The 12 kDa FK506-binding protein (FKBP12) constitutively binds to
the calcium release channel RyR1. Removal of FKBP12 using FK506 or
rapamycin causes an increased open probability and an increase in
the frequency of sub-conductance states in RyR1. Using cryo-electron
microscopy and single-particle image processing, we have determined
the 3D difference map of FKBP12 associated with RyR1 at 16 A resolution
that can be fitted with the atomic model of FKBP12 in a unique orientation.
This has allowed us to better define the surfaces of close apposition
between FKBP12 and RyR1. Our results shed light on the role of several
FKBP12 residues that had been found critical for the specificity
of the RyR1-FKBP12 interaction. As predicted from previous immunoprecipitation
studies, our results suggest that Gln3 participates directly in this
interaction. The orientation of RyR1-bound FKBP12, with part of its
FK506 binding site facing towards RyR1, allows us to propose how
FK506 is involved in the dissociation of FKBP12 from RyR1.