The nucleolar proteome and the (endosymbiotic) origin of the nucleus [2] (multiple letters)

Nucleoli are essential regions of the eukaryotic nuclei where the ribosomes are assembled. The human nucleolar proteome has been studied in the last two years, leading to the identification of several hundreds of proteins. (1,2) In a recent Bioessays article, Staub et al. (3) characterise a large set of conserved domains in these nucleolar proteins. The search of these domains in otherorganisms revealed acomplex pattern. Many exist in archaea, arguing for an archaeal origin of the nucleolar core machinery. A substantial bacterial contribution is also identified, while several domains are exclusively eukaryotic. These results support a chimeric origin of the nucleolus, with an ancestral core of archaeal origin supplemented with certain bacterial proteins and several proteins invented during the eukaryotic evolution. This mixed heritage is compatible with the diverse hypotheses that propose the amalgamation of archaea and bacteria to give rise to the first eukaryotes. However, these hypotheses differ in the mechanism explaining the merging of the archaeal and bacterial partners. Some propose an unspecified fusion (4) or the engulfment of an archaeon by a bacterium. (5,6) The most elaborate invoke symbioses between archaea and bacteria.(711)Another fundamental difference concerns the origin of the nucleus: is it a remnant of endosymbiotic archaea(5,6,1012)orjust an idiosyncratic eukaryotic invention?