The State 1 to State 2 transition in the photosynthetic membranes of plants and green algae involves the functional coupling of phosphorylated light-harvesting complexes of photosystem II (LHCII) to photosystem I (PSI). We present evidence suggesting that inChlamydomonas reinhardtiithis coupling may be aided by a hyper-phosphorylated form of the LHCII-like CP29 protein (Lhcbm4).