In recent work with large high symmetry viruses, single particle electron cryomicroscopy (cryoEM) has reached the milestone of determining near atomic resolution structures by allowing direct fitting of atomic models into experimental density maps. However, achieving this goal with smaller particles of lower symmetry remains extraordinarily challenging. Using a newly developed single electron counting detector, we confirm that electron beam induced motion significantly degrades resolution and, importantly, show how the combination of rapid readout and nearly noiseless electron counting allow image blurring to be corrected to subpixel accuracy. Thus, intrinsic image information can be restored to high resolution (Thon rings visible to ~3 Å). Using this approach we determined a 3.3 Å resolution structure of a ~700 kDa protein with D7 symmetry showing clear side chain density. Our method greatly enhances image quality and data acquisition efficiency - key bottlenecks in applying near atomic resolution cryoEM to a broad range of protein samples.

Purified preparations of scrapie prions contain a sialoglycoprotein of Mr 27,000-30,000, designated PrP 27-30, which is derived from the scrapie prion protein [Mr,33,000 Source of Scrapie Prions and Bioassay. A hamster-adapted isolate of the scrapie agent was passaged and prepared as described (1, 13).Preparation of the Subcellular Fractions. Weanling hamsters (LVG/LAK) were inoculated intracerebrally with 107 ID50 units of the scrapie agent. The brains were collected from hamsters sacrificed 60 days after infection and from age-matched uninfected animals. The brains were suspended in 0.32 M sucrose (10%, wt/vol) and homogenized with six bursts of 10 sec each by using a Polytron homogenizer set at medium speed. The homogenates were centrifuged in a Beckman 50.2 Ti rotor. Pellets were resuspended in 0.32 M sucrose solution and the volumes were adjusted to that of the supernatant. All solutions were kept on ice and all centrifugations were performed at 4°C. The fractions were adjusted to 10 mg of protein per ml with 0.32 M sucrose solutions. One-milliliter samples were centrifuged in the Beckman 50 Ti rotor at 38,000 rpm for 1 hr at 4°C. Pellets were resuspended and treated as described in Results. For digestion experiments, N-lauroylsarcosine (sarkosyl) was added from a 10% stock solution in 25 mM Tris'HCl/0.1 M NaCl, pH 7.4, to some of the samples. Control samples were diluted by the same amount of Tris buffer. Digestions were initiated by addition of an aliquot of proteinase K (2 mg/ml) in Tris buffer to give a final concentration of 0.1 mg/ml. After 30 min at room temperature, the digestions were terminated by addition of an aliquot of 0

The microtubule cytoskeleton of animal cells does not assemble spontaneously, but instead requires the centrosome. This organelle consists of a pair of centrioles surrounded by a complex collection of proteins known as the pericentriolar material (PCM). The PCM is required for microtubule nucleation. The minus, or slow-growing, ends of microtubules are embedded in the PCM and the plus, or fast-growing, ends project outwards into the cytoplasm during interphase, or into the spindle apparatus during mitosis. gamma-Tubulin is the only component of the PCM that is so far implicated in microtubule nucleation. Here we use immuno-electron microscopic tomography to show that gamma-tubulin is localized in ring structures in the PCM of purified centrosomes without microtubules. When these centrosomes are used to nucleate microtubule growth, gamma-tubulin is localized at the minus ends of the microtubules. We conclude that microtubule-nucleating sites within the PCM are ring-shaped templates that contain multiple copies of gamma-tubulin.

The gamma-tubulin ring complex (gammaTuRC) is a protein complex of relative molecular mass approximately 2.2 x 10(6) that nucleates microtubules at the centrosome. Here we use electron-microscopic tomography and metal shadowing to examine the structure of isolated Drosophila gammaTuRCs and the ends of microtubules nucleated by gammaTuRCs and by centrosomes. We show that the gammaTuRC is a lockwasher-like structure made up of repeating subunits, topped asymmetrically with a cap. A similar capped ring is also visible at one end of microtubules grown from isolated gammaTuRCs and from centrosomes. Antibodies against gamma-tubulin label microtubule ends, but not walls, in centrosomes. These data are consistent with a template-mediated mechanism for microtubule nucleation by the gammaTuRC.