Protein Transport into and Across Haloarchaeal Cytoplasmic Membranes

Abstract

Several pathways have evolved in the three domains of life to facilitate membrane protein insertion and the transport of proteins across lipid membranes. Haloarchaea employ the universally conserved Sec pathway, which transports unfolded proteins, for the transport of biologically important substrates into and across the membrane. However, they also extensively employ the twin arginine translocation (Tat) system, which transports substrates across the lipid bilayer in a folded conformation. Most haloarchaeal Tat substrates appear to be anchored to cytoplasmic membranes via lipid modifications. In silico analyses suggest that the prominent use of the Tat pathway and the lipid tethering of Tat substrates are traits unique to halophilic archaea. We will discuss the selective pressures that may have led to these unique adaptations as well as possible explanations for why they are not observed in halobacteria.