Mismatch recognition by the human MutS homologs hMSH2-hMSH6 is regulated by adenosine nucleotide binding, supporting the hypothesis that it functions as a molecular switch. Here we show that ATP-induced release of hMSH2-hMSH6 from mismatched DNA is prevented if the ends are blocked or if the DNA is circular. We demonstrate that mismmatched DNA provokes ADP-->ATP exchange, resulting in a discernible conformational transition that converts hMSH2-hMSH6 into a sliding clamp capable of hydrolysis-independent diffusion along the DNA backbone. Our results support a model for bidirectional mismatch repair in which stochastic loading of multiple ATP-bound hMSH2-hMSH6 sliding clamps onto mismatch-containing DNA leads to activation of the repair machinery and/or other signaling effectors similar to G protein switches.

Type II DNA topoisomerases actively reduce the fractions of knotted and catenated circular DNA below thermodynamic equilibrium values. To explain this surprising finding, we designed a model in which topoisomerases introduce a sharp bend in DNA. Because the enzymes have a specific orientation relative to the bend, they act like Maxwell's demon, providing unidirectional strand passage. Quantitative analysis of the model by computer simulations proved that it can explain much of the experimental data. The required sharp DNA bend was demonstrated by a greatly increased cyclization of short DNA fragments from topoisomerase binding and by direct visualization with electron microscopy.T ype II topoisomerases are essential enzymes that pass one DNA through another and thereby remove DNA entanglements. They make a transient double-stranded break in a gate segment (G segment) that allows passage by another segment (T segment) of the same or another DNA molecule (reviewed in refs. 1 and 2). Thus, these enzymes have the potential to convert real DNA molecules into phantom chains that freely pass through themselves to generate an equilibrium distribution of knots, catenanes, and supercoils.The actual picture is more complex and more interesting. The observed steady-state fractions of knotted, catenated, and supercoiled DNAs produced by type II topoisomerases are up to two orders of magnitude lower than at equilibrium (3). Thermodynamically, there is no contradiction in this finding because the enzymes use the energy of ATP hydrolysis. Active topology simplification by topoisomerases has an important biological consequence. It helps explain how topoisomerases can remove all DNA entanglements under the crowded cellular conditions which favor the opposite outcome. The challenge, though, is to understand how type II topoisomerases actively simplify DNA topology. Topology is a global property of circular DNA molecules, and yet it is determined by the much smaller topoisomerases, which can act only locally.Two models have been suggested to explain active simplification of DNA topology. First, if type II topoisomerases corral the T segment within a small loop of DNA containing the G segment, active disentanglement would result (3). However, it was pointed out when this model was suggested (3) that to account for the large effects observed, the loop trapping would need substantial energy input from ATP hydrolysis for the transport of the DNA along the enzymes, and these enzymes are energetically efficient (4). Moreover, no direct experimental data supporting the model have been presented.Second, a kinetic proofreading model proposed that two successive bindings of T segments are required for strand passage (5). The first binding event converts the enzyme bound with a G segment to an activated state. An assumption of the model is that segment collision in the knotted state occurs about 1͞P k times more often than in the unknotted state, where P k is the equilibrium probability of knotting. Our computer simulations below show that th...

Fas, a tumor necrosis factor family receptor, is activated by the membrane protein Fas ligand (FasL) expressed on various immune cells. Fas signaling triggers apoptosis and induces inflammatory cytokine production. Among the Fas induced cytokines, the IL-1β family cytokines require proteolysis to gain biological activity. Inflammasomes, which respond to pathogens and danger signals, cleave IL-1β cytokines via caspase-1. The mechanisms, by which Fas regulates IL-1β activation, however, remain unresolved. Here, we demonstrate that macrophages exposed to TLR ligands upregulate Fas, which renders them responsive to receptor engagement by Fas ligand. Fas signaling activates caspase-8 in macrophages and dendritic cells leading to the maturation of IL-1β and IL-18 independently of inflammasomes or Rip3. Hence, Fas controls a novel non-canonical IL-1β activation pathway in myeloid cells, which could play an essential role in inflammatory processes, tumor surveillance and control of infectious diseases.