A factor required for optimal assembly of photosystem II (PSII) which acts in the early stages of PSII assembly. Also plays a role in replacement of photodamaged D1 (psbA) (PubMed:18550538). May interact with precursor D1 to prevent its premature processing before association with D2 (psbD) (PubMed:18550538) (Probable). May also play a role in chlorophyll insertion into chlorophyll-binding proteins (PubMed:18550538) (Probable). Increasing levels of chlorophyll precursors partially suppresses deletion of this protein, supporting the idea that Ycf48 assists YidC in synthesis of chlorophyll-binding proteins (PubMed:30061392). Part of early PSII assembly complexes which includes D1 (psbA) and PsbI; not found in mature PSII. By two-hybrid analysis in yeast interacts with precursor and intermediate forms of D1, but less with mature D1 (PubMed:18550538). Binds to the lumenal side of PSI and PSII complexes. Coimmunoprecipitates with YidC (PubMed:30061392). Associated with a PSII precusor complex on the lumenal side of the thylakoid membrane. Eukaryotic orthologs often have inserts compared to cyanobacteria; insertion of a 19-residue sequence from C.merolae into the same position in this protein (at Val-176) has no visible effect on function. An Arg-rich patch composed of 4 residues on the 'top surface' of blades 4 and 5 is important for binding to assembling PSII centers; mutating each Arg residue individually has no phenotype whereas none of the triple mutants grow in high light (90-130 umol photon/m(2)/s), suggesting more than one residue is involved in binding to the lumenal side of assembling PSII centers. A 7-bladed beta-propeller torus, about 55 by 55 Angstroms, with a depth of about 25 Angstroms and a central pore. The last 3 residues are removed in the mature protein. Modified with diacylglycerol and acylation on Cys-29, with last 3 residues removed. Single fatty acid on Cys-29, with last 3 residues removed. Grows significantly slower than wild-type, 30-50% reduction in functional PSII, decreased chlorophyll content, significantly reduced replacement of photodamaged D1 (psbA) (PubMed:18550538). Grows poorly on low light (40-50 umol photon/m(2)/s) and not on higher light, reduced chlorophyll, increased levels of unassembled D1, substantially reduced levels of chlorophyll precursors (PubMed:30061392). Belongs to the Ycf48 family.