Gw. Cheong et al., LAMELLAR STACKING IN 3-DIMENSIONAL CRYSTALS OF CA2-ATPASE FROM SARCOPLASMIC-RETICULUM(), Biophysical journal, 70(4), 1996, pp. 1689-1699
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.