Halocins and sulfolobicins: the emerging story of archaeal protein and peptide antibiotics.

Production of antibiotic peptides and proteins is a near-universal feature of living organisms regardless of phylogenetic classification. Bacteriocins (proteinaceous antimicrobials from the domain Bacteria) have been studied for over 75 years, and the eucaryocins (proteinaceous antimicrobials from the domain Eucarya) since the early 1960s. However, one domain of organisms, the Archaea, containing hyperthermophiles, extreme halophiles and the methanogens, is just beginning to be scrutinized for the production of peptide antibiotics. Production of archaeal proteinaceous antimicrobials (archaeocins) from extreme halophiles (halocins) is a nearly universal feature of the rod-shaped haloarchaea. Halocin activity is first detectable in culture supernatants at the beginning of the transition into stationary phase, concomitant with an induction of transcription of the structural gene. Halocins are diverse in size, consisting of proteins as large as 35 kDa and peptide "microhalocins" as small as 3.6 kDa. The 36 amino acids of microhalocin HalS8 are located in the interior of a 311-residue pro-protein from which they are liberated by an unknown mechanism. Microhalocins are hydrophobic and robust, withstanding heat, desalting and exposure to organic solvents. Unlike the peptide bacteriocins and the eucaryocins, microhalocins possess a large number of neutral residues and are not cationic, leaving their mechanism(s) of action mostly a mystery. While microhalocins affect a variety of haloarchaeal genera (kingdom Euryarchaeota), they also exhibit cross-kingdom toxicity, inhibiting or killing Sulfolobus species (kingdom Crenarchaeota). Finally, archaeocins also are produced by the hyperthermophile "Sulfolobus islandicus". These 20-kDa protein antibiotics are not excreted into the environment, but are associated with small particles apparently derived from the cell's S-layer.