The Plant Glycosyltransferase Family GT64: In Search of a Function

The glycosyltransferase 64 (GT64) family includes members from a diverse range of species including human, Xenopus and Drosophila as well as plant species including Arabidopsis, the moss Physcomitrella and poplar. The majority of the animal GT64 proteins are bimodular, consisting of an N-terminal GT47 domain linked to a GT64 domain. The animal GT64 domain has been found to have GlcNAc transferase activity and functions during heparan sulphate bio-synthesis. However, the biochemical activity of the plant GT64 proteins has yet to be established. Arabidopsis has three GT64 members which have been named ECTOPICALLY PARTING CELLS1 (EPC1), EPC-LIKE1 (EPC-L1) and EPC-LIKE2 (EPC-L2). The EPC1 and EPC-L1 proteins are comprised of a single GT64 domain whereas the EPC-L2 protein has an additional N-terminal domain which does not show homology to any other protein other than EPC-L2 homologues from other plants. Mutation of the Arabidopsis GT64 EPC1 results in a plant with severely reduced stature, demonstrating the importance of its function for normal plant development. Neutral sugar analysis of the epc1 cell wall shows a reduction in galactose but there is currently no enzymatic activity demonstrated to explain this alteration. In contrast to EPC1, EPC-L2 is specifically expressed in reproductive tissues and mutations in the gene result in defects in pollen development, reduced seed formation and embryo abortions at the late globular stage.