Domain structures of the MS ring component protein (FliF) of the flagellar basal body of Salmonella typhimurium.

The proximal end of the flagellar basal body consists of a structure called the MS ring complex: two (M and S) rings with different thicknesses closely apposed and a rod extending from the center of the S ring. It has been shown that the MS ring complex consists of multiple copies of single protein FliF (molecular mass 61 kDa). We analyzed the domains of FliF to elucidate how a single protein can be used to construct a complicated particle with several distinct sub-structures. Tryptic digestion of the MS ring complex gave rise to a structure which lacked most of the M ring portion by electron microscopy and showed a major band at 25 kDa by SDS/gel electrophoresis. Amino acid sequence analysis of this band showed that both terminal regions of FliF have been digested, leaving a semi-stable peptide starting from Phe120 and ending at around 400. In addition, we constructed a truncated fliF gene which encodes a FliF lacking 103 amino acid residues from the C terminus. Amplification of the truncated FliF gave rise to a ring complex lacking the rim of the M ring. From these results we assign both terminal regions of FliF to the M ring. Possible domain structures of FliF corresponding to the S ring and the rod are also discussed.