Nucleus-Encoded Protein BFA1 Promotes Efficient Assembly of the Chloroplast ATP Synthase Coupling Factor 1

F-type ATP synthases produce nearly all of the ATP found in cells. The catalytic module F 1 commonly comprises an α 3 β 3 hexamer surrounding a γ/ε stalk. However, it is unclear how these subunits assemble to form a catalytic motor. In this work, we identified and characterized a chloroplast protein that interacts with the CF 1 β, γ, and ε subunits of the chloroplast ATP synthase and is required for assembly of its F 1 module. We named this protein BIOGENESIS FACTOR REQUIRED FOR ATP SYNTHASE1 (BFA1) and determined its crystal structure at 2.8-Å resolution. BFA1 is comprised primarily of two interacting β-barrels that are oriented nearly perpendicularly to each other. The contact region between BFA1 and the CF 1 β and γ subunits was further mapped by yeast two-hybrid assays. An in silico molecular docking analysis was performed and revealed close fitting contact sites without steric conflicts between BFA1 and CF 1 β/γ. We propose that BFA1 acts mainly as a scaffold protein promoting the association of a CF 1 α/β heterodimer with CF 1 γ. The subsequent assembly of other CF 1 α/β heterodimers may shift the position of the CF 1 γ subunit to complete assembly of the CF 1 module. This CF 1 assembly process is likely to be valid for other F-type ATP synthases, as their structures are highly conserved.