The signal transduction ATPases with numerous domains (STAND) family of proteins can be subdivided, according to the characteristics of the nucleotide-binding oligomerization domain (NOD), into three groups: the AP subfamily that includes APAF-1, an apoptotic mediator; the NACHT subfamily members that are involved in mammalian innate immunity; and the MalT subfamily of bacterial transcription factors. Although the presence of ATP is critical for the activation of these proteins, which are generally monomeric in the inactive state and oligomerize upon activation, it has been unclear what role ATP plays in this process and whether ATP hydrolysis was involved. Marquenet and Richet determined that the Escherichia coli MalT transcription factor cycles between an ADP-bound inactive form and an ATP-bound active form and that intrinsic ATP hydrolysis was required to return the protein to the inactive state. This is quite similar to the mechanism by which heterotrimeric guanine nucleotide-binding proteins (G proteins) and small guanosine triphosphatases (GTPases) function, except that the latter bind guanine nucleotides instead of adenosine nucleotides. Endogenous inhibitors of MalT--MalY, Aes, and MalK--bind and stabilize the monomeric form and, in the presence of ATP, maltotriose stimulates the oligomerization and formation of active MalT. The authors compared the activities of mutant MalT proteins that bound, but could not hydrolyze ATP, with the activity of wild-type MalT in vivo. MalT-D129A was most extensively studied. In vivo, MalT-D129A was hyperactive, stimulating transcription of a reporter to higher levels than the wild-type protein, and the activity of the ATP hydrolysis mutant was also not inhibited by MalK or MalY. Gel filtration assays showed that MalT-D129A did not bind effectively to MalY and that MalT-D129A existed in an oligomeric state with no detectable monomers in the presence or absence of added maltotriose or ATP. The wild-type protein bound MalY and existed as only monomers in the absence of maltotriose and ATP and as both oligomeric and monomeric forms in the presence of maltotriose and ATP. Assays to detect the bound nucleotide showed that MalT-D129A was bound to ATP in a 1:1 stoichometry, suggesting that it purified in an ATP-bound state. In contrast, wild-type MalT in the absence of ATP and maltotriose, either alone or when bound to MalY, was bound to ADP. Maltotriose stimulated the exchange of ADP for ATP on wild-type MalT, which was detected either using nonhydrolyzable analogs of ATP or with radiolabeled ATP. Thus, maltotriose appears to switch MalT from an inactive ADP-bound form to an activated ATP-bound form, and hydrolysis of ATP is necessary for MalT cycling and regulation. It will be interesting to see if other NOD proteins show similar adenine nucleotide exchange and hydrolysis as part of their signaling mechanism.
E. Marquenet, E. Richet, How integration pf positive and negative regulatory signals by a STAND signaling protein depends on ATP hydrolysis. Mol. Cell 28, 187-199 (2007). [PubMed]