell adhesion. Mutation of an analogous central residue (Trp-2) in E-cadherin also abrogates the adhesive capacity of that molecule. We also determined the crystal structure of a Ca2+-complexed two-domain fragment from N-cadherin. This structure, like its E-cadherin counterpart, does not adopt the strand dimer conformation. This suggests the possibility that classical cadherins might stably exist in both dimeric and monomeric forms. Data from several laboratories imply that lateral dimerization or clustering of cadherins may increase their adhesivity. We suggest the possibility that the strand dimer may play a role in this activation.

The recent spread of next-generation sequencing techniques has facilitated transcriptome analyses of non-model plants. As a result, many of the genes encoding enzymes related to the production of specialized metabolites have been identified. Compounds derived from 2,3-oxidosqualene (the common precursor of sterols, steroids and triterpenoids), a linear compound of 30 carbon atoms produced through the mevalonate pathway, are called triterpenes. These include essential sterols, which are structural components of biomembranes; steroids such as the plant hormones, brassinolides and the toxin in potatoes, solanine; as well as the structurally diverse triterpenoids. Triterpenoids containing one or more sugar moieties attached to triterpenoid aglycones are called triterpenoid saponins. Triterpenoid saponins have been shown to have various medicinal properties, such as anti-inflammatory, anticancerogenic and antiviral effects. This review summarizes the recent progress in gene discovery and elucidates the biochemical functions of biosynthetic enzymes in triterpenoid saponin biosynthesis. Special focus is placed on key players in generating the structural diversity of triterpenoid saponins, cytochrome P450 monooxygenases (P450s) and the UDP-dependent glycosyltransferases (UGTs). Perspectives on further gene discovery and the use of biosynthetic genes for the microbial production of plant-derived triterpenoid saponins are also discussed.

No abstract

Leucocin A is a class IIa bacteriocin produced by Leuconostoc spp. that has previously been shown to inhibit the growth of Listeria monocytogenes. A spontaneous resistant mutant of L. monocytogenes was isolated and found to be resistant to leucocin A at levels in excess of 2 mg/ml. The mutant showed no significant cross-resistance to nontype IIa bacteriocins including nisaplin and ESF1-7GR. However, there were no inhibition zones found on a lawn of the mutant when challenged with an extract containing 51,200 AU of pediocin PA-2 per ml as determined by a simultaneous assay on the sensitive wild-type strain. DNA and protein analysis of the resistant and susceptible strains were carried out using silver-stained amplified fragment length polymorphism (ssAFLP) and one-and two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), respectively. Two-dimensional SDS-PAGE clearly showed a 35-kDa protein which was present in the sensitive but absent from the resistant strain. The N-terminal end of the 35-kDa protein was sequenced and found to have an 83% homology to the mannose-specific phosphotransferase system enzyme IIAB of Streptococcus salivarius.Bacteriocins are proteinaceous antimicrobial peptides synthesized by bacteria and are usually active against strains closely related to the producing organism (13, 21). There is increasing interest in the use of bacteriocins from lactic acid bacteria in foods (20,22). For example, the class I bacteriocin nisin was approved for use in 1969 and has been applied to several foods (6). Class II bacteriocins are characteristically small, heat-stable peptides (15), some of which contain the consensus motif YGNGV and include leucocin A and pediocin PA-2. These were isolated from food-related lactic acid bacteria and have potential as food preservatives. With the addition of nisin (and other bacteriocins) into the environment, there is a concomitant interest in resistance to these antimicrobial compounds (4,5,10,18,19,33). The mechanism of resistance has not been fully established, but has been attributed to cell membrane and S-layer changes.Leucocin A is produced by several Leuconostoc spp. It inhibits the growth of the important potential food-borne pathogen Listeria monocytogenes (11,24). Resistance to class IIa bacteriocins has been reported to be a stable phenomenon (8,30). Transposon-mediated inactivation of 54 in L. monocytogenes has rendered it resistant to mesentericin Y105 (a class IIa bacteriocin) (31). There is, however, very little known about the molecular basis of resistance in naturally isolated strains.In an attempt to discover resistance-associated phenomena at both the DNA and proteiomic levels, amplified fragment length polymorphism (AFLP) and two-dimensional (2-D) gel electrophoresis were employed, respectively. AFLP is a genome fingerprinting technique based on the selective amplification of a subset of DNA fragments generated by restriction enzyme digestion (34). 2-D gel electrophoresis is a powerful tool for the analysis of complex p...