Amino acid sequence of spinach ferredoxin:thioredoxin reductase variable subunit.

Ferredoxin:thioredoxin reductase (FTR) is an iron-sulfur protein, which, in the presence of ferredoxin and thioredoxin, catalyses the light-dependent activation of several photosynthetic enzymes. Spinach FTR consists of two dissimilar polypeptide chains, A and B, present in equal amounts. Whereas subunit B seems to be responsible for the catalytic activity, subunit A has no known catalytic function. We found earlier that the N-terminus of subunit A, also called the variable subunit, shows terminal redundancy and that 2-3 of its serine residues are phosphorylated [Tsugita, A. Yano, K., Gardet-Salvi, L. & Schürmann, P. (1991) Protein Sequence Data Anal. 4, 9-13]. We now report the complete amino acid sequence of subunit A, determined by conventional protein sequencing methods. The polypeptide chain with a calculated molecular mass of 12,669 Da consists of 112 amino acids and has a calculated isoelectric point of 5.4. The analysis of the sequence supports the idea that this subunit has no catalytic function. The comparison with a known cyanobacterial FTR reveals about 58% similarity and the striking presence of a N-terminal extension in the spinach protein. This extension may be responsible for the reported size variability of this subunit.