Subunit structure of junctional feet in triads of skeletal muscle: a freeze-drying, rotary-shadowing study.

Isolated heavy sarcoplasmic reticulum vesicles retain junctional specializations (feet) on their outer surface. We have obtained en face three-dimensional views of the feet by shadowing and replicating the surfaces of freeze-dried isolated vesicles. Feet are clearly visible as large structures located on raised platforms. New details of foot structure include a four subunit structure and the fact that adjacent feet do not abut directly corner to corner but are offset by half a subunit. Feet aligned within rows were observed to be rotated at a slight angle off the long axis of the row creating a center-to-center spacing (32.5 nm) slightly less than the average diagonal of the feet (35.3 nm). Comparison with previous information from thin sections and freeze-fracture showed that this approach to the study of membranes faithfully preserves structure and allows better visualization of surface details than either thin-sectioning or negative-staining.