 In addition, the β-turns and loops in the strands contribute to the flexibility of the barrel.In particular, PPIB is a 21 kDa protein which contains a C-terminal ER retention motif that directs the protein to the ER organelle, while its N-terminal extension attaches it to its substrates.PPIB is a member of the peptidyl-prolyl cis-trans isomerase (PPIase) family.