Nucleotide and negatively charged lipid-dependent vesicle aggregation caused by SecA. Evidence that SecA contains two lipid-binding sites

SecA which is an overall acidic protein was found to induce an increase in the turbidity of a solution of vesicles consisting of negatively charged phospholipids. This increase was found to be due to an aggregation of the vesicles mediated by SecA. The SecA-mediated vesicle aggregation was not found for zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphocholine and showed a large dependence on both temperature and ionic strength. Furthermore it was shown that ATP and to a lesser extent ADP+P(i) were able to reduce the SecA-mediated vesicle aggregation, while no effect could be seen for a non-hydrolysable ATP analog AMP-PNP. Using the steady state fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene present in 1,2-dioleoyl-sn-glycero-3-phosphoglycerol vesicles we could show that SecA inserts in the bilayer. Monolayer studies confirmed that SecA is able to cause close contact between two membranes and gave a direct insight into the different types of lipid-protein interactions involved. From our results we propose that the SecA monomer possesses two lipid-binding sites which in the functional dimer conformation are responsible for the SecA-mediated vesicle aggregation.