Elongation and Contraction of Scallop Sarcoplasmic Reticulum (SR): ATP Stabilizes Ca²⁺-ATPase Crystalline Array Elongation of SR Vesicles

The Ca²⁺-ATPase is an integral transmembrane Ca²⁺ pump of the sarcoplasmic reticulum (SR). Crystallization of the cytoplasmic surface ATPase molecules of isolated scallop SR vesicles was studied at various calcium concentrations by negative stain electron microscopy. In the absence of ATP, round SR vesicles displaying an assembly of small crystalline patches of ATPase molecules were observed at 18 µM [Ca²⁺ ]. These partly transformed into tightly elongated vesicles containing ATPase crystalline arrays at low [Ca²⁺ ] (≤1.3 µM). The arrays were classified as ‘’tetramer”, “two-rail” (like a railroad) and ‘’monomer”. Their crystallinity was low, and they were unstable. In the presence of ATP (5 mM) at a low [Ca²⁺ ] of ~0.002 µM, “two-rail” arrays of high crystallinity appeared more frequently in the tightly elongated vesicles and the distinct tetramer arrays disappeared. During prolonged (~2.5 h) incubation, ATP was consumed and tetramer arrays reappeared. A specific ATPase inhibitor, thapsigargin, prevented both crystal formation and vesicle elongation in the presence of ATP. Together with the second part of this study, these data suggest that the ATPase forms tetramer units and longer tetramer crystalline arrays to elongate SR vesicles, and that the arrays transform into more stable “two-rail” forms in the presence of ATP at low [Ca²⁺ ].