Four IPO protomers form a compact tetrameric association by swapping their extended N termini from residues 1 to 10. We analyzed the tetrameric association of IPO-Me-Glc. As shown in Figure 2B , the 2 extended N termini from monomer A in blue and monomer B in purple swap with each other. The interacting interface between the four IPO protomers is formed by the extended N termini. Consequently, a larger buried interface between monomers A and B is 1,522 Å 2 . The residues located at the interface are 2-10, 12, 15-30, 59-67, 91-92, 98, 121, 134, 137, 139-140, 146, 150, and 152 in monomer A (as shown in red box in Figure 2C ). In total, 13 hydrogen bonds are formed by the residues Leu5, His8, Asn19, Gln22, Ser25, Arg27, Asp60, Ile61, Thr63, Thr121, Asn139 and Tyr150 in the interface between monomers A and B. The buried interface between monomer C and monomer D is 1,554 Å 2 . Furthermore, the buried interface between monomers A and C is 755 Å 2 , which is mainly contributed by the interacting residues of N-terminal residues 4 to 17 and C-terminal residues 91, 121-126, 128, and 151. In addition, the interface between monomers D and B is 731 Å 2 .