Sec-independent protein translocation in Escherichia coli - A distinct andpivotal role for the TatB protein

In Escherichia coli, transmembrane translocation of proteins can proceed by a number of routes. A subset of periplasmic proteins are exported via the Tat pathway to which proteins are directed by N-terminal "transfer peptides " bearing the consensus (S/T)RRXFLK "twin-arginine" motif, The Tat system i nvolves the integral membrane proteins TatA, TatB, TatC, and TatE, Of these , TatA, TatB, and TatE are homologues of the Hcf106 component of the Delta pH-dependent protein import system of plant thylakoids, Deletion of the tat B gene alone is sufficient to block the export of seven endogenous Tat subs trates, including hydrogenase-g. Complementation analysis indicates that wh ile TatA and TatE are functionally interchangeable, the TatB protein is fun ctionally distinct. This conclusion is supported by the observation that He licobacter pylori tatA will complement an E. coli tatA mutant, but not a ta tB mutant. Analysis of Tat component stability in various tot deletion back grounds shows that TatC is rapidly degraded in the absence of TatB suggesti ng that TatC complexes, and is stabilized by, TatB.