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.