The cardiac
isoform of the
ryanodine receptor (
RyR2) from dog binds predominantly a 12.6-kDa
isoform of the
FK506-binding protein (
FKBP12.6), whereas
RyR2 from other species binds both
FKBP12.6 and the closely related
isoform FKBP12. The role played by
FKBP12.6 in modulating
calcium release by
RyR2 is unclear at present. We have used cryoelectron microscopy and three-dimensional (3D) reconstruction techniques to determine the binding position of
FKBP12.6 on the surface of canine
RyR2.
Buffer conditions that should favor the "open" state of
RyR2 were used. Quantitative comparison of 3D reconstructions of
RyR2 in the presence and absence of
FKBP12.6 reveals that
FKBP12.6 binds along the sides of the square-shaped cytoplasmic region of the receptor, adjacent to domain 9, which forms part of the four clamp (corner-forming) structures. The location of the
FKBP12.6 binding site on "open"
RyR2 appears similar, but slightly displaced (by 1-2 nm) from that found previously for
FKBP12 binding to the skeletal muscle
ryanodine receptor that was in the
buffer that favors the "closed" state. The conformation of
RyR2 containing bound
FKBP12.6 differs considerably from that depleted of
FKBP12.6, particularly in the transmembrane region and in the clamp structures. The x-ray structure of
FKBP12.6 was docked into the region of the 3D reconstruction that is attributable to bound
FKBP12.6, to show the relative orientations of
amino acid residues (Gln-31, Asn-32, Phe-59) that have been implicated as being critical in interactions with
RyR2. A thorough understanding of the structural basis of RyR2-FKBP12.6 interaction should aid in understanding the roles that have been proposed for
FKBP12.6 in
heart failure and in certain forms of
sudden cardiac death.