The transfer of reductive energy and pace of proteome turnover: a theory of integrated catabolic control

Cotranslational proteolysis dominates glutathione homeostasis to support proper growth and development

The earliest proteolytic event affecting most proteins is the excision of the initiating Met (NME). This is an essential and ubiquitous cotranslational process tightly regulated in all eukaryotes. Currently, the effects of NME on unknown complex cellular networks and the ways in which its inhibition leads to developmental defects and cell growth arrest remain poorly understood. Here, we provide insight into the earliest molecular mechanisms associated with the inhibition of the NME process in Arabidopsis thaliana. We demonstrate that the developmental defects induced by NME inhibition are caused by an increase in cellular proteolytic activity, primarily induced by an increase in the number of proteins targeted for rapid degradation. This deregulation drives, through the increase of the free amino acids pool, a perturbation of the glutathione homeostasis, which corresponds to the earliest limiting, reversible step promoting the phenotype. We demonstrate that these effects are universally conserved and that the reestablishment of the appropriate glutathione status restores growth and proper development in various organisms. Finally, we describe a novel integrated model in which NME, protein N-alpha-acylation, proteolysis, and glutathione homeostasis operate in a sequentially regulated mechanism that directs both growth and development.

Nonequilibrium thermodynamics of thiol/disulfide redox systems: a perspective on redox systems biology

Understanding the dynamics of redox elements in biologic systems remains a major challenge for redox signaling and oxidative stress research. Central redox elements include evolutionarily conserved subsets of cysteines and methionines of proteins which function as sulfur switches and labile reactive oxygen species (ROS) and reactive nitrogen species (RNS) which function in redox signaling. The sulfur switches depend on redox environments in which rates of oxidation are balanced with rates of reduction through the thioredoxins, glutathione/glutathione disulfide, and cysteine/cystine redox couples. These central couples, which we term redox control nodes, are maintained at stable but nonequilibrium steady states, are largely independently regulated in different subcellular compartments, and are quasi-independent from each other within compartments. Disruption of the redox control nodes can differentially affect sulfur switches, thereby creating a diversity of oxidative stress responses. Systems biology provides approaches to address the complexity of these responses. In the present review, we summarize thiol/disulfide pathway, redox potential, and rate information as a basis for kinetic modeling of sulfur switches. The summary identifies gaps in knowledge especially related to redox communication between compartments, definition of redox pathways, and discrimination between types of sulfur switches. A formulation for kinetic modeling of GSH/GSSG redox control indicates that systems biology could encourage novel therapeutic approaches to protect against oxidative stress by identifying specific redox-sensitive sites which could be targeted for intervention.

Dose-dependent modulation of the T cell proteome by ascorbic acid

To investigate the hypothesis that the micronutrient ascorbic acid can modulate the functional genome, T cells (CCRF-HSB2) were treated with ascorbic acid (up to 150 microM) for up to 24 h. Protein expression changes were assessed by two-dimensional electrophoresis. Forty-one protein spots which showed greater than two-fold expression changes were subject to identification by matrix-assisted laser desorption ionisation time of flight MS. The confirmed protein identifications were clustered into five groups; proteins were associated with signalling, carbohydrate metabolism, apoptosis, transcription and immune function. The increased expression of phosphatidylinositol transfer protein (promotes intracellular signalling) within 5 min of ascorbic acid treatment was confirmed by Western blotting. Together, these observations suggest that ascorbic acid modulates the T cell proteome in a time- and dose-dependent manner and identify molecular targets for study following antioxidant supplementation in vivo.