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.
Subunit structure of junctional feet in triads of skeletal muscle: a freeze-drying, rotary-shadowing study.
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D G Ferguson, H W Schwartz, C Franzini-Armstrong; Subunit structure of junctional feet in triads of skeletal muscle: a freeze-drying, rotary-shadowing study.. J Cell Biol 1 November 1984; 99 (5): 1735–1742. doi: https://doi.org/10.1083/jcb.99.5.1735
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