FIG 8.

Diagram of the two secretion substrate specificity phases that are switched from the early phase to the late phase by FliK. Secreted proteins of the rod-hook class (represented by FlgD, FlgE, and FliK) are secreted through the early-phase gate, while flagellin-HAP proteins (represented by FlgM, FlgK, and FlgL) are secreted through the late-phase gate. FliK and FlgM are regulatory proteins, while the other secreted substrates are structural. At a certain point in hook elongation, FliK switches substrate specificity from the early phase to the late phase, presumably through a productive interaction with the C terminus of FliK and the FlhB component of the secretion apparatus in the cytoplasmic membrane.