Control of mitochondrial gene expressionControl of mitochondrial gene expression

Nuclear encoded mitochondrial gene expression

Mitochondrial encoded genes

Must be coordinated

All of the enzymes required for DNA replicationAnd transcription are encoded in nucleus

Retrograde regulation - General term for mitochondrial signalling

Defined as cellular responses to changes in functionalstate of mitochondria

Anterograde regulation Defined as transfer of information and materialfrom the nucleus (via cytoplasm) to mitochondria

Aerobic metabolism:(oxidative metabolism)

Glycolysis TCA cycle oxidative phosphorylation

Anaerobic metabolism:(fermentation)

Glucose Acetyl CoA Ethanol

Diauxic shift:

Metabolic change as fermentable carbon source is used up from…

Glucose Fermentative (Glycolysis Ethanol)

Oxidative Metabolism(Ethanol TCA cycle)

to…

yeast carbon metabolism

NADH + CO2

One pyruvate molecule is completely oxidised to CO2

4-Carbons

3-Carbons

CO2

6-Carbons

NADH + CO2

NADH

FADH

Outline of Tricarboxylic Acid Cycle

The NADH and FADH produced are oxidised by the respiratory electron transport chain

metabolic remodelling

Metabolic pathways were altered in respiratory deficient cellsIntermediates of TCA cycle needed for synthesis of amino acids and nucleotides

Metabolic pathways altered:

OAA and Acetyl-CoA supply

Acetyl-CoA hydrolase

in enzymes involved in fluxand conversion of metabolites madeby fatty acid oxidation to TCA and glyoxylate cycle intermediates.

in nutrient and metabolite transporters.

in enzymes for reoxidation of NADH

reconfigure metabolism byrecruiting peroxisomal activities,small molecule transport systems and lipid, sugar and amino acid turnover to get more OAA and Acetyl-CoA.

CIT2 gene expression upregulated 50 fold

Positive regulators =Rtg1, Rtg2 and Rtg3

Negative regulators = Mks1, Lst8p, Bmh1p, Bmh2p

Rtg1 an Rtg3 - basic helix-loop-helix-leucine zipper (bhlh-Zip) transcription factors thatheterodimerise and activate transcription of genes that contain an R Box -GTCAC

Rtg2 acts upstream of Rtg1 and 3sensor of mitochondrial dysfunctiontransducer of signals

Off state - Rtg1 and 3 sequestered in cytoplasm- Rtg 3 phosphorylated at multiple sites

On state - Rtg3 becomes partially dephosphorylayed- enters nucleus (with Rtg1) to activate transcription of genes with an R box

Thus it is the location that controls activity

Note in mutants lacking Rtg2 the retrograde response is completely lost indicating that Rtg2 plays a crucialrole in activating Rtg1 and 3

Regulators of Rtg2

Genetic screens to identify

Mks1p - negative regulator- Absence means CIT2 expressionconstitutively high- Phosphoprotein

Retrograde pathway on - Mks1p is largely dephosphorylatedIn a complex with Rtg2.

Retrograde pathway off - Mks1p is highly phosphoryalyedand not bound to Rtg2, but to 14-3-3- proteins Bmh1p and Bmh2p - as yet not known how this complex negatively regulates

Lst8p - negative regulator of retrograde response

- Acts at two pointsupstream of Rtg2downstream of Rtg2

Shown by the fact two classes of mutants exists for Lst8p - class I cannot bypass need for Rtg2 = upstream-class II - no need for Rtg2 = downstream

TOR signalling in yeast (target for rapamycin)growth controlnutrient signalling

RTG pathway activated when TOR signalling is inactivated

Lst8p is an integral component of Tor1 and 2 complexesActs as a positive regulator in these complexes

Details not yet clear on interaction of TOR and RTG pathway but thought to be indirect as RTG response still takes place if TOR pathway active

Retrograde response in mammalian cells