Abstract
The so-called Tat (twin-arginine translocation) system transports completely folded proteins across cellular membranes of archaea, prokaryotes and plant chloroplasts. Tat-directed proteins are distinguished by a conserved twin-arginine (RR-) motif in their signal sequences. Many Tat systems are based on the membrane proteins TatA, TatB and TatC, of which TatB and TatC are known to cooperate in binding RR-signal peptides and to form higher-order oligomeric structures. We have now elucidated the fine architecture of TatBC oligomers assembled to form closed intramembrane substrate-binding cavities. The identification of distinct homonymous and heteronymous contacts between TatB and TatC suggest that TatB monomers coalesce into dome-like TatB structures that are surrounded by outer rings of TatC monomers. We also show that these TatBC complexes are approached by TatA protomers through their N-termini, which thereby establish contacts with TatB and membrane-inserted RR-precursors.
Highlights
The so-called Tat system transports completely folded proteins across cellular membranes of archaea, prokaryotes and plant chloroplasts
Our results suggest that dome-like TatB structures form the core of intramembrane substrate-binding cavities that are surrounded by outer rings of TatC monomers
We demonstrate here that central positions in the TM4 and TM2 of TatC establish contacts with TatB. Their distance to the TM5-based binding area invokes the attachment of a second TatB molecule on the concave face of each TatC (Fig. 2b,c)
Summary
The so-called Tat (twin-arginine translocation) system transports completely folded proteins across cellular membranes of archaea, prokaryotes and plant chloroplasts. Tat (twin-arginine translocation) machineries exist in the plasma membranes of bacteria and archaea and the thylakoid membranes of plant chloroplasts, across which they transport folded proteins that contain the consensus motif S-R-R-x-F-L-K in their signal sequences[1,2,3,4,5]. These Tat machineries are constructed from TatA- and TatC-type membrane proteins. It has been suggested that multiple TatA monomers associate either via their transmembrane[8] or their amphipathic helices[9,10] to form size-fitting pores for the Tat substrates. We propose that TatA laterally enters those substrate–TatBC complexes
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