A factor required for optimal assembly of photosystem II (PSII), acting in the early stages of PSII assembly. Also plays a role in replacement of photodamaged D1 (psbA). Assists YidC in synthesis of chlorophyll-binding proteins. Interacts with the D1 protein (crystallized with PsbA1 or PsbA3), via the latter's C-terminal prepropeptide, may interact with parts of the mature D1 protein as well. Associated with a PSII precusor complex on the lumenal side of the thylakoid membrane. A 7-bladed beta-propeller torus, about 54 by 46 Angstroms, with a depth of about 25 Angstroms and a central pore. Belongs to the Ycf48 family.