The mysterious microbes that gave rise to complex life

Archaeal lipids

The major archaeal membrane glycerolipids are distinguished from those of bacteria and eukaryotes by the contrasting stereochemistry of their glycerol backbones, and by the use of ether-linked isoprenoid-based alkyl chains rather than ester-linked fatty acyl chains for their hydrophobic moieties. These fascinating compounds play important roles in the extremophile lifestyles of many species, but are also present in the growing numbers of recently discovered mesophilic archaea. The past decade has witnessed significant advances in our understanding of archaea in general and their lipids in particular. Much of the new information has come from the ability to screen large microbial populations via environmental metagenomics, which has revolutionised our understanding of the extent of archaeal biodiversity that is coupled with a strict conservation of their membrane lipid compositions. Significant additional progress has come from new culturing and analytical techniques that are gradually enabling archaeal physiology and biochemistry to be studied in real time. These studies are beginning to shed light on the much-discussed and still-controversial process of eukaryogenesis, which probably involved both bacterial and archaeal progenitors. Puzzlingly, although eukaryotes retain many attributes of their putative archaeal ancestors, their lipid compositions only reflect their bacterial progenitors. Finally, elucidation of archaeal lipids and their metabolic pathways have revealed potentially interesting applications that have opened up new frontiers for biotechnological exploitation of these organisms. This review is concerned with the analysis, structure, function, evolution and biotechnology of archaeal lipids and their associated metabolic pathways.

Widespread PRC barrel proteins play critical roles in archaeal cell division

Cell division is fundamental to all cellular life. Most of the archaea employ one of two alternative division machineries, one centered around the prokaryotic tubulin homolog FtsZ and the other around the endosomal sorting complex required for transport (ESCRT). However, neither of these mechanisms has been thoroughly characterized in archaea. Here, we show that three of the four PRC (Photosynthetic Reaction Center) barrel domain proteins of Haloferax volcanii (renamed Cell division proteins B1/2/3 (CdpB1/2/3)), play important roles in division. CdpB1 interacts directly with the FtsZ membrane anchor SepF and is essential for division, whereas deletion of cdpB2 and cdpB3 causes a major and a minor division defect, respectively. Orthologs of CdpB proteins are also involved in cell division in other haloarchaea. Phylogenetic analysis shows that PRC barrel proteins are widely distributed among archaea, including the highly conserved CdvA protein of the crenarchaeal ESCRT-based division system. Thus, diverse PRC barrel proteins appear to be central to cell division in most if not all archaea. Further study of these proteins is expected to elucidate the division mechanisms in archaea and their evolution.

Origin of eukaryotes: What can be learned from the first successfully isolated Asgard archaeon

The origin of cellular complexity characterizing eukaryotic cells remains a central unresolved issue in the study of diversification of cellular life on Earth. The isolation by Imachi et al.1 of a member of the Asgard archaea2 - a contemporary relative of organisms thought to have given rise to eukaryotic cells about 2 billion years ago - now promises new insight. The complete genome sequence of the isolated Lokiarchaeum strain confirms that the eukaryotic signature proteins (ESPs) previously identified in the Lokiarchaeota3 and other Asgard archaea2 are indeed encoded by these archaeal genomes and do not represent contamination from eukaryotes. These ESPs encode homologs of eukaryotic actins, small GTPases and the ESCRT complex proteins and are required for the functioning of complex eukaryotic cells. The new, slowly growing, anaerobic laboratory strain allows a first direct look at these organisms and provides key insights into the morphology and metabolism of an Asgard archaeal organism. The work has provided valuable information for other laboratories that aim to isolate and characterize related organisms from other environments.