Hexameric structure of the ATPase motor subunit of magnesium chelatase in chlorophyll biosynthesis.

Magnesium chelatase (MgCh) is a heterotrimeric enzyme complex, composed of two AAA+ family subunits that can assembly into a double ring structure and a large catalytic subunit. The small AAA+ subunit has ATPase activity and can self-oligomerize into a ring structure, while the other AAA+ subunit lacks independent ATPase activity. Previous structural studies of the ATPase motor subunit of MgCh from a bacteriochlorophyll-synthesizing bacterium have identified a unique ATPase clade, but the model of oligomeric assembly is unclear. Here we present the hexameric structure of the MgCh ATPase motor subunit from the chlorophyll-synthesizing cyanobacterium Synechocystis sp. PCC 6803. This structure reveals details of how the hexameric ring is assembled, and thus provides a basis for further studying the heterotrimeric complex.