Disordered Motifs and Domains in Cell Control
Poster Session II
29-POS
Board 5
Disorder Allosterically Propagates and Modulates Calcium Ligation of Synaptotagmin I
Michael E. Fealey
1,2
, Anne Hinderliter
1
.
1
University of Minnesota, Minneapolis, USA,
2
University of Minnesota Duluth, Duluth, MN,
USA.
Synaptotagmin I (Syt I) is a vesicle localized integral membrane protein responsible for sensing
the calcium influx that triggers fast synchronous release of neurotransmitter. Syt I consists of a
transmembrane helix, a cytosolic 60 residue tether region, and two C2 domains that bind calcium
and acidic phospholipids. How Syt I senses and subsequently allosterically conveys its calcium
ligation state to other fusion machinery proteins and membrane lipids to synchronize exocytosis
is incompletely understood. We believe that allosteric propagation of calcium ligation is
mediated in part by intrinsic disorder. The tether region between the transmembrane helix and
first C2 domain (C2A) consists of 83% disorder-promoting residues suggesting it represents an
intrinsically disordered region with potential for modulating activity of the adjacent C2 domains.
We probed this tether region for intrinsic disorder and found that, in aqueous solution, it is
unstructured with the dramatic allosteric effect of switching C2A’s calcium binding mode from
one devoid of cooperativity to one with cooperativity. With further analysis we discovered that
in the presence of lipid vesicles whose composition mimics a synaptic vesicle, the extent of
allosteric coupling between the tether (which becomes weakly folded) and C2A regions is
enhanced. The energetic sign of this allosteric coupling was positive, indicating the tether and
C2A are more stable together than apart at the membrane. This is a key feature of the allosteric
coupling as it provides a means of transducing the calcium ligation signal along the synaptic
vesicle surface through tether region ordering. Together, these finding indicate that Syt I harbors
an intrinsically disordered region that not only has potential to modulate calcium sensing for
triggered release, but also provides a means to allosterically propagate calcium ligation to other
lipid and protein modulators of membrane fusion.
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