LAMELLAR STACKING IN 3-DIMENSIONAL CRYSTALS OF CA2-ATPASE FROM SARCOPLASMIC-RETICULUM()

Electron microscopy of multilamellar crystals of Ca2+-ATPase currently offers the best opportunity for obtaining a high-resolution structure of this ATP-driven ion pump. Under certain conditions small, wormlike crystals are formed and provide views parallel to the lamellar plane, from which parameters of lamellar stacking can be directly measured. Assuming that molecular packing is the same, data from these views cou ld supplement those obtained by tilting large, thin platelike crystals . However, we were surprised to discover that the lamellar spacing was variable and depended on the amount of glycerol present during crysta llization (20% versus 5%). Projection maps (h,0,J) from these wormlike crystals suggest different molecular contacts that give rise to the d ifferent lamellar spacings. Based on an orthogonal projection map (h,k ,0) from collapsed, wormlike crystals and on x-ray powder patterns, we conclude that molecular packing within the lamellar plane is the same as that in thin, platelike crystals and is unaffected by glycerol. Fi nally, the orientation of molecules in the lamellar plane was characte rized from freeze-dried, shadowed crystals. Comparing the profile of m olecules in these multilamellar crystals with that previously observed in helical tubes induced by vanadate gives structural evidence of the conformational change that accompanies binding of calcium to Ca2+-ATP ase.