Novel Cell Surface Anchoring Mechanism Of Prokaryotic Secreted Protein
The microbial cell surface is decorated with a variety of protein structures that play important roles in key cellular processes such as providing cell stability, facilitating interactions between cells, and interacting with the environment. One important feature of the biosynthesis of these structures is the proper anchoring of proteins to the cell surface. In silico work recently predicted a novel protein anchoring mechanism for a subset of surface proteins that contain a conserved C-terminal tripartite architecture, which consists of a conserved motif, followed by a hydrophobic (H) domain, and positively charged amino acids. Using the well-studied model archaeon Haloferax volcanii S-layer glycoprotein (SLG), previously thought to be anchored in the membrane via its C-terminal hydrophobic domain, I have shown that its H-domain instead is processed and that this processing is dependent on a novel enzyme, the archaeosortase (ArtA). The characterization of a strain lacking ArtA is also described, demonstrating its substrates proper anchoring is important for maintaining cell stability, morphology, motility, and mating efficiency. I also have demonstrated that the SLG is anchored to the membrane via a covalent lipid anchor, the first C-terminal lipid-anchoring mechanism described for any prokaryote. Furthermore, I have shown that this process is also ArtA-dependent and that the substrate conserved PGF motif is critical for processing and lipid anchoring. This novel anchoring mechanism is not limited to SLG, as I have demonstrated Hvo_0405, is also processed in an ArtA- dependent manner and showed that the conserved tripartite architecture does not necessarily need to lie at the C-terminus. Interestingly, I was able to show that Hvo_0405, unlike SLG, which is a Sec substrate, is transported across the H. volcanii membrane via the Twin Arginine (Tat) pathway demonstrating that Sec and Tat substrates can be C-terminally processed in an ArtA-dependent manner. Considering that ArtA homologs are conserved among diverse organisms of both prokaryotic domains, data obtained from my work will also serves as foundation for future studies of cell surface anchoring in other archaea and bacteria.
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