Signal Transmission through the HtrII Transducer Alters the Interaction of Two α-Helices in the HAMP Domain

A conformational change of the transducer HtrII upon photoexcitation of the associated photoreceptor sensory rhodopsin II (SRII) was investigated by monitoring the kinetics of volume changes and the diffusion coefficient (D) of the complex during the photochemical reaction cycle. To localize the region of the transducer responsible, we truncated it at various positions in the cytoplasmic HAMP (histidine kinases, adenylyl cyclases, methyl-accepting chemotaxis proteins, and phosphatases) domain. The truncations do not alter receptor binding, which is dependent primarily on membrane-embedded domain interactions. We found that the light-induced reduction in D occurs in transducers of lengths 120 and 157 residues (Tr120 and Tr157), which are both predicted to contain a HAMP domain consisting of two amphipathic α-helices (AS-1 and AS-2). In contrast, the change in D was abolished in a transducer of 114 amino acid residues (Tr114), which lacks a distal portion of the second α-helix AS-2. The volume changes in SRII–Tr114 are comparable in amplitude and kinetics with those in SRII–Tr120 and SRII–Tr157, confirming the integrity of the complex, which was previously concluded from the similar SRII binding affinity and similar blocking of SRII proton transport by full-length HtrII and Tr114. Our results indicate that a substantial conformational change occurs in the HAMP domain during SRII–HtrII signaling. The data presented here are the first demonstration of stimulus-induced conformational changes of a HAMP domain and provide evidence that the presence of AS-2 is crucial for the conformational alterations. The reduction in diffusion coefficient is likely to due to structural changes in the AS-1 and AS-2 helices such that hydrogen bonding with the surrounding water molecules is increased, thereby increasing friction with the solvent. Similar structural changes may be a general feature in HAMP domain switching, which occurs in diverse signaling proteins, including sensor kinases, taxis receptors/transducers, adenylyl cyclases, and phosphatases.