The Arabidopsis thaliana putative sialyltransferase resides in the Golgi apparatus but lacks the ability to transfer sialic acid

A common feature of the animal sialyltransferases (STs) is the presence of four conserved motifs, namely large (L), small (S), very small (VS) and motif III. Although sialic acid (SA) has not been detected in plants, three orthologues containing sequences similar to the ST motifs have been identified in the Arabidopsis thaliana L. database. In this study, we report that the At3g48820 gene (Gene ID: 824043) codes for a Golgi resident protein lacking the ability to transfer SA to asialofetuin or Gal beta 1,3GalNAc and Gal beta 1,4GlcNAc oligosaccharide acceptors. Restoration of deteriorated motifs S, VS and motif III by constructing chimeric proteins consisting of the 28-308 amino acid region of the A. thaliana At3g48820ST-like protein and the 264-393 amino acid region of the Oryza sativa L. AK107493ST-like protein, or of the 28-240 amino acid region of the At3g48820protein and the 204-350 amino acid region of the Homo sapiens L. alpha 2,3-ST (NP_008858) was not able to recover sialyltransferase activity. Altering the appropriate amino acid regions of the A. thaliana At3g48820 ST-like protein to those typical for the mammalian motif III (HHYWE) and VS motif (HDADFE) also did not have any effect. Our data, together with previous results, indicate that A. thaliana in particular, and plants in general, do not have transferases for SA. Substrates for the plant ST-like proteins might be compounds involved in secondary metabolism.