Mobility of voltage-dependent ion channels and lectin receptors in the sarcolemma of frog skeletal muscle

The mobility of lectin receptors and of two types of ion channels was studied in skeletal muscles of the frog Rana temporaria. Lectin receptors were labeled with fluorescent derivatives of succinyl-concanavalin A (Con A) or wheat germ agglutinin (WGA), and their mobility was measured by fluorescence recovery after photobleaching. Of the receptors for WGA, ~53% were free to diffuse in the plane of the membrane, with an average diffusion coefficient as found in other preparations (D = 6.4 × 10^-11 cm²/s). Con A receptors were not measurably mobile. The mobility of voltage-dependent Na and K (delayed rectifier) channels was investigated with the loose-patch clamp method, coupled with through-the-pipette photodestruction of channels by ultraviolet (UV) light. Na channels were not measurably mobile (D ≤ 10^-12 cm²/s). With K channels, photodestruction was followed by a small but consistent recovery of K current, which suggested that some K channels diffused in the plane of the membrane. Our results with K currents are best fit if 25% of the K channels diffuse with D = 5 × 10^-11 cm²/s, with the remainder being immobile. For both Na and K channels, photodestruction by UV was most effective at a wavelength of ~289 nm. At this wavelength, the energy density required for an e-fold reduction in the number of functional channels was 0.40 J/cm² for Na channels and 0.94 J/cm² for K channels. Irradiation at this wavelength and dose did not measurably diminish the mobility of WGA receptors; hence, the immobility of Na and most K channels is not due to UV irradiation. It is concluded that mobile and immobile membrane proteins coexist in the sarcolemma of frog skeletal muscle, and that voltage-dependent Na and K channels are singled out for immobilization.