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Emerging Concepts in Ion Channel Biophysics
Poster Abstracts
107
51-POS
Board 51
Linking Deprotonation to Channel Closure for Acid-sensing Ion Channels
Maria Musgaard
.
University of Oxford, Oxford, United Kingdom.
Acid-sensing ion channels (ASICs) are proton-gated sodium channels found throughout the
central and peripheral nervous systems and e.g. involved in pain signaling. ASICs open upon
acidification of the synaptic cleft. At least three states are involved in the functional cycle; the
closed (deprotonated), open (protonated), and desensitized (closed, protonated) states. Several
crystal structures for the protonated states have been solved, showing a trimeric channel with a
large extracellular domain (ECD) and a smaller transmembrane domain (TMD). The structure of
the deprotonated closed state is, however, still unknown. Thus, we lack structural understanding
of the coupling mechanism between protonation/deprotonation and channel opening/closure.
Two ECD proton-sensing regions have been suggested, around 20 Å and 60 Å from the TMD,
respectively, and our aim is to gain insight into how these proton sensors couple to channel
gating.
We have studied the influence of different protonation states, corresponding to different pH
values, on the overall protein dynamics by performing atomistic molecular dynamics
simulations. Our initial results, only including the ECD, illustrate that the isolated ECD is
structurally stable, and that deprotonation promotes motions in the ECD-TMD linker regions
which would cause channel closure in a full-length model with linker distances corresponding to
crystal structures of the desensitized state. On the contrary, for the protonated state the linker
regions remain separated, corresponding to the open state crystal structures. Changes around the
so-called finger-thump site upon deprotonation are observed, as expected for a proton-sensing
region, and our data suggest that this might lead to some overall changes in the flexible finger
domain.
We are currently investigating whether the full channel responds similarly to deprotonation, and
thus whether we can explain the coupling mechanism that links deprotonation to channel closure
and predict the structure of the deprotonated state.