The role of intrinsically disordered C‐terminal region of FliK in substrate specificity switching of the bacterial flagellar type III export apparatus

The bacterial flagellar export switching machinery consists of a ruler protein, FliK, and an export switch protein, FlhB and switches substrate specificity of the flagellar type III export apparatus upon completion of hook assembly. An interaction between the C‐terminal domain of FliK (FliK_C) and the C‐terminal cytoplasmic domain of FlhB (FlhB_C) is postulated to be responsible for this switch. FliK_C has a compactly folded domain termed FliK_T3S4 (residues 268–352) and an intrinsically disordered region composed of the last 53 residues, FliK_CT (residues 353–405). Residues 301–350 of FliK_T3S4 and the last five residues of FliK_CT are critical for the switching function of FliK. FliK_CT is postulated to regulate the interaction of FliK_T3S4 with FlhB_C, but it remains unknown how. Here we report the role of FliK_CT in the export switching mechanism. Systematic deletion analyses of FliK_CT revealed that residues of 351–370 are responsible for efficient switching of substrate specificity of the export apparatus. Suppressor mutant analyses showed that FliK_CT coordinates FliK_T3S4 action with the switching. Site‐directed photo‐cross‐linking experiments showed that Val‐302 and Ile‐304 in the hydrophobic core of FliK_T3S4 bind to FlhB_C. We propose that FliK_CT may induce conformational rearrangements of FliK_T3S4 to bind to FlhB_C.