Separation and characterization of stroma and grana membranes — evidence for heterogeneity in antenna size of both Photosystem I and Photosystem II

A rapid procedure to fractionate the thylakoid membrane into two well-separated vesicle populations, one originating from the grana and the other from the stroma-membrane region, has been developed. This was achieved by sonication of thylakoids present in an aqueous two-phase system followed by partitioning either by countercurrent distribution or by a batch procedure in three steps. The membrane populations were analysed according to their composition and photochemical activities. The grana membranes comprise, on chlorophyll basis, about 60% of the thylakoid material and are enriched in PS II, but also contain some PS I, while the stroma membranes comprise about 40% and are enriched in PS I, but also contain some PS II. Cytochrome f was slightly enriched in the grana-derived vesicle fraction. The properties of both PS I and PS II differ between the two populations. The PS I of the grana fraction (PS Iα) reached half-saturation at about half the light intensity of the PS I in the stroma-membrane fraction (PS Iβ). The rate of P-700 photooxidation under low light illumination was higher for PS Iα than for PS Iβ (30% larger rate constant), showing that PS Iα has a larger antenna. The PS II of the grana fraction (PS IIα) reached half-saturation at half the light intensity compared to the PS II of the stroma-membrane fraction (PS IIβ). The results show that the grana-derived membranes contain PS Iα and PS IIα which have larger functional antenna sizes than the corresponding PS Iβ and PS IIβ of the stroma membranes. The results suggest that the photosystems of the grana are designed to allow effective electron transport both at low and high light intensities, while the stroma-membrane photosystems mainly work at high light intensities as a supplement to the grana systems.