The major light-harvesting chlorophyll a/b complex of photosystem Ⅱ (LHCIIb) is a multifunctional complex in the thylakoid membrane.Ser123,located at the junction of helix C and the lumen-exposed surface,possesses great structural and functional significance [1].In this work,Ser123 was mutated to Pro (S123P),Thr (S123T),Gln (S123Q),Tyr (S123Y),or Gly (S123G) to study its molecular mechanism.