Role of FKBP12.6 in cADPR-induced activation of reconstituted ryanodine receptors from arterial smooth muscle

cADP ribose (cADPR) serves as second messenger to activate the ryanodine receptors (RyRs) of the sarcoplasmic reticulum (SR) and mobilize intracellular Ca2+ in vascular smooth muscle cells. However, the mechanisms mediating the effect of cADPR remain unknown. The present study was designed to determine whether FK-506 binding protein 12.6 (FKBP12.6), an accessory protein of the RyRs, plays a role in cADPR-induced activation of the RyRs. A 12.6-kDa protein was detected in bovine coronary arterial smooth muscle (BCASM) and cultured CASM cells by being immunoblotted with an antibody against FKBP12, which also reacted with FKBP12.6. With the use of planar lipid bilayer clamping techniques, FK-506 (0.01-10 muM) significantly increased the open probability (NPO) of reconstituted RyR/Ca(2+)release channels from the SR of CASM. This FK-506-induced activation of RyR/Ca2+ release channels was abolished by pretreatment with anti-FKBP12 antibody. The RyRs activator cADPR (0.1-0 muM) markedly increased the activity of RyR/Ca2+ release channels. In the presence of FK-506, cADPR did not further increase the NPO of RyR/Ca2+ release channels. Addition of anti-FKBP12 antibody also completely blocked cADPR- induced activation of these channels, and removal of FKBP12.6 by preincubation with FK-506 and subsequent gradient centrifugation abolished cADPR- induced increase in the NPO of RyR/Ca2+ release channels. We conclude that FKBP12.6 plays a critical role in mediating cADPR- induced activation of RyR/Ca2+ release channels from the SR of BCASM.