ArtA-Dependent Processing of a Tat Substrate Containing a Conserved Tripartite Structure That Is Not Localized at the C Terminus

Abstract

Most prokaryote-secreted proteins are transported to the cell surface using either the general secretion (Sec) or twin-arginine translocation (Tat) pathway. A majority of secreted proteins are anchored to the cell surface, while the remainder are released into the extracellular environment. The anchored surface proteins play a variety of important roles in cellular processes, ranging from facilitating interactions between cells to maintaining cell stability. The extensively studied S-layer glycoprotein (SLG) of Haloferax volcanii, previously thought to be anchored via C-terminal intercalation into the membrane, was recently shown to be lipidated and to have its C-terminal segment removed in processes dependent upon archaeosortase A (ArtA), a recently discovered enzyme. While SLG is a Sec substrate, in silico analyses presented here reveal that, of eight additional ArtA substrates predicted, two substrates also contain predicted Tat signal peptides, including Hvo_0405, which has a highly conserved tripartite structure that lies closer to the center of the protein than to its C terminus, unlike other predicted ArtA substrates identified to date. We demonstrate that, even given its atypical location, this tripartite structure, which likely resulted from the fusion of genes encoding an ArtA substrate and a cytoplasmic protein, is processed in an ArtA-dependent manner. Using an Hvo_0405 mutant lacking the conserved "twin" arginines of the predicted Tat signal peptide, we show that Hvo_0405 is indeed a Tat substrate and that ArtA substrates include both Sec and Tat substrates. Finally, we confirmed the Tat-dependent localization and signal peptidase I (SPase I) cleavage site of Hvo_0405 using mass spectrometry.IMPORTANCE The specific mechanisms that facilitate protein anchoring to the archaeal cell surface remain poorly understood. Here, we have shown that the proteins bound to the cell surface of the model archaeon H. volcanii, through a recently discovered novel ArtA-dependent anchoring mechanism, are more structurally diverse than was previously known. Specifically, our results demonstrate that both Tat and Sec substrates, which contain the conserved tripartite structure of predicted ArtA substrates, can be processed in an ArtA-dependent manner and that the tripartite structure need not lie near the C terminus for this processing to occur. These data improve our understanding of archaeal cell biology and are invaluable for in silico subcellular localization predictions of archaeal and bacterial proteins.

Keywords: C-terminal processing; Haloferax volcanii; Tat substrates; archaea; archaeosortase; cell surface; exosortase; protein secretion; secretion systems.