Ca2+-atpase molecules as a calcium-sensitive membrane-endoskeleton of sarcoplasmic reticulum

Jun Nakamura, Yuusuke Maruyama, Genichi Tajima, Yuto Komeiji, Makiko Suwa, Chikara Sato

Research output: Contribution to journalArticlepeer-review

5 Citations (Scopus)


The Ca2+-transport ATPase of sarcoplasmic reticulum (SR) is an integral, transmembrane protein. It sequesters cytoplasmic calcium ions released from SR during muscle contraction, and causes muscle relaxation. Based on negative staining and transmission electron microscopy of SR vesicles isolated from rabbit skeletal muscle, we propose that the ATPase molecules might also be a calcium-sensitive membrane-endoskeleton. Under conditions when the ATPase molecules scarcely transport Ca2+, i.e., in the presence of ATP and ≤ 0.9 nM Ca2+, some of the ATPase particles on the SR vesicle surface gathered to form tetramers. The tetramers crystallized into a cylindrical helical array in some vesicles and probably resulted in the elongated protrusion that extended from some round SRs. As the Ca2+ concentration increased to 0.2 µM, i.e., under conditions when the transporter molecules fully carry out their activities, the ATPase crystal arrays disappeared, but the SR protrusions remained. In the absence of ATP, almost all of the SR vesicles were round and no crystal arrays were evident, independent of the calcium concentration. This suggests that ATP induced crystallization at low Ca2+ concentrations. From the observed morphological changes, the role of the proposed ATPase membrane-endoskeleton is discussed in the context of calcium regulation during muscle contraction.

Original languageEnglish
Article number2624
Pages (from-to)1-17
Number of pages17
JournalInternational Journal of Molecular Sciences
Issue number5
Publication statusPublished - 2021 Mar 1


  • ATP
  • Ca-ATPase
  • Calcium
  • Cell dynamics
  • Cell morphology
  • Membrane endoskeleton
  • Ryanodine receptor
  • Transmission electron microscopy
  • Two-dimensional crystallization


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