Promiscuous interactions of human septins: the GTP binding domain of SEPT7 forms filaments within the crystal

We describe the purification, crystallization and structure for the GTP-binding domain of human septin 7 (SEPT7G). We show that it forms filaments within the crystal lattice which employ both the G and NC interfaces, similar to those seen in the hetero-filament of SEPT2/6/7. The NC interface is considered promiscuous as it is absent from the hetero-filament. Such promiscuity could provide the potential for permuting monomers along a filament in order to generate diversity in hetero-polymers. On the other hand, our results suggest that the G and NC interfaces may be necessary but insufficient for determining correct hetero-filament assembly.