This Week in Science

Science  11 Feb 2011:
Vol. 331, Issue 6018, pp. 647
  1. Fickle Crab

      CREDIT: X-RAY: NASA/CXC/J.HESTER (ASU), OPTICAL: NASA/ESA/J.HESTER AND A. LOLL (ASU), INFRARED: NASA/JPL-CALTECH/R.GEHRZ (UNIV. OF MINN.)

      One of the most-studied astronomical objects in the sky, the Crab Nebula, is the remnant of a supernova explosion, recorded on Earth in 1054 C.E. (see the Perspective by Bernardini). Because of its steady emission and its brightness, the Crab Nebula has been used traditionally as a reference to calibrate observations of other sources in the sky. Based on data from two different space telescopes, Tavani et al. (p. 736, published online 6 January) and Abdo et al. (p. 739, published online 6 January) describe gamma-ray flaring episodes that show that the Crab is not as steady at high energies as once believed. These observations also pose challenges to the theoretical models of particle acceleration that are usually applied to the Crab and other supernova remnants.

    1. Squishing Methanol Monohydrate

        Most materials will expand when heated and will compress in all directions when subject to a uniform pressure. A few exceptions are known, such as water, which shows a maximum in density around 4°C, and some framework-like molecules or foams that can show expansion in one or two directions when compressed. Despite being a very simple molecular system, methanol monohydrate exhibits very unusual temperature and pressure-dependent mechanical properties. Fortes et al. (p. 742; see the Perspective by Grima et al.) show that the molecule has remarkably strong anisotropic thermal expansion (positive and negative) and negative linear compressibility, which can be attributed to the way the hydroxyl-water chains are bridged with the methyl groups.

      1. Deadly Switch

          Numerous bacterial species, such as Neisseria meningitidis, thrive in the mouth, throat, and gut, or on the skin, and propagate from one host to the next. Most of the time cohabitation with these commensals has no pathological consequences and can even be beneficial. However, commensal species are occasionally responsible for severe invasive infections, including septicemia and meningitis. Chamot-Rooke et al. (p. 778) describe the details of a molecular switch that takes place during the infection cycle of N. meningitidis that allows it to disseminate. The process depends on a regulated posttranslational modification of the N. meningitidis pilin, the major component of a filamentous adhesive structure, the type IV pilus. This modification caused bacteria to maintain their adhesive properties to the host cell surface but to lose their ability to stay attached to bacterial microcolonies, so that a small proportion of bacteria are thus free to colonize new hosts and propagate.

        1. Stand Up and Walk

            CREDIT: WARD ET AL.

            In early human evolution, walking upright required several evolutionary changes and adaptations. Understanding this evolution has been hampered by a sparse fossil record—particularly, a lack of key bones from feet across a number of species. Australopithecus is the first human that is thought to have been a fully terrestrial biped and is thought to have made the famous footprints at Laetoli in Tanzania. Ward et al. (p. 750) now describe a key element of the foot of A. afarensis, a complete fourth metatarsal, one of the long bones connecting the toe to the base of the foot. The bone's morphology indicates that this early human's foot was stiff and had a well-formed longitudinal arch, as seen in modern humans.

          1. An Amorphous Route to Smaller Band Gaps

              Titanium dioxide (TiO2) is an excellent photo catalyst when excited by ultraviolet (UV) light. Most of the solar spectrum is at longer wavelengths than UV, and much effort has been directed toward finding ways to decrease the band gap of TiO2 (for example, incorporating dopant atoms). Chen et al. (p. 746, published online 20 January) show that hydrogenation of the surface layers of nanophase TiO2 creates a black material that absorbs at longer wavelengths. Under simulated solar irradiation, this material was faster than bulk TiO2 at photooxidizing organic compounds such as methylene blue and could generate hydrogen from water containing methanol even when the UV part of the solar spectrum was removed with a filter.

            1. Digit Identity in Bird Wing

                For many years, there has been considerable controversy regarding bird wing digit identity. Embryological evidence has suggested that these digits represent digits 2, 3, and 4 in the pentadactyl ground state, whereas the fossil record supports a pattern of posterior digit reduction in the theropod ancestors of birds and predicts the bird digits to represent digits 1, 2, and 3. Tamura et al. (p. 753) examined the developmental mechanism of the posterior-most (third) digit in the chick wing and found that this digit was not developmentally comparable to digit 4 in the chick hindlimb but should instead be defined as digit 3. These findings reconcile developmental patterns with comparative morphological data and help to clarify the relationships between birds and their dinosaur ancestors.

              1. More or Less Durable

                  The ability to measure protein half-lives on a large scale in human cells should contribute to our understanding of a variety of physiological and pathological processes. In order to meet this goal, Eden et al. (p. 764, published online 13 January; see the Perspective by Plotkin) developed a method called bleach-chase. Bleach-chase was used to reveal an unexpectedly simple response of fluorescently tagged protein half-lives to stresses and to drugs that stop cell division: Long-lived proteins become longer-lived. It appears that changes in cell growth cause changes in the intra cellular dilution rates of proteins, which are not balanced by corresponding changes in active protein degradation.

                1. Double-Trouble Mutation

                    Many endocrine tumors do not metastasize but, nonetheless, can adversely affect health because the tumor cells secrete excessive amounts of hormones. Aldosterone-producing adrenal adenomas (APAs), for example, can cause severe hypertension. The molecular mechanism coupling uncontrolled cell proliferation with hormone production in these tumors has been unclear. Using an exome-sequencing approach to identify faulty genes in human APAs, Choi et al. (p. 768; see the Perspective by Funder) found that about one-third of the tumors examined had somatic mutations in the gene encoding the KCNJ5 potassium channel. The mutations allowed inappropriate permeation of sodium into the channel, resulting in cell depolarization. Because cell depolarization in the adrenal cortex is a signal for both aldosterone production and cell growth, disruption of this potassium channel explains the two prominent features of this kind of tumor.

                  1. 40S Ribosome Structure

                      CREDIT: RAMAKRISHNAN

                      Atomic resolution structures of prokaryotic ribosomes have provided understanding of how this macromolecular machine coordinates protein synthesis. Although the core function of the ribosome is conserved, the eukaryotic ribosome is much larger, with more complex regulation. Rabl et al. (p. 730, published online 23 December; see the Perspective by Ramakrishnan) report the structure of the Tetrahymena thermophila small ribosomal subunit (40S) in complex with eukaryotic initiation factor 1 (eIF1) that reveals the fold of the ribosomal RNA and all ribosomal proteins and defines interactions with eIF1. The structure provides insight into the evolution of eukaryotic ribosomes and is a step toward a more precise understanding of how translation is initiated in eukaryotes.

                    1. Recall Equals Retention

                        Teachers make use of various strategies to help students retain the most from the texts they read. One strategy, elaborative concept mapping, asks the students to diagram the relationships between the concepts they draw out of the text. Working with university students, Karpicke and Blunt (p. 772, published online 20 January) compared this strategy with another approach, retrieval practice, which asks students to recall in writing, in no particular order, as much as they can from what they have just read. Although the students usually expected the mapping approach to result in better learning, in fact, the retrieval practice approach worked better.

                      1. Parasites Ad Infinitum

                          Leishmania protozoan parasites cause a diversity of tropical and subtropical diseases, ranging from disseminated visceral infections to disfiguring facial lesions. Ives et al. (p. 775) found that the Leishmania guyanensis parasites that cause mucocutaneous lesions are themselves infected by viruses, which seem to act as the trigger for pathology. The host's Toll-like receptors 3 and 7 are activated in virus-infected L. guyanensis infections to promote expression of proinflammatory mediators, and the parasites are then able to persist within the inflammatory cells. The viruses might thus offer a tractable target for diagnostic, drug, or vaccine development for a group of widespread but neglected infections.

                        1. An Organized Tail

                            Mutations in five genes are associated with usher syndrome 1 (USH1) deaf-blindness in humans. A significant number of these mutations affect amino acids in tandem domains (the MyTH4-FERM domains) in the tail of myosin VIIa that are known to bind to another USH1 protein called Sans. Wu et al. (p. 757) determined the crystal structure of the MyTH4-FERM domains, complexed to a central domain from Sans. The MyTH4 and FERM domains form a supramodule. The FERM domain is tightly bound to a conserved region of the Sans fragment, and a second conserved region probably binds weakly to the MyTH4 domain. The structure provides a mechanistic basis for many of the disease-causing mutations.

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