Identification and evolutionary analysis of the nucleolar proteome of Giardia lamblia

The nucleoli, including their proteomes, of higher eukaryotes have been extensively studied, while few studies about the nucleoli of the lower eukaryotes - protists were reported. Giardia lamblia, a protist with the controversy of whether it is an extreme primitive eukaryote or just a highly evolved parasite, might be an interesting object for carrying out the nucleolar proteome study of protists and for further examining the controversy.

Using bioinformatics methods, we reconstructed G. lamblia nucleolar proteome (GiNuP) and the common nucleolar proteome of the three representative higher eukaryotes (human, Arabidopsis, yeast) (HEBNuP). Comparisons of the two proteomes revealed that: 1) GiNuP is much smaller than HEBNuP, but 78.4% of its proteins have orthologs in the latter; 2) More than 68% of the GiNuP proteins are involved in the "Ribosome related" function, and the others participate in the other functions, and these two groups of proteins are much larger and much smaller than those in HEBNuP, respectively; 3) Both GiNuP and HEBNuP have their own specific proteins, but HEBNuP has a much higher proportion of such proteins to participate in more categories of nucleolar functions.

For the first time the nucleolar proteome of a protist - Giardia was reconstructed. The results of comparison of it with the common proteome of three representative higher eukaryotes -- HEBNuP indicated that the simplicity of GiNuP is most probably a reflection of primitiveness but not just parasitic reduction of Giardia, and simultaneously revealed some interesting evolutionary phenomena about the nucleolus and even the eukaryotic cell, compositionally and functionally.