Crystallization of the Ca2+-ATPase of Sarcoplasmic Reticulum by Calcium and Lanthanide Ions

Two-dimensional crystalline arrays of Ca2+-ATPase molecules develop in sarcoplasmic reticulum vesicles exposed to Ca2+ or lanthanide ions. The Ca2+- or lanthanide-induced crystals are presumed to represent the E1 conformation of the Ca2+-ATPase, and their crystal form is clearly different from the earlier described E2 crystals induced by Na3VO4 in the presence of ethylene glycol bis(beta aminoethyl ether)-N,N,N',N'-tetraacetic acid (Taylor, K. A., Dux, L., and Martonosi, A. (1984) J. Mol. Biol. 174, 193-204). Analysis of the crystalline arrays by negative staining or freeze-fracture electron microscopy reveals obliquely oriented rows of particles corresponding to individual Ca2+-ATPase molecules. Computer analysis of the negatively stained lanthanide-induced crystalline Ca2+-ATPase arrays shows that the molecules are arranged in a P1 lattice. The pear-shaped profiles of Ca2+-ATPase molecules seen in projection in the density maps are similar to those seen in vanadate-induced crystals. The space group and unit cell dimensions of the E1 crystals are consistent with Ca2+-ATPase monomers as structural units, while the vanadate-induced E2 crystals form by lateral aggregation of chains of Ca2+-ATPase dimers. The transition between the E1 and E2 conformations may involve a shift in the monomer-oligomer equilibrium of the Ca2+-ATPase. The formation of E1 crystals by PrCl3 is promoted by inside negative membrane potential, presumably through stabilization of the E1 conformation of the enzyme. Cleavage of the Ca2+-ATPase by trypsin into two major fragments (A and B) did not interfere with the Ca2+- or the Pr3+-induced crystallization

The effects of osmotic stress on the viscoelastic and physical properties of articular chondrocytes.

The metabolic activity of chondrocytes in articular cartilage is influenced by alterations in the osmotic environment of the tissue, which occur secondary to mechanical compression. The mechanism by which osmotic stress modulates cell physiology is not fully understood and may involve changes in the physical properties of the membrane or the cytoskeleton. The goal of this study was to determine the effect of the osmotic environment on the mechanical and physical properties of chondrocytes. In isoosmotic medium, chondrocytes exhibited a spherical shape with numerous membrane ruffles. Normalized cell volume was found to be linearly related to the reciprocal of the extracellular osmolality (Boyle van't Hoff relationship) with an osmotically active intracellular water fraction of 61%. In deionized water, chondrocytes swelled monotonically until lysis at a mean apparent membrane area 234 +/- 49% of the initial area. Biomechanically, chondrocytes exhibited viscoelastic solid behavior. The instantaneous and equilibrium elastic moduli and the apparent viscosity of the cell were significantly decreased by hypoosmotic stress, but were unchanged by hyperosmotic stress. Changes in the viscoelastic properties were paralleled by the rapid dissociation and remodeling of cortical actin in response to hypoosmotic stress. These findings indicate that the physicochemical environment has a strong influence on the viscoelastic and physical properties of the chondrocyte, potentially through alterations in the actin cytoskeleton