title: Multiclass maximum-likelihood symmetry determination and motif reconstruction of 3-D helical objects from projection images for electron microscopy.
abstract: Many micro- to nano-scale 3-D biological objects have a helical symmetry. Cryo electron microscopy provides 2-D projection images where, however, the images have low SNR and unknown projection directions. The object is described as a helical array of identical motifs, where both the parameters of the helical symmetry and the motif are unknown. Using a detailed image formation model, a maximum-likelihood estimator for the parameters of the symmetry and the 3-D motif based on images of many objects and algorithms for computing the estimate are described. The possibility that the objects are not identical but rather come from a small set of homogeneous classes is included. The first example is based on 316 128 × 100 pixel experimental images of Tobacco Mosaic Virus, has one class, and achieves 12.40-Å spatial resolution in the reconstruction. The second example is based on 400 128 × 128 pixel synthetic images of helical objects constructed from NaK ion channel pore macromolecular complexes, has two classes differing in helical symmetry, and achieves 7.84- and 7.90-Å spatial resolution in the reconstructions for the two classes.
authors.affiliation: School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907, USA. lee299@purdue.edu
authors.forename: Seunghee
authors.initials: S
authors.lastname: Lee
authors.forename: Peter C
authors.initials: PC
authors.lastname: Doerschuk
authors.forename: John E
authors.initials: JE
authors.lastname: Johnson
mesh_terms: D000465:Algorithms; D020285:Cryoelectron Microscopy / Q000379:methods*; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D021621:Imaging, Three-Dimensional / Q000295:instrumentation; D016013:Likelihood Functions; D008958:Models, Molecular; D015221:Potassium Channels / Q000737:chemistry; D015222:Sodium Channels / Q000737:chemistry; D014027:Tobacco Mosaic Virus / Q000737:chemistry
chemical_list: D015221:Potassium Channels; D015222:Sodium Channels