Lineage-specific expansions provide genomic complexity among sea urchin GTPases

In every organism, GTP-binding proteins control many aspects of cell signaling. Here, we examine in silico several GTPase families from the Strongylocentrotus purpuratus genome: the monomeric Ras superfamily, the heterotrimeric G proteins, the dynamin superfamily, the SRP/SR family, and the "protein biosynthesis" translational GTPases. Identified were 174 GTPases, of which over 90% are expressed in the embryo as shown by tiling array and expressed sequence tag data. Phylogenomic comparisons restricted to Drosophila, Ciona, and humans (protostomes, urochordates, and vertebrates, respectively) revealed both common and unique elements in the expected composition of these families. Galpha and dynamin families contain vertebrate expansions, consistent with whole genome duplications, whereas SRP/SR and translational GTPases are highly conserved. Unexpectedly, Ras superfamily analyses revealed several large (5+) lineage-specific expansions in the sea urchin. For Rho, Rab, Arf, and Ras subfamilies, comparing total human gene numbers to the number of sea urchin genes with vertebrate orthologs suggests reduced genomic complexity in the sea urchin. However, gene duplications in the sea urchin increase overall numbers such that total sea urchin gene numbers approximate vertebrate gene numbers for each monomeric GTPase family. These findings suggest that lineage-specific expansions may be an important component of genomic evolution in signal transduction.