Phylogeny, sequence conservation, and functional complementation of the SBDS protein family

The Shwachman-Bodian-Diamond syndrome (SBDS) protein family occurs widely in nature, although its function has not been determined. Comprehensive database searches revealed SBDS homologues from 159 species, including examples from all sequenced archaeal and eukaryotic genomes and all eukaryotic kingdoms. Sequence alignment with ClustalX and MUSCLE algorithms led to the identification of conserved residues that occurred predominantly in the amino-terminal FYSH domain where they appeared to contribute to protein folding or stability. Only SBDS residue Gly91 was invariant in all species. Four distantly related protists were found to have two divergent SBDS genes in their genomes. In each case, phylogenetic analyses and the identification of shared sequence features suggested that one gene was derived from lateral gene transfer. We also identified a shared C-terminal zinc finger domain fusion in flowering plants and chromalveolates that may shed light on the function of the protein family and the evolutionary histories of these kingdoms. To assess the extent of SBDS functional conservation, we carried out complementation studies of SBDS homologues and interspecies chimeras in Saccharomyces cerevisiae. We determined that the FYSH domain was widely interchangeable among eukaryotes, while domain 2 imparted species specificity to protein function. Domain 3 was largely dispensable for function in our yeast complementation assay. Overall, the phylogeny of SBDS was shared with a group of proteins that were markedly enriched for RNA metabolism and/or ribosome-associated functions. These findings link Shwachman-Diamond syndrome to other bone marrow failure syndromes with defects in nucleolus-associated processes, including Diamond-Blackfan anemia, cartilage-hair hypoplasia, and dyskeratosis congenita.