Crystal structure of the complex between human CD8αα and HLA-A2

. The absolute hand of the map was determined by ensuring that the angle of tilted views of the two-dimensional crystal was recorded with the correct sign. As a check for the assignment of the sign of the tilt angles, we used two-dimensional crystals of bacteriorhodopsin (purple membrane) for which the hand is known. Images recorded from tilted, glucoseembedded purple membrane samples were subjected to the same protocol as our analysis of AQP1, and the resulting phases were compared with the reported phases for purple membrane 26 . Search for non-crystallographic symmetry was performed by calculating the rotation function using the program POLARRFN in the CCP4 package 27 . A non-crystallographic peak in the membrane plane was consistently seen for all resolution ranges (data not shown). This peak corresponds to a 2-fold rotation axis inclined by 88 with respect to the unit cell a (or b) axis and has the maximum strength for a radius of integration of 40 A ˚ .