Adaptation of protein secretion to extremely high‐salt conditions by extensive use of the twin‐arginine translocation pathway

Abstract

SummaryHalophilic archaea thrive in environments with salt concentrations approaching saturation. However, little is known about the way in which these organisms stabilize their secreted proteins in such 'hostile' conditions. Here, we present data suggesting that the utilization of protein translocation pathways for protein secretion by the Halobacteriaceae differs significantly from that of non-haloarchaea, and most probably represents an adaptation to the high-salt environment. Although most proteins are secreted via the general secretion (Sec) machinery, the twinarginine translocation (Tat) pathway is mainly used for the secretion of redox proteins and is distinct from the Sec pathway, in that it allows cytoplasmic folding of secreted proteins. TATFIND (developed in this study) was used for systematic whole-genome analysis of Halobacterium sp. NRC-1 and several other prokaryotes to identify putative Tat substrates. Our analyses revealed that the vast majority of haloarchaeal secreted proteins were predicted substrates of the Tat pathway. Strikingly, most of these putative Tat substrates were non-redox proteins, the homologues of which in non-haloarchaea were identified as putative Sec substrates. We confirmed experimentally that the secretion of one such putative Tat substrate depended on the twin-arginine motif in its signal sequence. This extensive utilization of the Tat pathway in haloarchaea suggests an evolutionary adaptation to high-salt conditions by allowing cytoplasmic folding of secreted proteins before their secretion.

The Bacterial Twin-Arginine Translocation Pathway

“…Halophilic archaea such as Halobacterium sp. NRC-1 and Haloferax volcanii are predicted to utilize Tat for the export of >90% of their secreted proteins (41,107). A functional Tat pathway is not required for the growth of E. coli, other γ-proteobacteria, or B. subtilis under most conditions; however, it is essential for the viability of H. volcanii even in rich media (41).…”

Insights into ABC Transport in Archaea

“…2). Halobacteria present an interesting case: it has been proposed that probably nearly all extracellular proteins are secreted by the twin arginine (Tat) translocation pathway (Bolhuis, 2002;Rose et al, 2002). Proteins recognized by the Tat translocation pathway contain a double arginine at their extreme N-termini.…”

Post-translational Secretion of Fusion Proteins in the Halophilic Archaea Haloferax volcanii

“…As such, components involved in the post-translational targeting and translocation of secretory proteins across the archaeal plasma membrane may remain to be identified. With this in mind, it is noteworthy that two recent studies have reported extensive use of the alternative Tat secretion pathway, involved in posttranslational translocation in bacteria, in Haloarchaea (39,40).…”