The book reproduces 55 of more than 300 articles written by the author, representing milestones in methods development of single-particle cryo-EM as well as important results obtained by this technique in the study of biological macromolecules and their interactions. Importantly, neither symmetries nor ordered arrangements (as in two-dimensional crystals, helical assemblies, icosahedral viruses) are required. Although the biological applications are mainly in the area of ribosome structure and function, the elucidation of membrane channel structures and their activation and gating mechanisms are represented, as well. The book is introduced by a commentary that explains the original development of concepts, describes the contributions of the author's colleagues and students, and shows how challenges were overcome as the technique matured. Along the way, the ribosome served as an example for a macromolecule with intricate structure and conformational dynamics that pose challenges for three-dimensional visualization. Toward the end of the book bringing us to the present time molecular structures with near-atomic resolution are presented, and a novel type of computational analysis, manifold embedding, is introduced. Single-particle cryo-EM is currently revolutionizing structural biology, presenting a powerful alternative to X-ray crystallography as a means to solve the structure of biological macromolecules. The book presents in one place a number of articles containing key advances in mathematical and computational methods leading up to the present time. Secondly, the development of the technique over the years is reflected by ever-expanding discoveries in the field of ribosome structure and function. Thirdly, as all histories of ideas, the history of concepts pertaining to this new method of visualization is fascinating all in itself.
With the advent of direct electron detectors, cryo-EM has become a popular choice for molecular structure determination. Among its advantages over X-ray crystallography are (1) no need for crystals, (2) much smaller sample volumes, and (3) the ability to determine multiple structures or conformations coexisting in one sample. In principle, kinetic experiments can be done using standard cryo-EM, but mixing and freezing grids require several seconds. However, many biological processes are much faster than that time scale, and the ensuing short-lived states of the molecules cannot be captured. Here, we lay out a detailed protocol for how to capture such intermediate states on the millisecond time scale with time-resolved cryo-EM.
Mitochondrial diseases produce profound neurological dysfunction via mutations affecting mitochondrial energy production, including the relatively common Leigh syndrome (LS). We recently described an LS case caused by a pathogenic mutation in USMG5, encoding a small supernumerary subunit of mitochondrial ATP synthase. This protein is integral for ATP synthase dimerization, and patient fibroblasts revealed an almost total loss of ATP synthase dimers. Here, we utilize in situ cryoelectron tomography (cryo-ET) in a clinical case-control study of mitochondrial disease to directly study mitochondria within cultured fibroblasts from a patient with LS and a healthy human control subject. Through tomographic analysis of patient and control mitochondria, we find that loss of ATP synthase dimerization due to the pathogenic mutation causes profound disturbances of mitochondrial crista ultrastructure. Overall, this work supports the crucial role of ATP synthase in regulating crista architecture in the context of human disease.
I developed an interest in electron optics when I worked with Ernst Kinder on my master thesis project in Physics at the University of Munich. The subject of my thesis was backscattering of electrons on the surface of liquid gold, an ambitious undertaking that forced me to construct a vacuum chamber, a crucible to heat up the gold, a detector, and an electron gun. In 1943, working with the electron microscope, Kinder had studied butterfly wings, which as he realized gained their brilliant colors from interference of light on
gratings formed by tiny scales, arranged in regular order...
In the past 4 years, because of the advent of new cameras, many ribosome structures have been solved by cryoelectron microscopy (cryo-EM) at high, often near-atomic resolution, bringing new mechanistic insights into the processes of translation initiation, peptide elongation, termination, and recycling. Thus, cryo-EM has joined X-ray crystallography as a powerful technique in structural studies of translation. The significance of this new development is that structures of ribosomes in complex with their functional binding partners can now be determined to high resolution in multiple states as they perform their work. The aim of this article is to provide an overview of these new studies and assess the contributions they have made toward an understanding of translation and translational control.
Translation initiation in eukaryotes is a highly regulated and rate-limiting process. It results in the assembly and disassembly of numerous transient and intermediate complexes involving over a dozen eukaryotic initiation factors (eIFs). This process culminates in the accommodation of a start codon marking the beginning of an open reading frame at the appropriate ribosomal site. Although this process has been extensively studied by hundreds of groups for nearly half a century, it has been only recently, especially during the last decade, that we have gained deeper insight into the mechanics of the eukaryotic translation initiation process. This advance in knowledge is due in part to the contributions of structural biology, which have shed light on the molecular mechanics underlying the different functions of various eukaryotic initiation factors. In this review, we focus exclusively on the contribution of structural biology to the understanding of the eukaryotic initiation process, a long-standing jigsaw puzzle that is just starting to yield the bigger picture.
The GTPase EF-Tu in ternary complex with GTP and aminoacyl-tRNA (aa-tRNA) promotes rapid and accurate delivery of cognate aa-tRNAs to the ribosomal A site. Here we used cryo-EM to study the molecular origins of the accuracy of ribosome-aided recognition of a cognate ternary complex and the accuracy-amplifying role of the monitoring bases A1492, A1493 and G530 of the 16S rRNA. We used the GTPase-deficient EF-Tu variant H84A with native GTP, rather than non-cleavable GTP analogues, to trap a near-cognate ternary complex in high-resolution ribosomal complexes of varying codon-recognition accuracy. We found that ribosome complexes trapped by GTPase-deficicent ternary complex due to the presence of EF-TuH84A or non-cleavable GTP analogues have very similar structures. We further discuss speed and accuracy of initial aa-tRNA selection in terms of conformational changes of aa-tRNA and stepwise activation of the monitoring bases at the decoding center of the ribosome.
Cryo-electron microscopy (EM) directly images the distribution of electrostatic potential
within macromolecules, and thus can provide much more information about atomic
charges than X-ray crystallography normally does. The electron scattering length of an
isolated ion is quite different from that of the corresponding neutral atom. The difference
is very large at small scattering angles where the effects of electron distributions are the
largest, but becomes smaller at high scattering angle where nuclear charge determines
outcomes. For this reason, in cryo-EM maps that have been solved at resolutions lower
than ~ 2.5 Å, peaks that correspond to anions will always be less prominent than those
that correspond to cations, and may even be negative. Furthermore, if a map of this kind
is smeared computationally after the fact, which reduces its effective resolution, anion
peaks will diminish in size, cation peaks will grow, and peaks that represent uncharged
atoms will remain about the same. These effects can be used to determine the sign of the
charges carried by the ions associated with a macromolecule, and even to estimate their
magnitudes. ESP value for a cation in a cation-anion pair is smaller than the value of the
cation in isolation, but ESP value for the anion in the ionic pair is greater than the value
of the anion in isolation. The experimental range of ESP value for Mg2+ relative to that of
the closest C1’ atom are found to be between 0.57 and 1.27.
We have evidence from a number of cryo-EM studies that molecules in solution exist in a continuous distribution of conformational states, far larger in number than the discrete ones identified by the standard methods of maximum likelihood classification such as Relion.(1) This finding, if it holds up, is particularly interesting when we wish to study a processive molecular machine that either actively “runs” with the functional ligands and energy quota (GTP and ATP) supplied or “idles” in the thermal equilibrium in the absence of ligands, because it promises the chance to uncover free-energy landscapes and functional pathways experimentally, without any model assumptions.
AMPA-subtype ionotropic glutamate receptors mediate fast excitatory neurotransmission throughout the central nervous system. Gated by the neurotransmitter glutamate, AMPA receptors are critical for synaptic strength and dysregulation of AMPA receptor-mediated signalling is linked to numerous neurological diseases. Here, we use cryo-electron microscopy to solve the structures of AMPA receptor-auxiliary subunit complexes in the apo, antagonist and agonist-bound states and elucidate the iris-like mechanism of ion channel opening. The ion channel selectivity filter is formed by the extended portions of the re-entrant M2 loops, while the helical portions of M2 contribute to extensive hydrophobic interfaces between AMPA receptor subunits in the ion channel. We show how the permeation pathway changes upon channel opening and identify conformational changes throughout the entire AMPA receptor that accompany activation and desensitization. Our findings provide a framework for understanding gating across the family of ionotropic glutamate receptors and the role of AMPA receptors in excitatory neurotransmission.