Two-component signaling in the AAA + ATPase DctD: binding Mg2+ and BeF3- selects between alternate dimeric states of...

A Crystallogral structure is described for the Mg2+-BeF3--bound receiver domain of Sinorhizobium meliloti DctD bearing amino acid substitution E121K. Differences between the apo- and ligand-bound active sites are similar to those reported for other receiver domains. However, the off and on states of the DctD receiver domain are characterized by dramatically different dimeric structures, which supports the following hypothesis of signal transduction. In the off state, the receiver domain and coiled-coil linker form a dimer that inhibits oligomerization of the AAA+ ATPase domain. In this conformation, the receiver domain cannot be phosphorylated or bind Mg2+ and BeF3-. Instead, these modifications stabilize an alternative dimeric conformation that repositions the subunits by approximately 20 A, thus replacing the a4-b5-a5 interface with an a4-b5 interface. Reoriented receiver domains permit the ATPase domain to oligomerize and stimulate open complex formation by the s54 form of RNA polymerase. NtrC, which shares 38% sequence identity with DctD, works differently. Its activated receiver domain must facilitate oligomerization of its ATPase domain. Significant differences exist in the signaling surfaces of the DctD and NtrC receiver domains that may help explain how triggering the common two-component switch can variously regulate assembly of a AAA+ ATPase domain.