Conformational stabilization and crystallization of the SecA translocation ATPase from Bacillus subtilis

SecA is the peripheral membrane-associated subunit of the enzyme complex `preprotein translocase' which assists the selective transport of presecretory proteins into and across bacterial membranes. The SecA protein acts as the molecular motor that drives the translocation of presecretory proteins through the membrane in a stepwise fashion concomitant with large conformational changes accompanying its own membrane insertion/retraction reaction cycle coupled to ATPase activity. The high flexibility of SecA causes a dynamic conformational heterogeneity which presents a barrier to growth of crystals of high diffraction quality. As shown by fluorescence spectroscopy, the T_m of the endothermic transition of cytosolic SecA from Bacillus subtilis is shifted to higher temperatures in the presence of 30% glycerol, indicating stabilization of the protein in its compact membrane-retracted conformational state. High glycerol concentrations are also necessary to obtain three-dimensional crystals suitable for X-­ray diffraction analysis, suggesting that stabilization of the conformational dynamics of SecA may be required for effective crystallization. The SecA crystals grow as hexagonal bipyramids in the trigonal space group P3₁12; they possess unit-cell parameters a = 130.8, b = 130.8, c = 150.4 Å at 100 K and diffract X-rays to approximately 2.70 Å using a high-flux synchrotron-radiation source.