Alternative Oxidase Isoforms Are Differentially Activated by Tricarboxylic Acid Cycle Intermediates

Area of Science:

• Plant molecular biology
• Mitochondrial respiration
• Enzyme regulation

Background:

• The cyanide-insensitive alternative oxidase (AOX) is crucial for plant respiration, involved in oxygen reduction and regulated by redox and 2-oxo acids.
• Arabidopsis thaliana has five AOX isoforms (AOX1A-D, AOX2), with AOX1D expression increasing in aox1a mutants but failing to fully compensate, suggesting isoform-specific regulation.
• Understanding these regulatory differences is key to elucidating AOX functional specialization.

Purpose of the Study:

• To investigate the differential posttranslational regulation of Arabidopsis AOX isoenzymes.
• To identify specific tricarboxylic acid cycle intermediates that modulate AOX1A, AOX1C, and AOX1D activity.
• To determine if structural modifications can alter AOX isoform regulatory properties.

Main Methods:

• Utilized a refined in vitro system to test the activity of purified AOX1A, AOX1C, and AOX1D proteins.
• Assessed the impact of seven tricarboxylic acid cycle intermediates (citrate, isocitrate, 2-oxoglutarate, succinate, fumarate, malate, oxaloacetate) on enzyme activity.
• Investigated the effect of substituting variable cysteine residues at position III on AOX isoform regulation.

Main Results:

• AOX1C activity was insensitive to all tested organic acids.
• AOX1A and AOX1D were both activated by 2-oxoglutarate; AOX1A showed additional activation by oxaloacetate.
• Substitution of cysteine residues at position III did not enable AOX isoforms to mimic each other's regulatory responses.

Conclusions:

• Arabidopsis AOX isoforms exhibit differential fine-tuning by tricarboxylic acid cycle metabolites, primarily influenced by their amino-terminal regions.
• These distinct regulatory mechanisms, particularly concerning 2-oxo acid interactions, underpin the functional specialization of AOX isoenzymes.
• The observed regulatory differences explain the inability of AOX isoforms to functionally compensate for one another in vivo.