POSTERS - BLAST X - University of Utah

BLAST X Tue. Morning Session
CRYSTAL STRUCTURE OF FLIT, A BACTERIAL FLAGELLAR EXPORT CHAPERONE FOR
THE FILAMENT CAP PROTREN HAP2 (FliD)
Katsumi Imada, Tohru Minamino, Miki Kinoshita and Keiichi Namba
Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, Osaka 565-
0871, Japan
The bacterial flagellum is a filamentous organelle responsible for motility. Since the
flagellum extends from the cytoplasm to the cell exterior, the external component proteins have
to be exported from the cytoplasm. The protein subunits are exported by the flagellar specific
export apparatus, which is a member of the type III secretion system. The export apparatus is
believed to be located within the C-ring of the flagellar basal body and consists of at least six
integral membrane proteins (FlhA, FlhB, FliO, FliP, FliQ, FliR) and three soluble proteins (FliH,
FliI, FliJ). In addition to these proteins, other cytoplasmic proteins (FlgN, FliA, FliS, FliT) act as
substrate-specific chaperons that facilitate the export of their substrates.
FliT is a flagellar export chaperone for FliD (HAP2), which forms a capping complex at
the distal end of the flagellar filament and promotes incorporation of flagellin subunits into the
growing filament, and prevents FliD from premature aggregation in the cytoplasm. FliT is not
only involved in protein export but also in regulation of flagellar gene expression. FliT negatively
regulates transcription of the flagellar class 2 operons by binding to FlhD4C2 complex, which is a
transcriptional activator.
We have determined a crystal structure of FliT at 3.2 Å resolution. The structure and
following genetic and biochemical studies have revealed that the C-terminal region of FliT
regulates its interactions with other flagellar proteins. We will discuss the molecular mechanisms
of protein export and gene expression based on the FliT structure.
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