We cloned the mature SsTroA domain (residues 36-317), and the protein was purified using GST-affinity column and a subsequent gel-filtration chromatography (Fig. S1A ). Samples collected from elution fractions were separated by 12% SDS-PAGE (Fig. S1A ) and the results show that SsTroA has a molecular mass of ,34 kDa, in consistence with the theoretical monomer molar mass of 32 kDa. An analytical FPLC SuperdexH 200 profile revealed that SsTroA exists as a mixture of monomer and dimer in solution, though mainly as a monomer (Fig. S2A) . To determine the native mass of SsTroA (unaffected by protein shape), quantitative hydrodynamic analyses were performed by analytical ultracentrifugation. Analysis of the sedimentation velocity profiles using the SEDFIT program yielded the weight average molar mass of the major peak is 31.8 kDa ( Figure S2B ), which accounts for 90% of the total protein (i.e., nearly identical to the theoretical molar mass of 32 kDa). Another peak at 62.6 kDa (6.8% of the total) is believed to correspond to dimers. The other ,3.2% of the protein has a significantly-higher sedimentation coefficient, but the amount of this subpopulation did not change significantly as a function of protein concentration. Thus, our data suggest that under the solution conditions used, a small portion of SsTroA is involved in the formation of stable complexes. This is consistent with the evidence that periplasmic SBPs exist in a monomer/dimer equilibrium, with monomers having higher affinity for the substrate [36, 37] .