Nature Structural Biology pp 48 - 53

The bite of a black widow spider can be deadly. The venom of this spider contains numerous proteins that are toxic to insects or mammals. One of these, alpha-latrotoxin, causes massive release of neurotransmitters, severely impairing the victim's cardiovascular and neuromuscular systems.

Neurotransmitter release from cells is normally tightly regulated by several protein signaling pathways as well as by the level of calcium in the cell. Alpha-latrotoxin molecules are thought to be multifunctional in their attack on this system: they not only interact with several components of the signaling pathways but also form pores in the cell membrane to allow calcium ions to enter. Together, these activities make alpha-latrotoxin a formidable enemy - and have also helped to make it a useful tool for studying nerve function.

Despite its widespread use in research for over 20 years, how this toxin forms membrane pores has not been known until now. The assembly pathway and structure of pore-forming alpha-latrotoxin have been determined by Yuri Ushkaryov and colleagues of Imperial College in London and are reported in the January issue of Nature Structural Biology.

Ushkaryov and his colleagues discovered that the active form of alpha-latrotoxin is composed of four individual molecules of the toxin that are tightly associated together in a symmetric structure called a tetramer. They also ascertained that two dimers (each containing two molecules of the toxin) assemble into one tetramer, and that assembly is stimulated by the addition of calcium. The structures of both the dimer and the tetramer were determined, and their comparison shows that tetramer assembly causes distinct structural changes in the individual toxin molecules - changes that appear to maximize positive interactions of the tetramer with the cell membrane.

This group determined the structures of the dimer and tetramer to a resolution of 18 and 14 angstroms, respectively. The tetramer structure is remarkably visually appealing—it resembles a windmill and clearly shows a hole down its center, through which calcium ions probably pass. The researchers further speculate that the 'arms' of the 'windmill' could provide surfaces for interactions with the cellular signaling machinery.

Technically, the achievements of Ushkaryov and colleagues are impressive because they were able to determine the structure of both tetrameric and dimeric alpha-latrotoxin by a technique called single particle cryo-electron microscopy. Unlike X-ray crystallography, which requires a protein complex to be assembled into a crystalline array for analysis, single particle cryo-electron microscopy allows the molecules under scrutiny to be randomly oriented in a thin layer of solution. Such samples are frozen and then subjected to an electron beam for visualization. Individual complexes (called particles) can be detected by this method only if they are above a certain size (in most cases, several hundred kilodaltons). The dimeric structure of alpha-latrotoxin is only 260 kilodaltons, which is clearly on the borderline for detection.

The results of Ushkaryov and coworkers suggest a plausible mechanism of alpha-latrotoxin pore formation. In the scientific community, their studies should stimulate research into the toxin's many mechanisms of action. In the general public, their accomplishments should be appreciated for their beautiful illustration of how the function of a molecule is determined by its structure. These results are discussed by Helen Saibil in an accompanying News and Views.

Nature Structural Biology pp 28 - 33

While today most reactions in the cell are catalyzed by proteins, there is the idea that long ago these reactions were carried out by RNA molecules. This 'RNA world' hypothesis gained support in 1982 when Sidney Altman and Thomas Cech discovered that RNA molecules known as ribozymes could in fact catalyze chemical reactions in the complete absence of proteins (This discovery earned them the Nobel Prize in Chemistry in 1989).

Once DNA is transcribed into messenger RNA it must be translated into protein. This process involves the ribosome, tRNAs and aminoacyl-tRNA synthetases, whose amino acid specificities determine the genetic code. To test the feasibility of the RNA world scenario, Suga and coworkers at the State University of New York at Buffalo asked whether the activity of the aminoacyl-tRNA synthetases could actually be performed by a ribozyme.

Such a ribozyme was generated in the test tube by Darwinian selective pressure on a large population of synthetic RNA molecules. The molecule that emerged as the winner has two distinct activities - it can (i) recognize a specific amino acid and (ii) transfer that aminoacyl group to itself and then to a specific tRNA molecule. Thus, two of the key functions of an aminoacyl-tRNA synthetase have now been shown to be performed by a ribozyme.

This finding, together with previous studies showing that ribozymes could catalyze amide and peptide bond formation, bolsters the idea that the contemporary system of translation involving many proteins and RNAs could have evolved from a simple set of reactions catalyzed by ribozymes. Paul Schimmel and Shana O. Kelley discuss these new results in an accompanying News & Views.

Nature Structural Biology pp 44 - 47

Photosynthesis by higher plants and cyanobacteria produces simple carbohydrates (for example, glucose) and molecular oxygen that sustain the majority of life forms on this planet. At the core of photosynthesis is the conversion of light energy into chemical energy; this conversion process is performed by two complex molecular assemblies, photosystems I and II.

Light absorbed by the photosystems generates high energy electrons that are eventually harvested in high energy chemical compounds such as ATP. Electron transfer within the photosystems proceeds through structurally organized components, including pigment-containing antenna and light harvesting complexes, as well as the photosynthesis reaction center. In photosystem II, the electron transfer process also drives the oxidation of water to generate molecular oxygen in the associated oxygen evolving complex.

To understand how light energy is utilized by plants for photosynthesis, it is essential to understand how electrons are transferred within the photosystems and the structural organization underlying this process. The overall structure of photosystem II, including the associated oxygen evolving complex, has now been determined by cryo-electron microscopy in the laboratory of James Barber at the Imperial College of Science, Technology and Medicine, UK. This structure, while not at a resolution displaying atomic details, provides a framework to identify the locations of several important component protein complexes and a possible pathway of electron transfer from photosystem II to the oxygen evolving complex. This structure thus brings us one step closer to understanding how light energy is harnessed.