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. Part of early PSII assembly complexes which includes D1 (psbA) and PsbI; not found in mature PSII. Binds to the lumenal side of PSII complexes. Interacts with YidC. Associated with a PSII precusor complex on the lumenal side of the thylakoid membrane. A 7-bladed beta-propeller torus, about 55 by 55 Angstroms, with a depth of about 25 Angstroms and a central pore. Belongs to the Ycf48 family.