We have developed a procedure to isolate, from skeletal muscle, enriched terminal cisternae of sarcoplasmic reticulum (SR), which retain morphologically intact junctional "feet" structures similar to those observed in situ. The fraction is largely devoid of transverse tubule, plasma membrane, mitochondria, triads (transverse tubules junctionally associated with terminal cisternae), and longitudinal cisternae, as shown by thin-section electron microscopy of representative samples. The terminal cisternae vesicles have distinctive morphological characteristics that differ from the isolated longitudinal cisternae (light SR) obtained from the same gradient. The terminal cisternae consist of two distinct types of membranes, i.e., the junctional face membrane and the Ca2+ pump protein-containing membrane, whereas the longitudinal cisternae contain only the Ca2+ pump protein-containing membrane. The junctional face membrane of the terminal cisternae contains feet structures that extend approximately 12 nm from the membrane surface and can be clearly visualized in thin section through using tannic acid enhancement, by negative staining and by freeze-fracture electron microscopy. Sections of the terminal cisternae, cut tangential to and intersecting the plane of the junctional face, reveal a checkerboardlike lattice of alternating, square-shaped feet structures and spaces each 20 nm square. Structures characteristic of the Ca2+ pump protein are not observed between the feet at the junctional face membrane, either in thin section or by negative staining, even though the Ca2+ pump protein is observed in the nonjunctional membrane on the remainder of the same vesicle. Likewise, freeze-fracture replicas reveal regions of the P face containing ropelike strands instead of the high density of the 7-8-nm particles referable to the Ca2+ pump protein. The intravesicular content of the terminal cisternae, mostly Ca2+-binding protein (calsequestrin), is organized in the form of strands, sometimes appearing paracrystalline, and attached to the inner face of the membrane in the vicinity of the junctional feet. The terminal cisternae preparation is distinct from previously described heavy SR fractions in that it contains the highest percentage of junctional face membrane with morphologically well-preserved junctional feet structures.
Comparison of the membrane composition of Spiroplasma citri and the corn stunt Spiroplasma.
