Fig. 1

a Structural data. Example of a field of P-glycoprotein particles (darker grey) embedded in vitreous ice (lighter grey). Scale bar = 20 nm. The micrograph was filtered to 8 Å resolution for clarity. Various orientations of the particles in the ice gave rise to different projections. Encircled are examples of projections corresponding to side-on views of the molecule (i.e. perpendicular to the long axis), with a clear inward-facing conformation indicated (arrow). b Reference-free classification of the particle data set identified different projection classes and c 3D classification yielded 3 classes all corresponding to an inward-facing conformation of the protein. The class displayed in yellow was generated from the largest group of particles and this dataset was selected for further 3D refinement