Recognition force microscopy/spectroscopy of ion channels: applications to the skeletal muscle Ca2+ release channel (RYR1).

The skeletal muscle Ca2+ release channel (ryanodine receptor 1, RYR1) plays an important role in the excitation–contraction coupling process. We purified ryanodine receptor type 1 from rabbit white muscle and adsorbed it to mica sheets with the cytoplasmic side facing up. Single receptors of uniformly distributed size and shape of 10–12 nm height and 40–50 nm width, and occasionally some aggregates were seen in contact mode AFM images. These immobilized RYR1 were specifically recognized by rabbit anti-RYR1 (antibody#8) with at least 30% efficiency, as measured by an enzyme immunoassay with goat-anti-rabbit. Single specific antibody–antigen recognition events were detected with AFM tips to which an antibody#8 was tethered. In linear scans, the occurrence of antibody–antigen binding showed significant lateral dependence, which allowed for the localization of binding sites with nm resolution. Variation of the loading rate in force spectroscopy experiments revealed a logarithmic dependence of the unbinding forces, ranging from 42 to 73 pN. From this dependence, a bond width of the binding pocket of L=0.2 nm and a kinetic off-rate of koff=12.7 s−1 was determined.