A direct analysis of lamellar x-ray diffraction from hydrated oriented multilayers of fully functional sarcoplasmic reticulum

Abstract

The profile structure of functional sarcoplasmic reticulum (SR) membranes was investigated by X-ray diffraction methods to a resolution of 10 A. The lamellar diffraction data from hydrated oriented multilayers of SR vesicles showed monotonically increasing widths for higher order lamellar reflections, indicative of simple lattice disorder within the multilayer. A generalized Patterson function analysis, previously developed for treating lamellar diffraction from lattice-disordered multilayers, was used to identify the autocorrelation function of the unit cell electron density profile. Subsequent deconvolution of this autocorrelation function provided the most probable unit cell electron density profile of the SR vesicle membrane pair. The resulting single membrane profile possesses marked asymmetry, suggesting that a major portion of the Ca++ -ATPase resides on the exterior of the vesicle. The electron density profile also suggests that the Ca++-dependent ATPase penetrates into the lipid hydrocarbon core of the SR membrane. Under conditions suitable for X-ray analysis, SR vesicles prepared as partially dehydrated oriented multilayers are shown to conserve most of their ATP-induced Ca++ uptake functionality, as monitored spectrophotometrically with the Ca++ indicator arsenazo III. This has been verified both in resuspensions of SR after centrifugation and slow partial dehydration, and directly in SR multilayers in a partially dehydrated state (20-30 percent water). Therefore, the profile structure of the SR membrane that we have determined may closely resemble that found in vivo.