Variable subunit of the ferredoxin-thioredoxin reductase (FTR), which catalyzes the two-electron reduction of thioredoxins by the electrons provided by reduced ferredoxin. Heterodimer of subunit A (variable subunit) and subunit B (catalytic subunit). Heterodimeric FTR forms a complex with ferredoxin and thioredoxin. Belongs to the ferredoxin thioredoxin reductase alpha subunit family.