The signal transduction ATPases with numerous domains (STAND) represent a newly recognized class of widespread, sophisticated ATPases that are related to the AAA+ proteins and that function as signaling hubs. These proteins control diverse biological processes in bacteria and eukaryotes, including gene expression, apoptosis, and innate immunity responses. They function as tightly regulated switches, with the off and on positions corresponding to a long-lived monomeric, ADP-bound form and a multimeric, ATP-bound form, respectively. Inducer binding to the sensor domain activates the protein by promoting ADP for ATP exchange, probably through removal of an intramolecular inhibitory interaction, whereas ATP hydrolysis turns off the protein. One key component of the switch is a three-domain module carrying the ATPase activity (nucleotide-binding oligomerization domain [NOD]). Analysis of the atomic structures of four crystallized nucleotide-bound NOD modules provides an unprecedented insight into the NOD conformational changes underlying the activation process.

The maltose system of Escherichia coli consists of a number of genes encoding proteins involved in the uptake and metabolism of maltose and maltodextrins. The system is positively regulated by MalT, its transcriptional activator. MalT activity is controlled by two regulatory circuits: a positive one with maltotriose as effector and a negative one involving several proteins. MalK, the ATP-hydrolyzing subunit of the cognate ABC transporter, MalY, an enzyme with the activity of a cystathionase, and Aes, an acetyl esterase, phenotypically act as repressors of MalT activity. By in vivo titration assays, we have shown that the N-terminal 250 amino acids of MalT contain the interaction site for MalY but not for MalK. This was confirmed by gel filtration analysis, where MalY was shown to coelute with the N-terminal MalT structural domain. Mutants in MalT causing elevated mal gene expression in the absence of exogenous maltodextrins were tested in their response to the three repressors. The different MalT mutations exhibited a various degree of sensitivity towards these repressors, but none was resistant to all of them. Some of them became nearly completely resistant to Aes while still being sensitive to MalY. These mutations are located at positions 38, 220, 243, and 359, most likely defining the interaction patch with Aes on the three-dimensional structure of MalT.The Escherichia coli maltose system consists of 10 genes encoding proteins dedicated to the uptake and the metabolism of maltose and maltodextrins (4). These genes are under the control of MalT, a specific transcriptional activator of 901 amino acids (aa). MalT belongs to a class of bacterial transactivators, the MalT or LAL family (11, 42). They are large proteins (Ͼ90 kDa), possess an ATP binding site near their N terminus, and share homology with LuxR near their C terminus. MalT binds and activates its target promoters (29) only in the presence of ATP (34) and the inducer maltotriose (28). MalT consists of four structural domains (11): domain 1 (DT1, aa 1 to 241) binds ATP, domains 2 (DT2, aa 250 to 436) and 3 (DT3, aa 437 to 806) bind the inducer, and domain 4 (DT4, aa 807 to 901) harbors the DNA binding site (11,43). MalT exists in an equilibrium between a monomeric (inactive) and a monomeric (active) form and is prone to multimerize. This equilibrium is shifted to the active form by the inducer maltotriose, which triggers a conformational change involving DT1, -2, and -3 and the linkers in between. The conformational change, which also requires ATP, is a step towards the formation of a high-order oligomer, the transcriptionally competent form of the protein (11, 36). Point mutations in malT (malT c ) have been isolated that confer a constitutive expression of the maltose regulon when maltodextrin is not present in the growth medium (12, 13) The in vitro analysis of two corresponding MalT c proteins revealed that, in contrast to the wild-type MalT, they could activate transcription from a MalT-dependent promoter in the absence of maltotriose but could st...

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