5 research outputs found
Clustering and Dynamics of Phototransducer Signaling Domains Revealed by Site-Directed Spin Labeling Electron Paramagnetic Resonance on SRII/HtrII in Membranes and Nanodiscs
In halophilic archaea the photophobic
response is mediated by the
membrane-embedded 2:2 photoreceptor/-transducer complex SRII/HtrII,
the latter being homologous to the bacterial chemoreceptors. Both
systems bias the rotation direction of the flagellar motor via a two-component
system coupled to an extended cytoplasmic signaling domain formed
by a four helical antiparallel coiled-coil structure. For signal propagation
by the HAMP domains connecting the transmembrane and cytoplasmic domains,
it was suggested that a two-state thermodynamic equilibrium found
for the first HAMP domain in <i>Np</i>SRII/<i>Np</i>HtrII is shifted upon activation, yet signal propagation along the
coiled-coil transducer remains largely elusive, including the activation
mechanism of the coupled kinase CheA. We investigated the dynamic
and structural properties of the cytoplasmic tip domain of <i>Np</i>HtrII in terms of signal transduction and putative oligomerization
using site-directed spin labeling electron paramagnetic resonance
spectroscopy. We show that the cytoplasmic tip domain of <i>Np</i>HtrII is engaged in a two-state equilibrium between a dynamic and
a compact conformation like what was found for the first HAMP domain,
thus strengthening the assumption that dynamics are the language of
signal transfer. Interspin distance measurements in membranes and
on isolated 2:2 photoreceptor/transducer complexes in nanolipoprotein
particles provide evidence that archaeal photoreceptor/-transducer
complexes analogous to chemoreceptors form trimers-of-dimers or higher-order
assemblies even in the absence of the cytoplasmic components CheA
and CheW, underlining conservation of the overall mechanistic principles
underlying archaeal phototaxis and bacterial chemotaxis systems. Furthermore,
our results revealed a significant influence of the <i>Np</i>HtrII signaling domain on the <i>Np</i>SRII photocycle
kinetics, providing evidence for a conformational coupling of SRII
and HtrII in these complexes