Phosphorylation of chloroplast membrane polypeptides

ILLUMINATION of chloroplast thylakoids leads to the formation of the so-called high energy state of the membrane1–3. The establishment of this state is accompanied by several structural changes within the membrane, including a conformational change in the coupling factor4, increased accessibility of photosystem II to the chemical probe p-diazonium benzene sulphonate5, and a reduction in the thickness of the partition between stacked thylakoids6. I describe here a rather different type of structural change that has not previously been reported for chloroplast membranes—protein phosphorylation. Like the above changes, protein phosphorylation is a reversible, energy-dependent membrane modification, but it differs from the other changes in that it takes the form of a specific chemical reaction involving certain identifiable chloroplast membrane polypeptides. The most conspicuous of these polypeptides is the light-harvesting chlorophyll a/b binding protein, the most abundant thylakoid polypeptide7.