Although archaea have a similar cellular organization as other prokaryotes, the lipid composition of their membranes and their cell surface is unique. Here we discuss recent developments in our understanding of the archaeal protein secretion mechanisms, the assembly of macromolecular cell surface structures, and the release of S-layer-coated vesicles from the archaeal membrane. 1. The Archaeal Cell Envelope The ability of many archaea to endure extreme conditions in hostile environments intrigues researchers to study the molecular mechanisms and specific adaptations involved. Very early, it was realized that the structure of the archaeal cell envelope differs substantially from that of bacteria [1]. With the only exception of Ignicoccus which exhibits an outer membrane enclosing a huge periplasmic space [2], known archaea possess only a single membrane. This cytoplasmic membrane is enclosed by an S-layer, a two-dimensional protein crystal that fully covers the cells (see review Jarrell et al. in this issue). In contrast to bacterial ester lipids, archaeal lipids consist of repeating isoprenyl groups linked to a glycerol backbone through an ether linkage [3, 4]. These lipids typically form diether bilayer membranes similar to membranes of eukarya and bacteria. Hyperthermo-acidophiles contain tetraether lipids that consist of C40 isoprenoid acyl chains that span the membrane entirely forming a monolayer membrane [5]. These membranes are extremely proton impermeable and enable these organisms to survive under conditions that the extracellular pH is up to 4 units below that of the cytoplasm [6]. Another peculiarity is that most of the extracellular proteins of archaea are glycosylated via N- and O-glycosylation. Finally, Archaea do not produce any murein, and only some methanogenic species are known to produce pseudomurein [7]. As the archaeal cell surface is so different from that of bacteria and eukarya, unique mechanisms must exist to form and shape it. Until recently most of our knowledge of protein secretion and on the assembly of the cell surface components in archaea was obtained by comparative genomic studies. However, in recent years tremendous progress has been made in our understanding of the assembly and function of cell surface structures and both the structural and functional basis of protein translocation across the archaeal membrane. Here we will discuss these topics with an emphasis on the cell surface structures. 2. Protein Secretion 2.1. Transport of Unfolded Proteins Across the Cytoplasmic Membrane The ability to transport proteins
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