The interface areas between Xaperones and their antigens are ranging from 600 to 900 Ų, very similar to the contact area of the interfaces of protein-protein interactions. It follows that Xaperones are suitable to stabilize the protomers of larger protein assemblies in one-to-one heterodimers.

We proved this principle by solving the structures of EpsI (red) and EpsJ (blue). Using a Xaperone (green) as a crystallization aid, the EpsI:EpsJ pseudopilin heterodimer (two components of the bacterial type 2 secretion system) was crystallized in 15 days compared with 11 months and 17 variants required for crystallization without Xaperone. 

Lam AY, Pardon E, Korotkov KV, Hol WG & Steyaert J (2009) Nanobody- aided structure determination of the EpsI:EpsJ pseudopilin heterodimer from Vibrio vulnificus. J Struct Biol 166, 8-15. 

In trypanosomes, many key RNA editing steps occur in 20S editosomes, which have a core of 12 proteins. Among these, the ‘‘interaction protein” KREPA6 (blue) performs a central role in maintaining the integrity of the editosome core. The use of Xaperones accelerated crystal growth of KREPA6 from Trypanosoma brucei dramatically. All three structures obtained are heterotetramers with a KREPA6 dimer in the center, and one Xaperone (red) bound to each KREPA6 subunit.

Wu M, Park Y-J, Pardon E, Turley S, Hayhurst A, Deng J, Steyaert J & Hol WGJ (2010) Structures of a key interaction protein from the Trypanosoma brucei editosome in complex with single domain antibodies. J. Struct. Biol., in press.