# The Cell Membrane of _Sulfolobus_ spp.--Homeoviscous Adaption and Biotechnological Applications
## 2 Cell Membrane and Lipids of _Sulfolobus_ Spp.
### Cell Membrane Structure  
Already, in 1973, the cell wall of _S. acidocaldarius_ was isolated and investigated for its compounds and biochemical characteristics. These fundamental experiments showed the high robustness against the treatment with enzymes and reagents; in particular, ethylenediamine tetraacetic acid (EDTA), sodium dodecyl sulfate (SDS), dimethyl sulfoxide (DMSO) and Triton X-100. The absence of peptidoglycan in the membrane distinguishes Archaea from Bacteria [29]. In addition to the cell membrane, the cell envelope of _Sulfolobus_ spp. consists of a proteinaceous layer called the surface (S-)layer. In _Sulfolobus_ spp., the proteins SlaA and SlaB [30] were identified as the components of this layer, in controversy to many other Archaea's S-layers consisting only of one protein. In Figure 1 a graphical illustration of the cell envelope is shown. SlaA serves as the sheath, whereas the SlaB protein makes up the shaft [30] and anchors the S-layer in the cytoplasmic membrane. The space in between, spanned by the proteins of the S-layer, represents the pseudoperiplasmic space [25]. The cell membrane, which encloses the cytoplasm, is composed of phospho- and glycolipids. A further crucial difference between archaeal and bacterial membrane lipids is the presence of ether bonds in the former instead of ester linkages. In most Archaea, a lipid bilayer comprised of glycerol diether lipids construct the cell membrane [25]. The core archaeal lipids are monomeric dialkyl glycerol diethers (commonly called archaeol or DGD). Their hydrophobic core of C\({}_{20}\) isoprenoids is linked to the glycerol backbone at the \(sn\)-2,3 positions (Figure 2A). Two diether moieties can form glycerol dialkyl glycerol tetraether (called caldarchaeol or GDGT), by head to head linking (Figure 2B). The hydrophobic core is then comprised of C\({}_{40}\) archaeal isoprenoids. Diether lipids form a bilayer in contrast to the monolayer formed by tetraether lipids. Both diether and tetraether structures exhibit variations in their polar head groups, cyclization and length of hydrophobic core (the latter is true for the archaeol of certain Archaea only). In _Sulfolobus_ spp. the constituents of the cell membranes are predominantly tetraether lipids arranged as a monolayer. Furthermore, Sulfolobales' membranes also contain calditol linked to the \(sn\)-1 position of the glycerol (Figure 2C and D). This is one of the major variants of caldarchaeol, and, based on early, inaccurate structure elucidation, is also known under the deceptive name glycerol dialkylnonitol tetraether (GDNT) [32; 33; 34; 35; 36]. Table 1 gives an overview of the membrane lipid composition of the order Sulfolobales, in comparison to other well studied archaeal members. The halophilic membrane of _Natronococcus_ is comprised of a mixture of C\({}_{20\text{-}20}\) diether lipids (lipids harboring two alkyl chains in the lipid core, with 20 carbon atoms per chain) and C\({}_{20\text{-}25}\) diether lipids (20 and 25 carbon atoms) [37], while _Sulfolobales_ mainly consist of C\({}_{40\text{-}40}\) tetraether lipids and partly of C\({}_{20\text{-}20}\) diether lipids [9]. Methanogens like the _Methanobacterium_ contain C\({}_{20\text{-}20}\) diether lipids, as well as C\({}_{40\text{-}40}\) tetraether lipids in varying ratios in their membrane [38]. GDNTs have only been found within the C  
Figure 1: Cell envelope profile of _Sulfolobus_ spp. The semi-permeable surface (S-)layer is comprised of the proteins SlaA (sheath) and SlaB (shaft). Both SlaA and SlaB are protein trimers and the contact area of these trimers is indicated with a dashed line. The salmon colored area represents the pseudoperiplasmic space. In _Sulfolobus_ spp., the membrane is predominantly comprised of membrane spanning tetraether lipids [25; 31]. \begin{table}