Structural Insights into Rcs Phosphotransfer: The Newly Identified RcsD-ABL Domain Enhances Interaction with the Response Regulator RcsB

The Rcs-signaling system is one of the most remarkable phosphorelay pathways in Enterobacteriaceae, comprising several membrane-bound and soluble proteins. Within the complex phosphotransfer pathway, the histidine phosphotransferase (HPt) domain of the RcsD membrane-bound component serves as a crucial factor in rnodulating the phosphorylation state of the transcription factor RcsB. We have identified a new domain, RcsD-ABL, located N terminally to RcsD-HPt that interacts with RcsB as well. We have determined its structure, characterized its interaction interface with RcsB, and built a structural model of the complex of the RcsD-ABL domain with RcsB. Our results indicate that the effector domain of RcsB, which normally binds to DNA, is recognized by RcsD-ABL, whereas the HPt domain interacts with the phosphoreceiver domain of RcsB.