Ca2+-ATPase Molecules as a Calcium-Sensitive Membrane-Endoskeleton of Sarcoplasmic Reticulum.

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

doi:10.3390/ijms22052624

Abstract

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.

Highlights

The sarcoplasmic reticulum (SR) plays a primary role in regulating cytoplasmic calcium concentrations in skeletal muscle cells [1,2,3]
Several studies of SR in vertebrates show (i) that the SR membrane system mainly consists of two regions of bulbous terminal cisternae and tubular longitudinal elements [10,11]; (ii) that the terminal cisterna is associated with the transverse tubules via bridging structures comprised of ryanodine receptor (RyR) channels and referred to as ”feet” [12,13,14]; and (iii) that the tubular elements of the SR pass through the Z line of myofibrils [10,11,15]
The calcium transport reaction of SR Ca2+ -ATPase molecules is coupled to large conformational changes in the molecule [32,33]

Summary

Introduction

The sarcoplasmic reticulum (SR) plays a primary role in regulating cytoplasmic calcium concentrations in skeletal muscle cells [1,2,3]. After ryanodine receptor (RyR) channels release Ca2+ from the SR to contract the muscle [3], Ca2+ -ATPase (110 kDa), an integral membrane protein [4,5], relaxes it again by transporting cytoplasmic calcium ions into the SR lumen, which lowers the cytoplasmic Ca2+ concentration [1,2]. The average particle density of the 40 Å domains on the SR surface (particles/μm2 ) is 3 to 4 times larger than the particle density of the 85 Å particles, suggesting that the ATPase molecules have a tetrameric structure in the membrane [18]

Methods

Results

Discussion

Conclusion