ASYMMETRICAL BLOCKADE OF THE CA2+ RELEASE CHANNEL (RYANODINE RECEPTOR) BY 12-KDA FK506 BINDING-PROTEIN

A soluble 12-kDa FK506 binding protein (FKBP12), the cellular receptor of the immunosuppressive drug FK506, is tightly associated with the Ca2+ release channel of rabbit skeletal muscle sarcoplasmic reticulum [Jayaraman, T., Brillantes, A, M., Timerman, A, P., Fleischer, S., Erdjument-Bromage, H., Tempst, P. and Marks, A. (1992) J. Biol. Chem. 267, 9474-9477], We have assessed the role of excess free FKBP12 in the function of single Ca2+ release channels incorporated into planar lipid bilayers. The addition of human recombinant FKBP12 (hFKBP12) to the cytoplasmic face of the Ca2+ release channel blocked the flow of cytoplasmic to luminal current (outward current) in a concentration-dependent manner but had no significant effect on the flow of luminal to cytoplasmic current (inward current), The luminal to cytoplasmic flow of current was modulated by Ca2+, Mg2+, ATP, caffeine, and ryanodine in the presence and absence of hFKBP12. An immunosuppressive drug, L-683,590, an analog of FK506, did not block or reverse the asymmetrical hFKBP12 blockade of single Ca2+ release channels in planar lipid bilayers, FKBP12 may play a role in regulation of the flow of ions into the lumen of the sarcoplasmic reticulum through the Ca2+ release channel.