Mass determination and estimation of subunit stoichiometry of the bacterial hook-basal body flagellar complex of Salmonella typhimurium by scanning transmission electron microscopy.

The basal body, a part of the rotary motor of the bacterial flagellum, is a multiprotein assembly that consists of four rings (denoted M, S, P, and L) and an axial rod (denoted R). From analysis of scanning transmission electron microscopy images of hook-basal body preparations isolated from Salmonella typhimurium, we have determined the masses of the basal body and three of its subcomplexes. The mass of the basal body (i.e., the four rings and rod) is 4400 +/- 490 kDa (mean +/- SD; n = 54). The mass of the LPR subcomplex (i.e., L and P rings and the whole rod) is 2600 +/- 380 kDa (n = 55), that of the L and P rings and the distal part of the rod is 2100 +/- 320 kDa (n = 25), and the mass of the L and P ring subcomplex is 1700 +/- 260 kDa (n = 514). These results, together with the masses of the component proteins, indicate that the rings contain approximately 26 subunits each and that the mass of the rod is consistent with a composition of approximately 6 copies each of three of the rod proteins FlgB, FlgC, and FlgF and approximately 26 copies of FlgG as determined by Jones et al. [Jones, C. J., Macnab, R. M., Okino, H. & Aizawa, S.-I. (1990) J. Mol. Biol. 212, 377-387] using quantitative gel electrophoresis. The results of Jones et al., together with ours, account for all proteins in the basal body to within approximately 5% (or 200 kDa).