Acyclic Polyisoprenoid Hydrocarbons: Fundamental Regulators of the Archaeal Membrane?
Résumé
Sterols and hopanoids, the typical membrane regulators of Eukaryotes and Bacteria, are absent from Archaea, which might instead use acyclic polyisoprenoid hydrocarbons (APH). Although a few studies evidenced that APH do modulate membrane physicochemical properties as other membrane regulators, little is known about these intriguing compounds in the third domain of life. In an effort to elucidate APH distribution, physiological and adaptive functions, and biosynthetic pathway in Archaea, we collected all the literature data available and evaluated the APH composition of 65 archaeal species homogeneously distributed within the phylogenetic tree and ecological niches of Archaea. Fully saturated to fully unsaturated 30, 35, and 40-carbon long APH were detected in most species investigated, regardless of their phylogenetic position, lipid composition, or preferential environmental conditions, although the chain length could be correlated with growth conditions. Despite such a wide occurrence of APH in Archaea, only rare homologues of the eukaryotic and bacterial APH biosynthetic pathways were found. How they synthesize these compounds thus remains elusive and lipid analysis still prevails as the only mean to assess the production and functions of these essential archaeal membrane components.
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