Negatively stained sarcoplasmic reticulum from the scallop Placopecten magellanicus presented a variety of crystalline forms, the most common being tubular structures. These were characterized by paired rows of morphological units, spaced at ,--120 A, running diagonally across the tubules. The orthogonal unit cell (120 x 55 A) contained two units, related by a twofold axis, which probably represented the part of the Ca2+-ATPase molecule projecting from the outer surface of the membrane.Fragmented sarcoplasmic reticulum (FSR) 1 prepared from rabbit skeletal muscle and membranes reconstituted in vitro from SR Ca2÷-ATPase have been studied extensively by electron microscopy and x-ray diffraction. Negatively stained rabbit FSR shows vesicles of average diameter 0.15 um (1) covered on their outer surface with projections ~60 A long and 30-40 A diam which represent part of the Ca2+-ATPase molecule (2-5). Freeze-fracturing studies show 85-A-diam particles embedded in the concave fracture face, also interpreted as the CaE+-ATPase (6). A ratio of 3-4 surface projections per 85-A inner particle has led to the suggestion that the ATPase has a tetrameric structure in the membrane (7). Xray studies have confirmed the asymmetrical distribution of mass across the SR membrane and that the diameter of the projections is ~35 ,~ (8-10). Rabbit SR vesicles do not, however, show sufficient order to allow a detailed structure analysis, although treatment with vanadate has recently been reported to cause an ordered surface array of subunits (11).Scallop SR consists of cisternae connected by tubular elements, and is closely associated with the sarcolemma (12). We have prepared FSR from the cross-striated adductor muscle of Placopecten magellanicus and examined it by electron microscopy. Negatively stained vesicles showed a crystalline appearance, and images of one class of tubular SR have been analyzed by two-dimensional reconstruction techniques.
MATERIALS AND METHODS
Preparation of Sarcoplasmic Reticulum Vesicles: Live seascallops (Pecten magellanicus) were obtained from the Marine Biological Laboratory, Woods Hole, MA. Strips of the striated adductor muscle were excised from the animal and chemically skinned in a buffered solution containing 0.05-0.1% wl/vol saponin, 100 mM NaCI, 8 mM MgSO4, 5 mM EGTA, 5
Photo-cross-linking techniques show that when scallop myosin or myofibrils are subjected to experimental conditions that cause relaxed muscles to go into rigor, the N-terminal portion of the regulatory light chain of myosin moves towards the essential light chain while the C-terminal portion stays in place. These changes occur on the myosin before combination with actin. Cross-linking of the N-terminal region to the essential light chain in rigor locks the myosin into a conformation such that calcium sensitivity of the actin-activated Mg-ATPase is lost.
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