PsbP does not require LHCII to bind the PSII core

Photosystem II of higher plants and green algae contains three extrinsic subunits PsbO, PsbP and PsbQ located on the lumenal surface where they stabilize the Mn-cluster of the oxygen evolving complex (OEC) and facilitate its catalytic activity. To isolate active PSII complexes from higher plants several methods are available, which include sucrose density centrifugation and column chromatography. However, when isolating pure PSII cores, depleted of light-harvesting antennae (LHCII), a total loss of PsbP and PsbQ usually occurs. By direct solubilization of stacked thylakoids with low concentration of n-dodecyl-beta-D-maltoside in combination with sucrose density gradient centrifugation containing glycine-betaine we isolated PSII cores completely depleted of LHCII, but retaining an almost full complement of PsbO and significant amount of PsbP (30%). Along the sucrose gradient PsbQ was completely lost in the first fractions, while PsbO was present mainly in fractions containing PSII, indicating respectively a weak binding to the PSII core of the former and a strong binding of the latter. Although some PsbP was partially lost along the gradient, this OEC protein was present in appreciable amount in PSII containing fractions, ruling out the possibility of its nonspecific co-migration with PSII and confirming that LHCII proteins are not a strict requirement for its assembly to the PSII core.