Expressed Sequence Tag Analysis of Vanadocytes in a Vanadium-Rich Ascidian, Ascidia sydneiensis samea

Some species in the family Ascidiidae accumulate vanadium at concentrations in excess of 350 mM, which corresponds to about 10⁷ times higher than that in seawater. In these species signet ring cells, with a single huge vacuole in which vanadium ion is contained, function as vanadium-accumulating cells, vanadocytes. To investigate the mechanism underlying this phenomenon, we performed an expressed sequence tag (EST) analysis of a complementary DNA library from vanadocytes of a vanadium-rich ascidian, Ascidia sydneiensis samea. We determined the nucleotide sequences of 1000 ESTs and performed a BLAST analysis against the SwissProt database. We found 93 clones of metal-related gene homologues, including the ferritin heavy subunit, hemocyanin, and metallothionein. Two ESTs, in particular, exhibited significant similarity to vanabins that have been extracted from A. sydneiensis samea blood cells as low molecular weight vanadium-binding proteins. We have named the genes encoding these ESTs vanabin3 and vanabin4. Immobilized metal ion affinity chromatography revealed that these novel vanabin homologues bind vanadium(IV) ions.