Involved in the assembly of the F(1) ATP synthase in chloroplast thylakoid membranes (PubMed:25775508). Functions downstream of the CPN60 chaperones to promote assembly of the catalytically active core of the chloroplast ATP synthase (PubMed:25775508). Assists the assembly of the ATP synthase gamma subunit into the active F(1) core downstream of CPN60-mediated folding, which is critical for the biogenesis of the chloroplast ATP synthase (PubMed:25775508). Interacts with ATPC1. Severe growth defects, and inability to flower and produce seeds.