Structural comparison of oxidized and reduced FKBP13 from Arabidopsis thaliana

Abstract

AtFKBP13, an immunophilin in the chloroplast thylakoid lumen, participates in redox-regulatory processes via a pair of conserved disulfide bonds that are present at the N- and C-termini of the protein. Characterization of this protein by structural and biochemical analysis has revealed a novel mechanism of redox regulation in the thylakoid lumen. The protein is active in its oxidized form but is inactivated after reduction by the thioredoxin system. This is in sharp contrast with the regulation of biosynthetic enzymes in the stroma of the chloroplast, where reduction of enzymes by thioredoxin activates their function. To understand how the reduced form of AtFKBP13 is stabilized and how reduction of the cysteine residues affects the molecular properties of the enzyme, we determined the crystal structure of reduced AtFKBP13 at 1.88 A. Comparison of the reduced structure and the oxidized form that we published earlier shows rearrangements in redox site regions, readjustments of hydrogen-bonding interactions and the secondary structure of the active site residues 50-53, and reduced accessibility of the catalytic residues involved in the peptidyl proline isomerase (PPIase) activity of this enzyme. We propose that redox-linked changes in the secondary structure of the PPIase domain are responsible for significant functional differences in this protein in the reduced and oxidized states.

Plant immunophilins: a review of their structure-function relationship

“…2A; PDB: 1U79; [27]), hereafter called AtFKBP13-S2. The disulfide bonds in the protein play a major role in regulating its own enzymatic activity by their redox states [90]. Under normal conditions, the protein remains in the oxidized state and displays PPIase activity.…”

The FKBP families of higher plants: Exploring the structures and functions of protein interaction specialists

“…Knockdown using RNA interference showed that AtFKBP16-2 was essential for assembly of functional NDH and for formation of the NDH-PSI supercomplex that carries out CEF [65]. Notably, FKBP13 and FKBP16-2 in higher plants are homologous duplicates possessing cysteine pairs at identical positions [7,8], which in AtFKBP13 form disulphide bonds under oxidising conditions that stabilise the active site and allow substrate-binding [66,67]. It is therefore likely that the substrate-binding activities of FKBP13 and probably FKBP16-2 are regulated by thylakoid redox signals, suggesting that these FKBPs may link redox with assembly and/ or activity of NDH and Cyt b 6 f, both of which are linked to CEF [65,68,69].…”

Crystal Structure of Arabidopsis Cyclophilin38 Reveals a Previously Uncharacterized Immunophilin Fold and a Possible Autoinhibitory Mechanism

“…The immunophilin with the highest PPIase activity in the thylakoid lumen is FKBP13, whose crystal structure has already been reported (Gopalan et al, 2004(Gopalan et al, , 2006, followed by CYP20-2. The spinach (Spinacia oleracea) homolog thylakoid lumen protein40 (TLP40) (82% sequence identity to At-CYP38) is known to be an active thylakoid lumenal PPIase with enzymatic activity observed in vitro (Fulgosi et al, 1998).…”