Immunophilin Modulation of Calcium Channel Gating

The FK506 binding protein (FKBP12) is the cytosolic receptor for the immunosuppressant drugs FK506 and rapamycin. Recently, we have shown that FKBP12 copurifies with the ryanodine receptor (RyR), a 565,000-Da protein with four subunits that form the intracellular calcium release channels of the sarcoplasmic reticulum and endoplasmic reticulum. To identify the cellular function of FKBP12, in the absence of the ligands rapamycin and FK506, we coexpressed RyR and FKBP12 in insect cells. By measuring the single-channel properties of the RyR-FKBP complex reconstituted into planar lipid bilayers, we showed that FKBP12 modulates channel gating by decreasing channels with subconductance states, decreasing open probability after caffeine activation, and increasing mean open time. These effects were reversed by adding FK506 or rapamycin, both of which inhibit FKBP12 isomerase activity and dissociate the FKBP-RyR complex. These studies provided a natural cellular (ligand-independent) function for FKBP12 and established that the functional calcium release channel complex includes FKBP12. We also expressed recombinant RyR1 in Xenopus laevis oocytes that lack FKBP12. Functional studies showed that the properties of the cloned RyR1, expressed in oocytes, were comparable to those of the native RyR1. These studies showed that FKBP12 is not required for tetrameric formation of the channel structure or for insertion into an intracellular calcium-containing membrane. Both insect cells (Sf9) and Xenopus oocytes are excellent models for heterologous expression of FKBP12 and RyR. Combined with determination of the single-channel properties of the resulting complex reconstituted into planar lipid bilayers, these approaches are well suited to the study of the role of FKBP12 as a modulator of calcium channel function.