Deconjugation of Nedd8 from Cul1 Is Directly Regulated by Skp1-F-box and Substrate, and the COP9 Signalosome Inhibits Deneddylated SCF by a Noncatalytic Mechanism

COP9 signalosome (CSN) mediates deconjugation of the ubiquitin-like protein Nedd8 from the cullin subunits of SCF and other cullin-RING ubiquitin ligases (CRLs). This process is essential to maintain the proper activity of CRLs in cells. Here, we report a detailed kinetic characterization of CSN-mediated deconjugation of Nedd8 from SCF. CSN is an efficient enzyme, with a k_(cat) of ∼1 s^(−1) and K_mfor neddylated Cul1-Rbx1 of ∼200 nm, yielding a k_(cat)/K_m near the anticipated diffusion-controlled limit. Assembly with an F-box-Skp1 complex markedly inhibited deneddylation, although the magnitude varied considerably, with Fbw7-Skp1 inhibiting by ∼5-fold but Skp2-Cks1-Skp1 by only ∼15%. Deneddylation of both SCF^(Fbw7) and SCF^(Skp2-Cks1) was further inhibited ∼2.5-fold by the addition of substrate. Combined, the inhibition by Fbw7-Skp1 plus its substrate cyclin E was greater than 10-fold. Unexpectedly, our results also uncover significant product inhibition by deconjugated Cul1, which results from the ability of Cul1 to bind tightly to CSN. Reciprocally, CSN inhibits the ubiquitin ligase activity of deneddylated Cul1. We propose a model in which assembled CRL complexes engaged with substrate are normally refractory to deneddylation. Upon consumption of substrate and subsequent deneddylation, CSN can remain stably bound to the CRL and hold it in low state of reduced activity

Structural and kinetic analysis of the COP9-Signalosome activation and the cullin-RING ubiquitin ligase deneddylation cycle

The COP9-Signalosome (CSN) regulates cullin–RING ubiquitin ligase (CRL) activity and assembly by cleaving Nedd8 from cullins. Free CSN is autoinhibited, and it remains unclear how it becomes activated. We combine structural and kinetic analyses to identify mechanisms that contribute to CSN activation and Nedd8 deconjugation. Both CSN and neddylated substrate undergo large conformational changes upon binding, with important roles played by the N-terminal domains of Csn2 and Csn4 and the RING domain of Rbx1 in enabling formation of a high affinity, fully active complex. The RING domain is crucial for deneddylation, and works in part through conformational changes involving insert-2 of Csn6. Nedd8 deconjugation and re-engagement of the active site zinc by the autoinhibitory Csn5 glutamate-104 diminish affinity for Cul1/Rbx1 by ~100-fold, resulting in its rapid ejection from the active site. Together, these mechanisms enable a dynamic deneddylation-disassembly cycle that promotes rapid remodeling of the cellular CRL network

Estimation of Landscape Soil Water Losses from Satellite Observations of Soil Moisture

This study presents an observation-driven technique to delineate the dominant boundaries and temporal shifts between different hydrologic regimes over the contiguous United States (CONUS). The energy- and water-limited evapotranspiration regimes as well as percolation to the subsurface are hydrologic processes that dominate the loss of stored water in the soil following precipitation events. Surface soil moisture estimates from the NASA Soil Moisture Active Passive (SMAP) mission, over three consecutive summer seasons, are used to estimate the soil water loss function. Based on analysis of the rates of soil moisture dry-downs, the loss function is the conditional expectation of negative increments in the soil moisture series conditioned on soil moisture itself. An unsupervised classification scheme (with cross validation) is then implemented to categorize regions according to their dominant hydrological regimes based on their estimated loss functions. An east-west divide in hydrologic regimes over CONUS is observed with large parts of the western United States exhibiting a strong water-limited evapotranspiration regime during most of the times. The U.S. Midwest and Great Plains show transitional behavior with both water- and energy-limited regimes present. Year-to-year shifts in hydrologic regimes are also observed along with regional anomalies due to moderate drought conditions or above-average precipitation. The approach is based on remotely sensed surface soil moisture (approximately top 5 cm) at a resolution of tens of kilometers in the presence of soil texture and land cover heterogeneity. The classification therefore only applies to landscape-scale effective conditions and does not directly account for deeper soil water storage

Can Surface Soil Moisture Information Identify Evapotranspiration Regime Transitions?

The transition of evapotranspiration between energy‐ and water‐limitation regimes also denotes a nonlinear change in surface water and energy coupling strength. The regime transitions are primarily dominated by available moisture in the soil, although other micro‐meteorological factors also play a role. Remotely sensed soil moisture is frequently used for detecting evapotranspiration regime transitions during inter storm dry downs. However, its sampling depth does not include the entire soil profile, over which water uptake is dominated by plant root distribution. We use flux tower, surface (θ ( s ); observations at 5 cm), and vertically integrated in situ soil moisture ([Formula: see text]; 0–50 cm) observations to address the question: Can surface soil moisture robustly identify evapotranspiration regime transitions? Results demonstrate that θ ( s ) and θ ( v ) are hydraulically linked and have synchronized evapotranspiration regime transitions. As such, θ ( s ) and θ ( v ) capture comparable statistics of evapotranspiration regime prevalence, which supports the utility of remote‐sensing θ ( s ) for large‐scale land‐atmosphere exchange analysis