Initial attachment of bacteriophage P22 to the Salmonella host cell is known to be mediated by interactions between lipopolysaccharide (LPS) and the phage tailspike proteins (TSP), but the events that subsequently lead to DNA injection into the bacterium are unknown. We used the binding of a fluorescent dye and DNA accessibility to DNase and restriction enzymes to analyze DNA ejection from phage particles in vitro. Ejection was specifically triggered by aggregates of purified Salmonella LPS but not by LPS with different O-antigen structure, by lipid A, phospholipids, or soluble O-antigen polysaccharide. This suggests that P22 does not use a secondary receptor at the bacterial outer membrane surface. Using phage particles reconstituted with purified mutant TSP in vitro, we found that the endorhamnosidase activity of TSP degrading the O-antigen polysaccharide was required prior to DNA ejection in vitro and DNA replication in vivo. If, however, LPS was pre-digested with soluble TSP, it was no longer able to trigger DNA ejection, even though it still contained five O-antigen oligosaccharide repeats. Together with known data on the structure of LPS and phage P22, our results suggest a molecular model. In this model, tailspikes position the phage particles on the outer membrane surface for DNA ejection. They force gp26, the central needle and plug protein of the phage tail machine, through the core oligosaccharide layer and into the hydrophobic portion of the outer membrane, leading to refolding of the gp26 lazo-domain, release of the plug, and ejection of DNA and pilot proteins.

The heat shock protein Hsp33 is a very potent molecular chaperone with a distinctive mode of functional regulation; its activity is redox-regulated. In its reduced form all six cysteinyl residues of Hsp33 are present as thiols, and Hsp33 displays no folding helper activity. Exposure of Hsp33 to oxidizing conditions like H(2)O(2), however, rapidly converts Hsp33 into an efficient molecular chaperone. Activated Hsp33 binds tightly to refolding intermediates of chemically denatured luciferase and suppresses efficiently their aggregation in vitro. Matrix-assisted laser desorption/ionization-mass spectrometry peptide mapping in combination with in vitro and on target protein chemical modification showed that this activation process of Hsp33 is accompanied by the formation of two intramolecular disulfide bonds within Hsp33: Cys(232)-S-S-Cys(234) and Cys(265)-S-S-Cys(268). Cys(141), although not involved in disulfide bond formation, was found highly reactive toward chemical modifications. In contrast, Cys(239) is readily accessible under reducing conditions but becomes poorly accessible though still reduced when Hsp33 is in its active state. This indicates a significant conformational change during the activation process of Hsp33. Mass spectrometry, thus, unraveled a novel molecular mechanism by which alteration of the disulfide bond structure, as a result of changes in the cellular redox potential, results in the activation of a molecular chaperone.