Seasonal oscillations in water exchange between aquifers and the coastal ocean

Ground water of both terrestrial and marine origin flows into coastal surface waters as submarine groundwater discharge, and constitutes an important source of nutrients, contaminants and trace elements to the coastal ocean. Large saline discharges have been observed by direct measurements and inferred from geochemical tracers, but sufficient seawater inflow has not been observed to balance this outflow. Geochemical tracers also suggest a time lag between changes in submarine groundwater discharge rates and the seasonal oscillations of inland recharge that drive groundwater flow towards the coast. Here we use measurements of hydraulic gradients and offshore fluxes taken at Waquoit Bay, Massachusetts, together with a modelling study of a generalized coastal groundwater system to show that a shift in the freshwater-saltwater interface-controlled by seasonal changes in water table elevation-can explain large saline discharges that lag inland recharge cycles. We find that sea water is drawn into aquifers as the freshwater-saltwater interface moves landward during winter, and discharges back into coastal waters as the interface moves seaward in summer. Our results demonstrate the connection between the seasonal hydrologic cycle inland and the saline groundwater system in coastal aquifers, and suggest a potentially important seasonality in the chemical loading of coastal waters.     

New ages for human occupation and climatic change at Lake Mungo,Australia

Australia's oldest human remains, found at Lake Mungo, include the world's oldest ritual ochre burial (Mungo III) and the first recorded cremation (Mungo I). Until now, the importance of these finds has been constrained by limited chronologies and palaeoenvironmental information. Mungo III, the source of the world's oldest human mitochondrial DNA, has been variously estimated at 30 thousand years (kyr) old, 42-45 kyr old and 62 +/- 6 kyr old, while radiocarbon estimates placed the Mungo I cremation near 20-26 kyr ago. Here we report a new series of 25 optical ages showing that both burials occurred at 40 +/- 2 kyr ago and that humans were present at Lake Mungo by 50-46 kyr ago, synchronously with, or soon after, initial occupation of northern and western Australia. Stratigraphic evidence indicates fluctuations between lake-full and drier conditions from 50 to 40 kyr ago, simultaneously with increased dust deposition, human arrival and continent-wide extinction of the megafauna. This was followed by sustained aridity between 40 and 30 kyr ago. This new chronology corrects previous estimates for human burials at this important site and provides a new picture of Homo sapiens adapting to deteriorating climate in the world's driest inhabited continent.     

Transforming activity of polyoma virus middle-T antigen probed by site-directed mutagenesis

The ability of polyoma virus to transform cells results primarily from the action of one of the virus-coded early proteins, called middle-T antigen. Middle-T has an associated tyrosine-specific protein kinase activity that can be measured in vitro and results in the phosphorylation of middle-T itself. Almost all mutants so far tested that lack the ability to transform cells, also lack associated kinase activity. Attempts to map within middle-T the tyrosine residue(s) that are phosphorylated in vitro suggest that a likely site of phosphorylation is tyrosine 315 (refs 8-10 and unpublished results). The amino acid sequence preceding Tyr 315 includes a tract of six contiguous glutamic acid residues and bears some homology with that preceding the tyrosine phosphorylated in vivo in pp60v-src, the transforming protein of Rous sarcoma virus, and with a region in the polypeptide hormone, gastrin, preceding a tyrosine that is sulphated. Furthermore, although surprisingly large tracts of middle-T may be removed without affecting its transforming activity, mutants that lack the sequences corresponding to amino acids 311-318 inclusive are transformation defective. Because the likely site of phosphorylation, the homology with pp60v-src and gastrin and the sequence apparently required for transformation all overlap, it has generally been accepted that this region of middle-T may form part of an essential region, possibly an active site on the protein. Here we have used techniques of site-directed and site-specific mutagenesis to probe the sequence requirements in more detail. Contrary to expectation, the results obtained strongly suggest that Tyr 315 and conservation of the surrounding amino acid sequence are not essential for transformation.     

Functional cloning of TUG as a regulator of GLUT4 glucose transporter trafficking

Insulin stimulates glucose uptake in fat and muscle by mobilizing the GLUT4 glucose transporter. GLUT4 is sequestered intracellularly in the absence of insulin, and is redistributed to the plasma membrane within minutes of insulin stimulation. But the trafficking mechanisms that control GLUT4 sequestration have remained elusive. Here we describe a functional screen to identify proteins that modulate GLUT4 distribution, and identify TUG as a putative tether, containing a UBX domain, for GLUT4. In truncated form, TUG acts in a dominant-negative manner to inhibit insulin-stimulated GLUT4 redistribution in Chinese hamster ovary cells and 3T3-L1 adipocytes. Full-length TUG forms a complex specifically with GLUT4; in 3T3-L1 adipocytes, this complex is present in unstimulated cells and is largely disassembled by insulin. Endogenous TUG is localized with the insulin-mobilizable pool of GLUT4 in unstimulated 3T3-L1 adipocytes, and is not mobilized to the plasma membrane by insulin. Distinct regions of TUG are required to bind GLUT4 and to retain GLUT4 intracellularly in transfected, non-adipose cells. Our data suggest that TUG traps endocytosed GLUT4 and tethers it intracellularly, and that insulin mobilizes this pool of retained GLUT4 by releasing this tether.