23TK

Structure of the 328-692 fragment of FlhA (A489E) form 1

Summary for 23TK

• Entry DOI: 10.2210/pdb23tk/pdb
• Descriptor: Flagellar biosynthesis protein FlhA (1 entity in total)
• Functional Keywords: flagellar type iii secretion, protein transport
• Biological source: Salmonella enterica subsp. enterica serovar Typhimurium
• Total number of polymer chains: 2
• Total formula weight: 86632.77

Authors

Sakai, M.,Kinoshita, M.,Takekawa, N.,Minamino, T.,Imada, K. (deposition date: 2026-02-16, release date: 2026-10-07, Last modification date: 2026-10-21)

Primary citation

Sakai, M.,Kinoshita, M.,Takekawa, N.,Namba, K.,Imada, K.,Minamino, T.
Structural basis for substrate specificity switching revealed by the FlhA(A489E) suppressor mutation.
Biophys Physicobio., 23:e230030-e230030, 2026

PubMed Abstract: The flagellar type III secretion system (fT3SS) switches its substrate specificity from hook-type to filament-type upon completion of hook assembly. This process is regulated by the coordinated actions of FliK, FlhB, and FlhA, but the structural mechanism underlying substrate specificity switching remains unclear. Here, we determined the crystal structures of the C-terminal cytoplasmic domain of FlhA(FlhA) carrying the A489E substitution, which partially restores motility of the cleavage-defective (P270A) mutant. The structures were solved in two distinct conformational states: an open form at 3.28 Å resolution and a semi-closed form at 2.90 Å resolution. In parallel, AlphaFold3-based structural predictions suggested that the flexible C-terminal cytoplasmic tail of FlhB (FlhB) binds to a groove formed between the α2 helix and β2 strand of FlhA, overlapping with the intersubunit linker-binding site within the FlhA ring. Structural analyses further revealed that the A489E substitution rearranges a local salt-bridge network involving Arg-386 and Glu-483, leading to subtle positional shifts of the α2 helix and remodeling of the predicted FlhB-binding surface. These structural changes may alter the predicted interaction between FlhA and FlhB, thereby lowering the energetic barrier associated with conformational remodeling of the FlhA ring during substrate specificity switching. Together, our results provide structural insight into how local electrostatic remodeling modulates conformational dynamics of FlhA and regulates substrate specificity switching in the fT3SS.

PubMed: 42840504
DOI: 10.2142/biophysico.bppb-v23.0030

Experimental method

X-RAY DIFFRACTION (2.9 Å)