To define glycemic thresholds for activation of counterregulatory hormone secretion, initiation of symptoms (autonomic and neuroglycopenic), and onset of deterioration of cognitive function, we measured indexes of these responses during glycemic plateaus of 90, 78, 66, 54, and 42 mg/dl in 10 normal volunteers, with the use of the hyperinsulinemic glucose clamp technique. Activation of glucagon, epinephrine, norepinephrine, and growth hormone secretion began at arterialized venous plasma glucose concentrations of 68 +/- 1, 68 +/- 1, 65 +/- 1, and 67 +/- 2 (SE) mg/dl, respectively. Autonomic symptoms (anxiety, palpitations, sweating, irritability, and tremor) began at 58 +/- 2 mg/dl, which was significantly (P = 0.0001) lower. Neuroglycopenic symptoms (hunger, dizziness, tingling, blurred vision, difficulty thinking, and faintness) and deterioration in cognitive function tests began at 51 +/- 3 and 49 +/- 2 mg/dl, respectively, values that were both significantly (P = 0.018 and 0.004, respectively) lower than that for initiation of autonomic symptoms. We therefore conclude that there is a distinct hierarchy of responses to decrements in plasma glucose, such that the threshold for activation of counterregulatory hormone secretion occurs at higher plasma glucose levels than that for initiation of autonomic warning symptoms, which in turn occurs at higher plasma glucose levels than that for onset of neuroglycopenic symptoms and deterioration in cerebral function. Such a hierarchy would maximize the opportunity to avoid incapacitating hypoglycemia.

The crystal structure of the cyanobacterial cytochrome b 6 f complex has previously been solved to 3.0-Å resolution using the thermophilic Mastigocladus laminosus whose genome has not been sequenced. Several unicellular cyanobacteria, whose genomes have been sequenced and are tractable for mutagenesis, do not yield b 6 f complex in an intact dimeric state with significant electron transport activity. The genome of Nostoc sp. PCC 7120 has been sequenced and is closer phylogenetically to M. laminosus than are unicellular cyanobacteria. The amino acid sequences of the large core subunits and four small peripheral subunits of Nostoc are 88 and 80% identical to those in the M. laminosus b 6 f complex. Purified b 6 f complex from Nostoc has a stable dimeric structure, eight subunits with masses similar to those of M. laminosus, and comparable electron transport activity. The crystal structure of the native b 6 f complex, determined to a resolution of 3.0 Å (PDB id: 2ZT9), is almost identical to that of M. laminosus. Two unique aspects of the Nostoc complex are: (i) a dominant conformation of heme b p that is rotated 180°a bout the ␣-and ␥-meso carbon axis relative to the orientation in the M. laminosus complex and (ii) acetylation of the Rieske iron-sulfur protein (PetC) at the N terminus, a post-translational modification unprecedented in cyanobacterial membrane and electron transport proteins, and in polypeptides of cytochrome bc complexes from any source. The high spin electronic character of the unique heme c n is similar to that previously found in the b 6 f complex from other sources.

SUMMARY Multidrug transporters are ubiquitous efflux pumps that provide cells with defense against various toxic compounds. In bacteria, which typically harbor numerous multidrug transporter genes, the majority function as secondary multidrug/proton antiporters. Proton-coupled secondary transport is a fundamental process that is not fully understood, largely owing to the obscure nature of proton-transporter interactions. Here we analyzed the substrate/proton coupling mechanism in MdfA, a model multidrug/ proton antiporter. By measuring the effect of protons on substrate binding and by directly measuring proton binding and release, we show that substrates and protons compete for binding to MdfA. Our studies strongly suggest that competition is an integral feature of secondary multidrug transport. We identified the proton-binding acidic residue and show that, surprisingly, the substrate binds at a different site. Together, the results suggest an interesting mode of indirect competition as a mechanism of multi-drug/proton antiport.

Bacillus anthracis, the causative agent of anthrax, requires surface (S)-layer proteins for the pathogenesis of infection. Previous work characterized S-layer protein binding via the surface layer homology domain to a pyruvylated carbohydrate in the envelope of vegetative forms. The molecular identity of this carbohydrate and the mechanism of its display in the bacterial envelope are still unknown. Analyzing acid-solubilized, purified carbohydrates by mass spectrometry and NMR spectroscopy, we identify secondary cell wall polysaccharide (SCWP) as the ligand of S-layer proteins. In agreement with the model that surface layer homology domains bind to pyruvylated carbohydrate, SCWP was observed to be linked to pyruvate in a manner requiring csaB, the only structural gene known to be required for S-layer assembly. B. anthracis does not elaborate wall teichoic acids; however, its genome harbors tagO and tagA, genes responsible for the synthesis of the linkage unit that tethers teichoic acids to the peptidoglycan layer. The tagO gene appears essential for B. anthracis growth and complements the tagO mutant phenotypes of staphylococci. Tunicamycin-mediated inhibition of TagO resulted in deformed, S-layer-deficient bacilli. Together, these results suggest that tagO-mediated assembly of linkage units tethers pyruvylated SCWP to the B. anthracis envelope, thereby enabling S-layer assembly and providing for the pathogenesis of anthrax infections.

Review of the literature on the cognitive correlates and consequences of Type 2 diabetes reveals two very intriguing findings. Not only are verbal learning and memory skills most likely to be disrupted as compared to other cognitive skills (e.g. attention, executive function; psychomotor efficiency), but these mnestic deficits appear to be restricted to individuals with diabetes who are older than 60-65 years of age. Middle-aged adults with either Type 2 or Type 1 diabetes are apparently protected insofar as researchers have only infrequently reported learning and memory impairments in that age group. Why do older adults have such an increased risk of diabetes-associated memory dysfunction? In our view, this phenomenon is a consequence of a synergistic interaction between diabetes-related metabolic derangements and the structural and functional changes occurring in the central nervous system (CNS) that are part of the normal ageing process. To critically evaluate that possibility, we summarise what is known about learning and memory dysfunction in the adult with diabetes, examine the extent to which chronic hyperglycaemia may adversely affect the integrity of the CNS, and selectively review the literature on age-associated changes in brain morphology and cognitive function, paying special attention to the threshold theory of cognitive impairment.