Abstract
RyR1 is an intracellular calcium channel with a central role in muscle
contraction. We obtained a three-dimensional reconstruction of the
RyR1 in the closed state at a nominal resolution of approximately
10 A using cryo-EM. The cytoplasmic assembly consists of a series
of interconnected tubular structures that merge into four columns
that extend into the transmembrane assembly. The transmembrane assembly,
which has at least six transmembrane alpha-helices per monomer, has
four tilted rods that can be fitted with the inner helices of a closed
K$^+$ channel atomic structure. The rods splay out at the lumenal
side and converge into a dense ring at the cytoplasmic side. Another
set of four rods emerges from this ring and shapes the inner part
of the four columns. The resulting constricted axial structure provides
direct continuity between cytoplasmic and transmembrane assemblies,
and a possible mechanism for control of channel gating through conformational
changes in the cytoplasmic assembly.
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