Myoglobin-like aerotaxis transducers in Archaea and Bacteria

Haem-containing proteins such as haemoglobin and myoglobin play an essential role in oxygen transport and storage. Comparison of the amino-acid sequences of globins from Bacteria and Eukarya suggests that they share an early common ancestor, even though the proteins perform different functions in these two kingdoms. Until now, no members of the globin family have been found in the third kingdom, Archaea. Recent studies of biological signalling in the Bacteria and Eukarya have revealed a new class of haem-containing proteins that serve as sensors. Until now, no haem-based sensor has been described in the Archaea. Here we report the first myoglobin-like, haem-containing protein in the Archaea, and the first haem-based aerotactic transducer in the Bacteria (termed HemAT-Hs for the archaeon Halobacterium salinarum, and HemAT-Bs for Bacillus subtilis). These proteins exhibit spectral properties similar to those of myoglobin and trigger aerotactic responses.     

Evidence that humans evolved from a knuckle-walking ancestor

Bipedalism has traditionally been regarded as the fundamental adaptation that sets hominids apart from other primates. Fossil evidence demonstrates that by 4.1 million years ago, and perhaps earlier, hominids exhibited adaptations to bipedal walking. At present, however, the fossil record offers little information about the origin of bipedalism, and despite nearly a century of research on existing fossils and comparative anatomy, there is still no consensus concerning the mode of locomotion that preceded bipedalism. Here we present evidence that fossils attributed to Australopithecus anamensis (KNM-ER 20419) and A. afarensis (AL 288-1) retain specialized wrist morphology associated with knuckle-walking. This distal radial morphology differs from that of later hominids and non-knuckle-walking anthropoid primates, suggesting that knuckle-walking is a derived feature of the African ape and human clade. This removes key morphological evidence for a Pan-Gorilla clade, and suggests that bipedal hominids evolved from a knuckle-walking ancestor that was already partly terrestrial.     

NO is necessary and sufficient for egg activation at fertilization

The early steps that lead to the rise in calcium and egg activation at fertilization are unknown but of great interest--particularly with the advent of in vitro fertilization techniques for treating male infertility and whole-animal cloning by nuclear transfer. This calcium rise is required for egg activation and the subsequent events of development in eggs of all species. Injection of intact sperm or sperm extracts can activate eggs, suggesting that sperm-derived factors may be involved. Here we show that nitric oxide synthase is present at high concentration and active in sperm after activation by the acrosome reaction. An increase in nitrosation within eggs is evident seconds after insemination and precedes the calcium pulse of fertilization. Microinjection of nitric oxide donors or recombinant nitric oxide synthase recapitulates events of egg activation, whereas prior injection of oxyhaemoglobin, a physiological nitric oxide scavenger, prevents egg activation after fertilization. We conclude that nitric oxide synthase and nitric-oxide-related bioactivity satisfy the primary criteria of an egg activator: they are present in an appropriate place, active at an appropriate time, and are necessary and sufficient for successful fertilization.     

YidC mediates membrane protein insertion in bacteria

The basic machinery for the translocation of proteins into or across membranes is remarkably conserved from Escherichia coli to humans. In eukaryotes, proteins are inserted into the endoplasmic reticulum using the signal recognition particle (SRP) and the SRP receptor, as well as the integral membrane Sec61 trimeric complex (composed of alpha, beta and gamma subunits). In bacteria, most proteins are inserted by a related pathway that includes the SRP homologue Ffh, the SRP receptor FtsY, and the SecYEG trimeric complex, where Y and E are related to the Sec61 alpha and gamma subunits, respectively. Proteins in bacteria that exhibit no dependence on the Sec translocase were previously thought to insert into the membrane directly without the aid of a protein machinery. Here we show that membrane insertion of two Sec-independent proteins requires YidC. YidC is essential for E. coli viability and homologues are present in mitochondria and chloroplasts. Depletion of YidC also interferes with insertion of Sec-dependent membrane proteins, but it has only a minor effect on the export of secretory proteins. These results provide evidence for an additional component of the translocation machinery that is specialized for the integration of membrane proteins.     

Quasi-planar nucleus structure in apoferritin crystallization

First-order phase transitions of matter, such as condensation and crystallization, proceed through the formation and subsequent growth of 'critical nuclei' of the new phase. The thermodynamics and kinetics of the formation of these critical nuclei depend on their structure, which is often assumed to be a compact, three-dimensional arrangement of the constituent molecules or atoms. Recent molecular dynamics simulations have predicted compact nucleus structures for matter made up of building blocks with a spherical interaction field, whereas strongly anisotropic, dipolar molecules may form nuclei consisting of single chains of molecules. Here we show, using direct atomic force microscopy observations, that the near-critical-size clusters formed during the crystallization of apoferritin, a quasi-spherical protein, and which are representative of the critical nucleus of this system, consist of planar arrays of one or two monomolecular layers that contain 5-10 rods of up to 7 molecules each. We find that these clusters contain between 20 and 50 molecules each, and that the arrangement of the constituent molecules is identical to that found in apoferritin crystals. We anticipate that similarly unexpected critical nucleus structures may be quite common, particularly with anisotropic molecules, suggesting that advanced nucleation theories should treat the critical nucleus structure as a variable.     

Crystal structure of enteropathogenic Escherichia coli intimin-receptor complex

Intimin and its translocated intimin receptor (Tir) are bacterial proteins that mediate adhesion between mammalian cells and attaching and effacing (A/E) pathogens. Enteropathogenic Escherichia coli (EPEC) causes significant paediatric morbidity and mortality world-wide. A related A/E pathogen, enterohaemorrhagic E. coli (EHEC; O157:H7) is one of the most important food-borne pathogens in North America, Europe and Japan. A unique and essential feature of A/E bacterial pathogens is the formation of actin-rich pedestals beneath the intimately adherent bacteria and localized destruction of the intestinal brush border. The bacterial outer membrane adhesin, intimin, is necessary for the production of the A/E lesion and diarrhoea. The A/E bacteria translocate their own receptor for intimin, Tir, into the membrane of mammalian cells using the type III secretion system. The translocated Tir triggers additional host signalling events and actin nucleation, which are essential for lesion formation. Here we describe the the crystal structures of an EPEC intimin carboxy-terminal fragment alone and in complex with the EPEC Tir intimin-binding domain, giving insight into the molecular mechanisms of adhesion of A/E pathogens.