Fig. 4. Structure of the Bumper domain of the LR by the STA approach.

a The Bumper domain adopts two major conformations. By back-projecting the NPC particles onto the original tomograms, two major conformations of the Bumper domain are seen and named Bumper-6 and -7. The lengths of Bumper-6 and -7 are 29 and 34 nm, respectively. b Classification of the Bumper domains. The Bumper domains were re-cropped and classified. Bumper-6 and -7 represent 21.5% and 38.4% of the total particles classified. c Refined structure of Bumper-7 exhibits distinguishing features (upper panel). Two mutually perpendicular views are shown for an isolated Bumper-7 (middle and lower panels). d Refined structure of Bumper-6. In contrast to Bumper-7, the internal interface of Bumper-6 involves two pairs of promoter tips. e Mapping the Bumper domains onto the NPC particles. The reconstructions for Bumper-7 (marine), Bumper-6 (red) and the LR subunit (gray) were back-projected onto the original tomograms based on the refined coordinates of the individual particles. Shown here is a representative section of the nuclear membrane. The conformations of nearly all the classified Bumper domains are the same as those of the Bumper domains formed by independently back-projecting the LR subunits onto the original tomograms.