The recombinant AtPDIL5-1, a putative single domain PDI-like protein from Arabidopsis thaliana, binds an Fe-S cluster.

Protein disulfide isomerases (PDI) assist the in vivo protein folding by helping newly translated polypeptide chains to form native disulfide bonds. PDI structure consists in two tandem repeats of thioredoxin domains, a-b-b'-a', in which a and a' domains are catalytically active. This enzyme is the founding member of a family of proteins that vary in length and domain arrangement, but share the common structural feature of having at least one domain with a thioredoxin-like fold. In A. thaliana there are at least 13 putative PDI-like enzymes (PDIL) and AtPDIL5-1 is the only one displaying a single domain architecture (1). The human counterpart (HsERp18) has been biochemical characterized as an ER-resident disulfide oxidase (2). Our results indicate that rAtPDIL5-1 behaves differently possessing only a marginal in vitro PDIL activity. Moreover its predicted catalytic pocket presents an additional cysteine three amino acids before the putative catalytic couple and this triad is conserved in all plants single domain PDIL (3). The amino acid stretch (CxxxCxxC) is the signature of enzymes belonging to the SAM radical superfamily where it is involved in the binding of an 4Fe-4S cluster (4). Spectroscopic analysis of the anaerobically purified complexes, indicates that rAtPDIL5-1 is able to bind an Fe-S cluster when expressed in Escherichia coli. Since preliminary data on subcellular localization strongly suggest that AtPDIL5-1 is an ER resident protein and since no Fe-S cluster assembly machinery has been described for this compartment, our findings are particularly intriguing. (1) Ondzighi C., et al.; Plant Cell (2008), 20(8) 2205-20. (2) Alanen H.I., et al., J. Biol. Chem. (2003), 278(31) 28912-20. (3) Selles B., et al., Genomics (2011), 97, 37-50. (4) Frei P.A., et al., Crit. Rev. Biochem. Mol. Biol. (2008), 43(1) 63-88.