8Number of E3s per genome Saccharomyces cereviseae 68 Caenorhabditis elegansDrosophila melanogasterHomo sapiensArabidopsis thaliana686571895271156
9Advantages of proteolysis control in signal transduction Fast response to a change in signal intensity:direct control of protein activities in contrast to transcriptional regulation that involves transcription, transcript processing and translation steps before protein abundance is increased.2) Proteolysis control can rapidly increase as well as decrease a proteins activity (only an increase is possible with transcriptional regulation).3) Accurate reflection of signal intensity in response output: secundary modifications such as phosporylation/dephosphorylation can also directly change a proteins activity. However since such controls tend to be leaky, i.e. are the result of modification/demodification equilibria, their outcome depends on the initial abundance of the target protein.
10Why more E3s in plants?* More E3s means more proteolysis control of signaling.* Energetically wasteful?* Animals can side-step adverse environmental conditions.The sessile plant must endure.Plants need to be more sensitive to environmental changes.* Proteolysis control of signaling allows for quick responses to changes in signal intensities (changes in environmental conditions).* Proteolysis control also allows for an accurate response-strength to signal-intensity ratio.* Allows for a constant state of readiness.* Plants are less energy-limited.From Kepinski and Leyser, 2003
11Proteolysis control of signaling Signal transduction leads to destabilization of a repressor of the response or stabilization of a response activator.This is accomplished via secundary modification (phosphorylation or dephosphorylation) of the target protein that leads to or prevents its detection by a Ubiquitin ligase (E3). Alternatively, signaling directly controls E3 affinity for the target protein.Controlling the activity of a protein via its degradation rate allows for faster and more accurate responses to changing concentrations/intensities of the signal (changing environment).
12The Ub/26SP pathway and signaling Describe two mechanisms that can be used to transform a signal into a response via the regulated degradation of a repressor of this response. Show how increased signal intensity leads to an increased response output.
13The Ub/26SP pathway and signaling Describe two mechanisms that can be used to transform a signal into a response via the regulated degradation of an activator of this response. Show how increased signal intensity leads to an increased response output.
14* Signal (variable) DNA RNA Response repressor Response repressor E3 R Control of gene expression via conditional proteolysisEXAMPLE 1:Signal (variable)*DNARNAResponse repressorResponse repressorConstitutive expressionE3RespnroResponse (variable)
15* Signal (variable) DNA RNA Response activator Response activator R e Control of gene expression via conditional proteolysisEXAMPLE 2:Signal (variable)*DNARNAResponse activatorResponse activatorConstitutive expressionRespnactivroE3Response (variable)
17Signal (variable) E3 R e s p n a c t i v r o DNA RNA Control of gene expression via conditional proteolysisEXAMPLE 3:Signal (variable)E3RespnactivroDNARNAResponse activatorConstitutive expressionResponse (variable)
18Light responses (variable) Control of gene expression via conditional proteolysisEXAMPLE 3: PhotomorphogenesisLight (variable)COP1RespnactivroDNARNAHY5Constitutive expressionLight responses (variable)
19COP1 acts as an E3 to target HY5 for degradation RINGCoilWD-40 repeatsCOP1E2*UbbZIPHY5HY5UbUbE2UbUbDegradation via26S ProteasomeDegradation by the 26S proteasomeUbE1Ub(Osterlund et al.,2000)
21Signal (variable) E3 R e s p n r o DNA RNA Response repressor Control of gene expression via conditional proteolysisEXAMPLE 4:Signal (variable)E3RespnroDNARNAResponse repressorConstitutive expressionResponse (variable)
22Auxin Response (variable) Control of gene expression via conditional proteolysisEXAMPLE 4: Auxin response pathwayAuxin (variable)TIR1RespnroDNARNAAUX/IAA factorsConstitutive expressionAuxin Response (variable)
25Summary: important to remember How does a target protein become polyubiquitinated through the sequential action of E1, E2 and E3 enzymes?26S Proteasome: structure/function. How does the proteasome detect and then degrade target proteins?Where in the cell does the Ubiquitin/26S Proteasome pathway act?ATP requiring steps in the pathway? Energy is needed to establish specific proteolysis (as opposite to non-specific).Predict the effects of loss of function of different components of the pathway (proteasome --- pleiotropic; E3 --- highly specific phenotype).Why proteolysis control of signal transduction (what are the advantages)?Possible mechanisms of conditional protein degradation to control signal/response ratios (see examples 1-4).