Molecular mechanisms for interaction of glycine betaine with supra-molecular phycobiliprotein complexes

Glycine betaine (GB) is a biologically important small molecule protecting cells, proteins and enzymes in vivo and in vitro under environmental stresses. Recently, it was found that GB could also relax the structure and inactivate the function of phycobiliproteins and phycobilisome (PBS), a kind of supra-molecular complexes, in cyanobacterial cells. The molecular mechanisms for the opposite phenomena are quite ambiguous. Taking PBS and a trimeric or monomeric C-phycocyanin (C-PC) as models, the molecular mechanism for the interaction of GB with supra-molecular complexes or nuclear proteins was investigated. The energetic decoupling of PBS components induced by GB suggests that the PBS core-membrane linking polypeptide was the most sensitive site while the rod-core linker was the next. Biochemistry analysis proves that PBS structure was loosened but not dissociated into the components. On the basis of the results and structure knowledge, it was proposed that GB screened the electrostatic attraction of the opposite charges on a linker and a protein leading to a much looser structure. It was observed that GB induced a spectral blue shift for trimeric C-PC but a red shift for a monomeric C-PC (a nuclear protein), which were ascribed to GB’s screening of the electrostatic attraction of a linker to a protein and strengthening of the hydrophobic interaction between C-PC monomers. The trimers and monomers’ forming of the same products under high concentration of GB was ascribed to a compromise of the opposite interaction forces.