FlhB

FlhB is a core component of the bacterial flagellar type III secretion system, the export apparatus responsible for assembling the extracellular flagellum. It is an inner membrane protein with multiple N-terminal transmembrane segments that anchor it in the cytoplasmic membrane and a cytoplasmic C-terminal domain, commonly referred to as FlhB-C. FlhB is conserved across many Gram-negative bacteria that possess flagella.

FlhB participates in the export of flagellar components and plays a key role in a substrate specificity switch that coordinates the order of assembly. It resides in the export gate together with other components such as FlhA and the membrane-spanning proteins FliP, FliQ, and FliR. FlhB undergoes autoproteolytic processing at a conserved asparagine residue, producing an N-terminal fragment and a C-terminal FlhB-C fragment; this processing is required for switching secretion specificity from early rod and hook components to late filament proteins like flagellin. The switch is coordinated with FliK, a ruler protein that monitors hook length and helps trigger the transition when the hook reaches the correct size.

Genetic and biochemical analyses show that FlhB is essential for flagellar assembly and motility in many bacteria; mutations that affect autoproteolysis or FlhB-C function impair export and assembly. FlhB interacts with other export apparatus components, including FlhA, and is encoded within the flagellar gene cluster under control of the flagellar regulatory network.

FlhB is a widely conserved and essential element of the flagellar type III secretion system, serving both as an anchoring membrane protein and as a regulator of secretion specificity during assembly of the bacterial flagellum.