Heterogeneity within animal thioredoxin reductases. Evidence for alternative first exon splicing

Area of Science:

• Biochemistry
• Molecular Biology
• Genetics

Background:

• Animal thioredoxin reductases (TRs) are crucial selenocysteine-containing flavoenzymes involved in redox homeostasis.
• Mammalian TRs, including TR1 (cytosolic) and TR3 (mitochondrial), were previously characterized as homodimers of 55-57 kDa subunits.

Purpose of the Study:

• To investigate the heterogeneity of animal thioredoxin reductases (TRs).
• To identify novel forms and isoforms of TR1 and TR3.
• To elucidate the genetic mechanisms underlying TR heterogeneity.

Main Methods:

• Electrophoretic, immunoblot, and mass spectrometry analyses of TR1.
• Homology analyses of TR1 cDNA and mRNA.
• Comparative analysis of TR gene structures and expression in mouse, rat, human, and Drosophila.

Main Results:

• TR1 from various sources showed heterogeneity, including a 67-kDa band.
• A novel mouse TR1 cDNA encoding a 67-kDa selenoprotein subunit was identified.
• Multiple mRNA isoforms of TR1, TR3, and Drosophila TR resulted from alternative first exon splicing, leading to predicted mitochondrial and cytosolic proteins.
• Human TR3 gene overlaps with the COMT gene, involving shared first exon sequences.

Conclusions:

• Animal TRs exhibit remarkable heterogeneity, largely due to alternative first exon splicing, an evolutionarily conserved mechanism.
• This splicing mechanism contributes to the complex regulation of TR expression and their specific localization in organelles and cell types.