Mucin biosynthesis: bovine C2GnT-M gene, tissue-specific expression, and herpes virus-4 homologue

Mucin glycans are the major determinant of mucin functions. Mucin glycan branch structures, which increase structural heterogeneity and thus functional potential, are extended from beta6 N-acetylglucosaminides formed by beta6 N-acetylglucosaminyltransferases (beta6GnT). Core 2 beta6GnT-M (C2GnT-M) is the only branching enzyme that can synthesize all known mucin beta6 N-acetylglucosaminides. We report the cloning of four different bovine (b) C2GnT-M transcripts that are different only at 5'-untranslated regions. Two bC2GnT-M transcripts are found exclusively in tracheal epithelium and testis, whereas the other two are found in all other mucus-secreting tissues. The bC2GnT-M gene contains four exons spanning 5.3 kb, and the entire open reading frame is in one exon. The bC2GnT-M ORF has 95, 83, and 75% sequence identity to those of bovine herpes virus type 4 (BHV-4), human, and rat C2GnT-Ms, respectively. The homology between bovine and BHV-4 C2GnT-M genes is in the region between 170 nucleotides upstream from ATG start codon and 114 nucleotides downstream from TGA stop codon of the viral gene. Localized at the nonconserved region of the viral genome, the BHV-4 C2GnT-M gene is the only known viral C2GnT-M gene. The results suggest that BHV-4 acquired its C2GnT-M gene from the bovine gene. The mechanism of the viral acquisition of bC2GnT-M gene and the roles of the C2GnT-M gene in the survival and pathogenesis of this virus remain to be elucidated.