Steps toward catalytic antibodies
Highly specific recognition of antigens by antibodies results during affinity maturation, in which an expanding population of different B cell clones compete for the same antigen. Patten et al. ( p. 1086; see the Perspective by Davis, p. 1078) cloned and expressed nine affinity mutations from germline cells that occurred during the generation of an antibody that catalyzes ester hydrolysis. A kinetic analysis revealed that as the antibody evolved, its affinity for the transition-state analog increased by a factor of 104, and the reaction rate for the ester substrate increased by about a factor of 102. A crystal structure of the antibody revealed that these mutations were not in direct contact with the hapten and thus likely played a conformational role.
Some clays can swell to several times their original thickness in the presence of water, but the details of water adsorption between the layers within the clay are poorly understood. Karaborni et al. (p. 1102) present simulations of sodium-montmorillonite, an abundant clay that affects the stability of oil wells during drilling. Particularly strong swelling of this clay may result from the occurrence of two different types of stable structures, depending on the number of water layers between the clay layers. Switching between these two different stable states may facilitate the swelling of the clay.
On the level
Reconstructing past changes in sea level has proven difficult, yet it is critical for evaluating causes of sea-level changes (for example, tectonic versus climatic) and controls on deposition. Two controversial studies by Vail and Haq and their co-workers had correlated sequence boundaries and other features in marine stratigraphic records distributed globally to try to delineate past changes in sea level. Miller et al. (p. 1092) now test this interpretation by comparing the oxygen isotope curve, an indicator of sea-level changes due to glaciation, with ages of sequence boundaries in Oligocene-Miocene sediments along the New Jersey coast. The ages of unconformities are correlated with indications of a lowering of sea level inferred from oxygen isotope values.
Listening to the air
A recently completed network of 24 Global Positioning System satellites has been used to listen to the structure of the Earth's atmosphere using radio occultation. Kursinski et al. (p. 1107) determined preliminary vertical temperature and water vapor profiles that illustrate the enhanced resolution of this technique over limited ground-based balloon radiosonde and model data. Additional measurements should contribute to our understanding of the structure of the stratosphere and enhance weather forecasting.
CO in a comet coma
The development of sensitive millimeter-wave telescopes has allowed astronomers to observe distant comets to understand what produces their gaseous comas when they are too cold to sublimate water-ice. Jewitt et al. (p. 1110) have determined that the bright coma of comet Hale-Bopp is due to the outgassing of CO. This rare observation of CO outgassing may be a transient brightening that will evolve into an even brighter water outgassing, visible with the naked eye, as Hale-Bopp makes its closest approach to the sun in April 1997.
Locus control regions (LCRs) permit the tissue-specific expression of linked genes at high levels. Festenstein et al. (p. 1123) have studied the LCR from the human CD2 gene in transgenic mice. They find that the CD2 LCR was necessary to prevent variegated expression (expression in some cells of a tissue but not others) when the gene was inserted into centromeric heterochromatin. Such variegated expression was associated with a closed chromatin configuration. They conclude that the LCR functions by establishing or maintaining an open chromatin domain.
Regulation by release
Like other receptors, the transforming growth factor (TGF)-beta receptor is expected to transmit signals to the interior of the cell through its interaction with associated proteins. Wang et al. (p. 1120) searched for proteins that interacted with the TGF-beta receptor and found the alpha subunit of farnesyltransferase (FNTA). FNTA regulates the activity of small guanine nucleotide-binding proteins like Ras that participate in many signaling pathways. When TGF-beta bound to its receptor, FNTA was released. Thus, signaling by the TGF-beta receptor may be mediated, at least in part, by ligand-induced release of FNTA and consequent regulation of small guanine nucleotide-binding proteins.
Liver-specific multidrug transporter
Formation of bile requires the secretion of bile acids from the liver through a canalicular multiorganic anion transporter (cMOAT) that has been defined physiologically but whose molecular definition has remained elusive. Deficiencies in bile acid secretion leads to disease. Paulusma et al. (p. 1126) have described the isolation of the sequence that encodes cMOAT. The protein is related to known multidrug transporters and is specifically expressed in the canalicular membrane of liver cells.