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Proctor2007 - Age related decline of proteolysis, ubiquitin-proteome system

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Short description

Model of the Ubiquitin-Proteasome System

Description of the Model

This is a stochastic model of the ubiquitin-proteasome system for a generic pool of native proteins (NatP) which have a half-life of about 10 hours under normal conditions. It is assumed that these proteins are only degraded after they have lost their native structure due to a damage event. This is represented in the model by the misfolding reaction which depends on the level of reactive oxygen species (ROS) in the cell. Misfolded proteins (MisP) are first bound by an E3 ubiquitin ligase. Ubiquitin (Ub) is activated by E1 (ubiquitin-activating enzyme) and then passed to E2 (ubiquitin-conjugating enzyme). The E2 enzyme then passes the ubiquitin molecule to the E3/MisP complex with the net effect that the misfolded protein is monoubiquitinated and both E2 and E3 are released. Further ubiquitin molecules are added in a step-wise manner. When the chain of ubiquitin molecules is of length 4 or more, the polyubiquitinated misfolded protein may bind to the proteasome. The model also includes de-ubiquitinating enzymes (DUB) which cleave ubiquitin molecules from the chain in a step-wise manner. They work on chains attached to misfolded proteins both unbound and bound to the proteasomes. Misfolded proteins bound to the proteasome may be degraded releasing ubiquitin. Misfolded proteins including ubiquitinated proteins may also aggregate. Aggregates (AggP) may be sequestered (Seq_AggP) which takes them out of harm's way or they may bind to the proteasome (AggP_Proteasome). Proteasomes bound by aggregates are no longer available for protein degradation.

Related Publication
  • An in silico model of the ubiquitin-proteasome system that incorporates normal homeostasis and age-related decline.
  • Proctor CJ, Tsirigotis M, Gray DA
  • BMC systems biology , 3/ 2007 , Volume 1 , pages: 17
  • School of Clinical and Medical Sciences-Gerontology, Newcastle University, UK.
  • The ubiquitin-proteasome system is responsible for homeostatic degradation of intact protein substrates as well as the elimination of damaged or misfolded proteins that might otherwise aggregate. During ageing there is a decline in proteasome activity and an increase in aggregated proteins. Many neurodegenerative diseases are characterised by the presence of distinctive ubiquitin-positive inclusion bodies in affected regions of the brain. These inclusions consist of insoluble, unfolded, ubiquitinated polypeptides that fail to be targeted and degraded by the proteasome. We are using a systems biology approach to try and determine the primary event in the decline in proteolytic capacity with age and whether there is in fact a vicious cycle of inhibition, with accumulating aggregates further inhibiting proteolysis, prompting accumulation of aggregates and so on. A stochastic model of the ubiquitin-proteasome system has been developed using the Systems Biology Mark-up Language (SBML). Simulations are carried out on the BASIS (Biology of Ageing e-Science Integration and Simulation) system and the model output is compared to experimental data wherein levels of ubiquitin and ubiquitinated substrates are monitored in cultured cells under various conditions. The model can be used to predict the effects of different experimental procedures such as inhibition of the proteasome or shutting down the enzyme cascade responsible for ubiquitin conjugation.The model output shows good agreement with experimental data under a number of different conditions. However, our model predicts that monomeric ubiquitin pools are always depleted under conditions of proteasome inhibition, whereas experimental data show that monomeric pools were depleted in IMR-90 cells but not in ts20 cells, suggesting that cell lines vary in their ability to replenish ubiquitin pools and there is the need to incorporate ubiquitin turnover into the model. Sensitivity analysis of the model revealed which parameters have an important effect on protein turnover and aggregation kinetics.We have developed a model of the ubiquitin-proteasome system using an iterative approach of model building and validation against experimental data. Using SBML to encode the model ensures that it can be easily modified and extended as more data become available. Important aspects to be included in subsequent models are details of ubiquitin turnover, models of autophagy, the inclusion of a pool of short-lived proteins and further details of the aggregation process.
Carole Proctor

Metadata information

Curation status
  • Model originally submitted by : Carole Proctor
  • Submitted: Apr 2, 2007 9:45:08 PM
  • Last Modified: Aug 11, 2014 3:38:13 PM
  • Version: 2 public model Download this version
    • Submitted on: Aug 11, 2014 3:38:13 PM
    • Submitted by: Carole Proctor
    • With comment: Current version of Proctor2007 - Age related decline of proteolysis, ubiquitin-proteome system
  • Version: 1 public model Download this version
    • Submitted on: Apr 2, 2007 9:45:08 PM
    • Submitted by: Carole Proctor
    • With comment: Original import of BIOMD0000000105.xml.origin
Curator's comment:
(added: 02 Apr 2007, 19:58:29, updated: 02 Apr 2007, 19:58:29)
Figure3 (i) (iii) (v) have been reproduced by Gillespie2.