The Plug domain has been shown to be an independently folding subunit of the TonB-dependent receptors ([1]). It acts as the channel gate, blocking the pore until the channel is bound by ligand. At this point it under goes conformational changes opens the channel.

1: The plug domain of a neisserial TonB-dependent transporter retains structural integrity in the absence of its transmembrane beta-barrel. Oke M, Sarra R, Ghirlando R, Farnaud S, Gorringe AR, Evans RW, Buchanan SK; FEBS Lett 2004;564:294-300. PMID:15111112