Essential for photosystem II (PSII) biogenesis; required for assembly of an early intermediate in PSII assembly that includes D2 (psbD) and cytochrome b559. Restricted to the stromal lamelae. Translocation into the thylakoid lumen occurs via the Tat pathway (By similarity). The position of the signal peptide cleavage has not been experimentally proven (By similarity). A 7-bladed beta-propeller torus, about 54 by 55 Angstroms, with a depth of about 25 Angstroms and a central pore. Belongs to the Ycf48 family.