Essential for photosystem II (PSII) biogenesis; required for assembly of an early intermediate in PSII assembly that includes D2 (psbD) and cytochrome b559. 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.