Two new, putative alpha-1-3/4 fucosyltransferases ([alpha]1-3/4 Fuc-Ts), Fuc-TX and Fuc-TXI, were identified in the vertebrate genome and transcriptome sequence databases through sequence homology-based queries. These proteins have a significant sequence similarity to only [alpha]1-3/4 Fuc-Ts, and possess peptide motifs that are evolutionarily conserved among the known vertebrate [alpha]1-3/4 Fuc-Ts. However, Fuc-TX and Fuc-TXI lack the HH[R/W][D/E] sequence that determines the specificity for type 1 or 2 substrates among the known vertebrate enzymes, and Fuc-TXI proteins do not possess a transmembrane domain. The Fut10 and Fut11 genes that encode these proteins are expressed ubiquitously in the adult mouse and in the mouse embryo throughout development. Though a Fuc-T activity of the mouse proteins could not be detected, Fuc-TXI, but not Fuc-TX, was found to hydrolyze GDP-fucose. The interaction of Fuc-TXI with GDP-fucose was also confirmed by its binding to GDP-hexanolamine. In zebrafish, Fut11 transcripts could be detected during early embryonic development. A knock-down of Fuc-TXI in zebrafish embryos with Fut11-specific antisense morpholino oligonucleotides resulted in malformations of the posterior trunk and tail
