The Nitrogen Source‐Dependent 126‐kDa Protein From Synechococcus PCC 7942 Plasmalemma: A Trimer of the NrtA Nitrate‐Binding Protein

Expression of a 126‐kDa protein in the plasmalemma (cytoplasmic membrane) from Synechococcus PCC 7942 is dependent on the nitrogen source. Polyclonal antibody raised against the NrtA protein reacted with the 126‐kDa protein. Two peptide sequences from the 126‐kDa protein were retrieved in NrtA. FPLC purification of plasmalemma solubilised in Triton X‐100 gave a fraction consisting mainly of a 126‐kDa component (72.6%), as shown by sedimentation velocity. Equilibrium sedimentation of the same fraction gave evidence of the existence of an oligomeric structure (Ka = 2×10³ and 2.9 subunits). Thus, the 126‐kDa protein is considered as a trimeric arrangement of the 45‐kDa protein in the plasmalemma.