We solved the crystal structure of the periplasmic N-terminal domain of GspD (blue) from the bacterial type 2 secretion system secretin in complex with a Xaperone (red). The prime function of the Xaperone in promoting crystal growth is probably formation of a compact heterotetramer. Peri-GspD in the tetramer is more rigid than peri-GspD by itself, given the potentially flexible linker between the N1 and N2 subdomains.

Korotkov KV, Pardon E, Steyaert J & Hol WG (2009) Crystal structure of the N-terminal domain of the secretin GspD from ETEC determined with the assistance of a nanobody. Structure 17, 255-265.