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Description
Type 3 secretion (T3S) is the only secretion system in Biology known to switch from a given set of secretion substrates to a completely different set of protein secretion substrates. We devised a simple genetic selection that revealed the presence of a protein lock, a gatekeeper, that holds the apparatus into "early" secretion mode. Upon completion of an intermediate flagellum assembly structure, the hook-basal body (HBB), a secreted molecular ruler measures that the HBB has reached its terminal length of 80 nm. The C-terminus of the final secreting ruler performs protein intercourse with the gatekeeper that is holding the flagellar T3S apparatus in early substrate specific secretion mode. The result of this protein intercourse is the opening of the gatekeeper's hydrophobic core that allows it to escape from its held position, which results in the flipping the flagellar T3S-specificity switch.