Data processing

The helical crystals data sets have been published (Table 1; (33, 34)). The structure from helical crystals of SERCA alone was an average generated from five helical symmetry groups (−22, 6 reference symmetry). The structure from helical crystals of SERCA in the presence of PLN (Asn27-Ala mutant; canine sequence) was an average generated from two helical symmetry groups (−27, 9 reference symmetry). Density maps were calculated for the SERCA-PLN helical crystals without enforcing twofold symmetry. The two molecules composing the unit cell for the control structure (SERCA alone) were masked and aligned by cross correlation with the corresponding molecules from the SERCA-PLN map. After alignment, the two molecules composing the unit cell for the control structure were summed to generate a single map and back transformed to Fourier space to generate layer-line data with the SERCA-PLN helical symmetry (−27, 9 reference symmetry). Density maps were then calculated from the control and PLN layer-line data with and without twofold symmetry enforced. Because the control and PLN layer-line data were now of the same helical symmetry group, a difference map could be calculated by simple density scaling and pixel-by-pixel subtraction.

Crystallographic Data for Frozen-Hydrated Helical Crystals of SERCA in the Absence and Presence of PLN Is Summarized