Allosteric Activation of the Par-6 PDZ Via a Partial Unfolding Transition

Proteins exist in a delicate balance between the native and unfolded states, where thermodynamic stability may be sacrificed to attain the flexibility required for efficient catalysis, binding, or allosteric control. Partition-defective 6 (Par-6) regulates the Par polarity complex by transmitting a GTPase signal through the Cdc42/Rac interaction binding PSD-95/Dlg/ZO-1 (CRIB-PDZ) module that alters PDZ ligand binding. Allosteric activation of the PDZ is achieved by local rearrangement of the L164 and K165 side chains to stabilize the interdomain CRIB:PDZ interface and reposition a conserved element of the ligand binding pocket. However, microsecond to millisecond dynamics measurements revealed that L164/K165 exchange requires a larger rearrangement than expected. The margin of thermodynamic stability for the PDZ domain is modest (∼3 kcal/mol) and further reduced by transient interactions with the disordered CRIB domain. Measurements of local structural stability revealed that tertiary contacts within the PDZ are disrupted by a partial unfolding transition that enables interconversion of the L/K switch. The unexpected participation of partial PDZ unfolding in the allosteric mechanism of Par-6 suggests that native-state unfolding may be essential for the function of other marginally stable proteins

Physiologic adaptations of the tubuloglomerular feedback system

The maintenance of volume homeostasis is sufficiently important to mammalian terrestrial life that a large amount of evolutionary energy has been expended in the development of multiple control systems, each involved in regulating the volume and composition of internal body fluids. The kidney, which participates in most of these systems, has evolved physiologic attributes which enhance the efficiency of volume regulation. Perhaps the most fundamental of these attributes is a close coordination between the processes of glomerular filtration and tubular reabsorption. Such coordination is required to prevent the amplification of small fluctuations in glomerular filtration rate into large fluctuations in total body salt and water content.It was first suggested by Homer Smith that reabsorption of fluid from the nephron should increase as the delivery of tubular fluid into that segment increases [1]. When applied to the proximal tubule, this principle of flow-dependent transport has come to be referred to as “glomerulotubular balance” [2, 3]. Glomerulotubular balance depends upon intrinsic properties of the proximal nephron including the affinities and densities of various solute transporters and the differential permeabilities of the nephron to various solutes and water, and upon the trans-epithelial concentration gradients of these solutes [4–6]. By definition, glomerulotubular balance describes the functional dependence of tubular reabsorption on glomerular filtration rate independently of other neuro-humoral effectors of tubular transport. However, since glomerulotubular balance is a substrate-driven process, it cannot accomplish an increment in proximal tubular reabsorption which exceeds an increment in delivered load. Therefore, in the absence of effectors other than glomerulotubular balance the volume of fluid entering the distal nephron must be a monotonically increasing function of GFR [7].How then, may the kidney avert an unintentional diuresis should the hemodynamic forces favoring glomerular filtration combine to overwhelm the reabsorptive capacity of the nephron? In 1937 Goormaghtigh suggested that the juxtaglomerular apparatus might participate in the maintenance of volume homeostasis by generating some sort of signal in response to changes in the composition of distal tubular fluid [8]. The peculiar anatomic arrangement of the nephron would facilitate transmission of this signal to the upstream glomerulus and lead to alterations in the physiologic determinants of glomerular filtration. This hypothesis has been refined over the past three decades as substantial experimental data have accrued to support the existence of an operational system of tubuloglomerular feedback (TGF) [9]. Contemporary models of the TGF system, by analogy to negative feedback-driven control systems in engineering control theory, divide the system into three component processes [10]. The first of these components is a parameter which the system is designed to regulate, in this case, the rate at which tubular fluid transits the late proximal nephron or VLP. The second component includes the macula densa and surrounding interstitium which serve to detect differences between the current value of VLP and some internal set-point, and translate this information into an output command. The third component, or effector limb, of the TGF system is constituted by the contractile glomerular mesangium and glomerular arterioles which respond to the aforementioned output command by altering nephron filtration rate (SNGFR) to keep VLP in line with the system's internal set-point. When TGF is allowed to function as a closed-loop system [7], as is the case in vivo, its presence is, by nature, undetectable. However, when late proximal flow is uncoupled from nephron filtration by artificial microperfusion of the late proximal tubule, a dependence of SNGFR on VLP can be defined [11]. This relationship is referred to as the “TGF function”, or “gain” of the TGF system [7, 10]. This TGF function specifies a continuum of points in the VLP-SNGFR plane at which the nephron may operate. The actual operating point of the system exists at the point in this plane where the TGF and glomerulotubular balance functions intersect (Fig. 1).The TGF function may vary in response to the changing needs of the organism, both with regard to volume homeostasis and renal function. The altering of TGF under conditions of pregnancy, loss of renal mass, and a variety of other pathophysiologic conditions suggests that the juxtaglomerular apparatus is involved in events pertinent not merely to volume regulation but to overall renal growth and function

Benthic Foraminiferal response to sea level change in the mixed siliciclastic-carbonate system of southern Ashmore Trough (Gulf of Papua)

Ashmore Trough in the western Gulf of Papua (GoP) represents an outstanding modern example of a tropical mixed siliciclastic-carbonate depositional system where significant masses of both river-borne silicates and bank-derived neritic carbonates accumulate. In this study, we examine how benthic foraminiferal populations within Ashmore Trough vary in response to sea level–driven paleoenvironmental changes, particularly organic matter and sediment supply. Two 11.3-m-long piston cores and a trigger core were collected from the slope of Ashmore Trough and dated using radiocarbon and oxygen isotope measurements of planktic foraminifera. Relative abundances, principal component analyses, and cluster analyses of benthic foraminiferal assemblages in sediment samples identify three distinct assemblages whose proportions changed over time. Assemblage 1, with high abundances of Uvigerina peregrina and Bolivina robusta, dominated between ∼83 and 70 ka (early regression); assemblage 2, with high abundances of Globocassidulina subglobosa, dominated between ∼70 and 11 ka (late regression through lowstand and early transgression); and assemblage 3, with high abundances of neritic benthic species such as Planorbulina mediterranensis, dominated from ∼11 ka to the present (late transgression through early highstand). Assemblage 1 represents heightened organic carbon flux or lowered bottom water oxygen concentration, and corresponds to a time of maximum siliciclastic fluxes to the slope with falling sea level. Assemblage 2 reflects lowered organic carbon flux or elevated bottom water oxygen concentration, and corresponds to an interval of lowered siliciclastic fluxes to the slope due to sediment bypass during sea level lowstand. Assemblage 3 signals increased off-shelf delivery of neritic carbonates, likely when carbonate productivity on the outer shelf (Great Barrier Reef) increased significantly when it was reflooded. Benthic foraminiferal assemblages in the sediment sink (slopes of Ashmore Trough) likely respond to the amount and type of sediment supplied from the proximal source (outer GoP shelf)

A single nephron model of acute tubular injury: Role of tubuloglomerular feedback

A single nephron model of acute tubular injury: Role of tubuloglomerular feedback. A single nephron model of nephrotoxic tubular injury was established to examine the mechanism whereby acute tubular damage contributes to reductions in nephron filtration rate (SNGFR). Acute microperfusion of 0.5ng of uranyl nitrate (UN) into the early proximal tubule produced a significant reduction (16 to 30%) in SNGFR measured in both distal and proximal tubules of the same nephron and a decrease in absolute proximal reabsorption. Microperfused inulin was retained in the tubule suggesting this finding reflected a true reduction in SNGFR. Concurrent infusion of high dose furosemide (2 × 10-4M) and bumetanide (2 × 10-5M), but not low dose furosemide (2 × 10-5M), prevented the UN induced reduction in SNGFR. High dose furosemide begun after UN perfusion also prevented reduction in SNGFR. Continuous direct measurement of glomerular capillary hydrostatic pressure revealed no change. Distal intratubular Na+ and CI- concentration increased significantly after UN perfusion. Activation of tubuloglomerular feedback mechanisms best explains the reduction in glomerular ultrafiltration that is characteristic of nephrotoxic forms of tubular injury

Editor\u27s Corner - New Assistant Editors

It is a special pleasure to introduce two new assistant editors in the physical sciences. Chemistry and physics articles have not been abundant in recent issues and both editors are particularly interested in locating articles of practical value to classroom teachers. As each of the new editors states in his editorial, many readers have excellent ideas for classroom activities. Some of these activities may not be totally new but have a new approach which will prove helpful to other teachers. The entire editorial staff is eager to aid you in preparing articles for publication. Contact one of the editors to help you share your ideas with the profession

Structure of the lectin mannose 6-phosphate receptor homology (MRH) domain of glucosidase II an enzyme that regulates glycoprotein folding quality control in the endoplasmic reticulum

Here we report for the first time the three-dimensional structure of a mannose 6-phosphate receptor homology (MRH) domain present in a protein with enzymatic activity, glucosidase II (GII). GII is involved in glycoprotein folding in the endoplasmic reticulum. GII removes the two innermost glucose residues from the Glc3Man9GlcNAc2 transferred to nascent proteins and the glucose added by UDP-Glc:glycoprotein glucosyltransferase. GII is composed of a catalytic GIIα subunit and a regulatory GIIβ subunit. GIIβ participates in the endoplasmic reticulum localization of GIIα and mediates in vivo enhancement of N-glycan trimming by GII through its C-terminal MRH domain. We determined the structure of a functional GIIβ MRH domain by NMR spectroscopy. It adopts a β-barrel fold similar to that of other MRH domains, but its binding pocket is the most shallow known to date as it accommodates a single mannose residue. In addition, we identified a conserved residue outside the binding pocket (Trp-409) present in GIIβ but not in other MRHs that influences GII glucose trimming activity.Fil: Olson, Linda J.. Medical College Of Wisconsin; Estados UnidosFil: Orsi, Ramiro. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Bioquímicas de Buenos Aires. Fundación Instituto Leloir. Instituto de Investigaciones Bioquímicas de Buenos Aires; ArgentinaFil: Alculumbre, Solana G.. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Bioquímicas de Buenos Aires. Fundación Instituto Leloir. Instituto de Investigaciones Bioquímicas de Buenos Aires; ArgentinaFil: Peterson, Francis C.. Medical College Of Wisconsin; Estados UnidosFil: Stigliano, Ivan Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Bioquímicas de Buenos Aires. Fundación Instituto Leloir. Instituto de Investigaciones Bioquímicas de Buenos Aires; ArgentinaFil: Parodi, Armando Jose A.. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Bioquímicas de Buenos Aires. Fundación Instituto Leloir. Instituto de Investigaciones Bioquímicas de Buenos Aires; ArgentinaFil: D'alessio, Cecilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Bioquímicas de Buenos Aires. Fundación Instituto Leloir. Instituto de Investigaciones Bioquímicas de Buenos Aires; Argentina. Universidad de Buenos Aires; ArgentinaFil: Dahms, Nancy M.. Medical College Of Wisconsin; Estados Unido

Long-term optical photometric monitoring of the quasar SDSS J153259.96-003944.1

We report optical Cousins R and I band monitoring observations of the high redshift (z = 4.67) QSO SDSS J153259.96-003944.1 that does not show detectable emission lines in its optical spectrum. We show this object varies with a maximum amplitude of ~0.4 mag during a year and three months of monitoring. Combined with two other epochs of photometric data available in the literature, we show the object has gradually faded by ~0.9 mag during the period June 1998 - April 2001. A linear least squares fit to all available observations gives a slope of ~0.35 mag/yr which translates to ~1.9 mag/yr in the rest frame of the quasar. Such a variability is higher than that typically seen in QSOs but consistent with that of BL Lacs, suggesting that the optical continuum is Doppler boosted. Alternatively, within photometric errors, the observed lightcurve is also consistent with the object going through a microlensing event. Photoionization model calculations show the mass of the Broad Line Region to be few tens of solar mass similar to that of low luminosity Seyfert galaxies, but about orders of magnitude less than that of luminous quasars. Further frequent photometric/spectroscopic monitoring is needed to support or refute the different alternatives discussed here on the nature of SDSS J153259.96-003944.1.Comment: 7 pages with 4 figures, uses mn2e.cls, Accepted for publication in MNRA

Optical--to--X-ray emission in low-absorption AGN: Results from the Swift-BAT 9 month catalogue

(Abridged) We present simultaneous optical--to--X-ray spectral energy distributions (SEDs) from Swift's X-ray and UV--optical telescopes (XRT and UVOT) for a well-selected sample of 26 low-redshift (z<0.1) AGN from the Swift/BAT 9-month catalogue, the largest well-studied, hard X-ray selected survey of local AGN to date. Our subsample consists of AGN with low intrinsic X-ray absorption (N_H<10^22 cm^-2) and minimal spectral complexity, to more accurately recover the intrinsic accretion luminosity in these sources. We perform a correction for host galaxy contamination in all available UVOT filter images to recover the intrinsic AGN emission, and estimate intrinsic dust extinction from the resultant nuclear SEDs. Black hole mass estimates are determined from the host-galaxy 2MASS K-band bulge luminosity. Accretion rates determined from our SEDs are on average low (Eddington ratios <~ 0.1) and hard X-ray bolometric corrections cluster at ~10-20, in contrast with the higher values seen for quasars. An average SED for the 22 low accretion rate (Eddington ratio < 0.1) objects is presented, with and without correction for intrinsic extinction. We do not find a correlation of optical--to--X-ray spectral index with Eddington ratio, regardless of the optical reference wavelength chosen for defining the spectral index. The low accretion rates and bolometric corrections found for this representative low-redshift sample are of particular importance for studies of AGN accretion history.Comment: 25 pages, 22 figures, 4 tables, accepted for publication in MNRA