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When isolated, intact chloroplasts of pea (Pisum sativum) are incubated in the light with [32P]-orthophosphate, isotope is incorporated into several polypeptides. Among the most conspicuous phosphoproteins are two which form a very closely spaced doublet on dodecyl sulphate/polyacrylamide gels and co-electrophorese with the major polypeptide component of the light-harvesting chlorophyll a/b binding complex. Like the light-harvesting polypeptide, the phosphoprotein doublet is bound to thylakoids, sediments with the heavy particles released from thylakoids after digitonin treatment, is soluble in chloroform/methanol and has an apparent molecular weight of about 26000. The doublet also appears in the highly purified light-harvesting chlorophyll a/b binding complex isolated from thylakoids by hydroxylapatite chromatography. I conclude that two polypeptide components of the complex are phosphorylated. One of these components may be the major light-harvesting chlorophyll a/b binding protein.