This is the C-terminal domain (CTD) of Methanocaldococcus jannaschii peptidyl-prolyl cis/trans isomerase FKBP26. FKBP26 mediates protein folding. CTD has an alpha/beta-sandwich structure composed of three alpha-helices and a three-stranded, mixed-orientation beta-sheet. The CTD domain is responsible for dimerization of FKBP26 by through inter-subunit antiparallel pairing of the third beta-strands of the two CTDs. The CTD dimer forms a continuous, six-stranded mixed beta-sheet. A CTD-like structure is also found in the phylogenetically conserved NifU proteins (HIRIP5 in mouse and humans) [1].

1: Structural analysis of protein folding by the long-chain archaeal chaperone FKBP26. Martinez-Hackert E, Hendrickson WA; J Mol Biol. 2011;407:450-464. PMID:21262232