| SecA forms a physiological dimer with a dissociation constant that has previously been shown to vary with temperature and ionic strength. Despite high conservation of SecA between organisms, the currently published crystal structures do not show the same dimer symmetry even though the monomer subunits are very similar. We now present data showing that the distance between the two protomers in the dimer is different from all published structures.;The oligomeric state of SecA in solution is altered by ligands that it interacts with during protein translocation. Analytical ultracentrifugation and fluorescence anisotropy measurements show that the physiological dimer of SecA is monomerized by long-chain phospholipid analogues and increased KCl and MgCl2 concentration. SecA monomers can undergo a normal conformational reaction cycle, but many conditions used to produce monomerization disrupt the integrity of the protein. These interactions alter the complex equilibria between monomeric and oligomeric forms of SecA, supporting the possibility that changes in the oligomeric state of SecA could play a functional role in the protein translocation reaction. Interactions of SecA with diacyl-phospholipids can facilitate the endothermic conformational transition and influence the catalytic properties of the protein. |
