Crystal structure of the Sec18p N-terminal domain

Yeast Sec18p and its mammalian orthologue N-ethylmaleimide-sensitive fusion protein (NSF) are hexameric ATPases with a central role in vesicle traffic king. Aided by soluble adapter factors (SNAPs), Sec18p/NSF induces ATP-depe ndent disassembly of a complex of integral membrane proteins from the vesic le and target membranes (SNAP receptors), During the ATP hydrolysis cycle, the Sec18p/NSF homohexamer undergoes a large-scale conformational change in volving repositioning of the most N terminal of the three domains of each p rotomer, a domain that is required for SNAP-mediated interaction with SNAP receptors, Whether an internal conformational change in the N-terminal doma ins accompanies their reorientation with respect to the rest of the hexamer remains to be addressed. We have determined the structure of the N-termina l domain from Sec18p by x-ray crystallography. The Sec18p N-terminal domain consists of two beta-sheet-rich subdomains connected by a short linker. A conserved basic cleft opposite the linker may constitute a SNAP-binding sit e. Despite structural variability in the linker region and in an adjacent l oop, all three independent molecules in the crystal asymmetric unit have th e identical subdomain interface, supporting the notion that this interface is a preferred packing arrangement. However, the linker flexibility allows for the possibility that other subdomain orientations may be sampled.