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).
Jiyao W et al.(2023). "The conserved domain database in 2023.",