Serine acetyltransferase from Escherichia coli is a dimer of trimers

Equilibrium sedimentation studies show that the serine acetyltransferase (SAT) of Escherichia coli is a hexamer. The results of velocity sedimentation and quasi-elastic light scattering experiments suggest that the identical subunits are loosely packed and/or arranged in an ellipsoidal fashion. Chemical cross-linking studies indicate that the fundamental unit of quaternary structure is a trimer. The likelihood, therefore, is that in solution SAT exists as an open arrangement of paired trimers. Crystals of SAT have 32 symmetry, consistent with such an arrangement, and the cell density function is that expected for a hexamer. Electron microscopy with negative staining provides further evidence that SAT has an ellipsoidal subunit organization, the dimensions of the particles consistent with the proposed paired trimeric subunit arrangement. A bead model analysis supports the view that SAT has a low packing density and, furthermore, indicates that the monomers may have an ellipsoidal shape. Such a view is in keeping with the ellipsoidal subunit shape of trimeric LpxA, an acyltransferase with which SAT shares contiguous repeats of a hexapeptide motif.