The monomeric IPO from residues 1 to 154 shows a typical bprism fold found in the JRL family, with 12 b-sheets (b3-b14) and 2 additional short, extended, N-terminal b-strands (b1-b2) (Figure 2A and 2C) . Each b-prism fold comprises 3 Greek-key motifs forming 3 planes by 3 four-stranded b-sheets: plane 1 by b3 to b4 and b13 to b14; plane 2 by b5 to b8; plane 3 by b9 to b12.