Abstract
Human histo-blood group A transferase (AT) catalyzes the biosynthesis of oligosaccharide A antigen important in blood transfusion and cell/tissue/organ transplantation. This enzyme may synthesize Forssman antigen (FORS1) of the FORS blood group system when exon 3 or 4 of the AT mRNA is deleted and/or the LeuGlyGly tripeptide at codons 266–268 of AT is replaced by GlyGlyAla. The Met69Ser/Thr substitutions also confer weak Forssman glycolipid synthase (FS) activity. In this study, we prepared the human AT derivative constructs containing any of the 20 amino acids at codon 69 with and without the GlyGlyAla substitution, transfected DNA to newly generated COS1(B3GALNT1 + A4GALT) cells expressing an enhanced level of globoside (Gb4), the FS acceptor substrate, and immunologically examined the FORS1 expression. Our results showed that all those substitution constructs at codon 69 exhibited FS activity. The combination with GlyGlyAla significantly increased the activity. The conserved methionine residue in the ABO, but not GBGT1, gene-encoded proteins may implicate its contribution to the separation of these genes in genetic evolution. Surprisingly, with increased Gb4 availability, the original human AT with the methionine residue at codon 69 was also demonstrated to synthesize FORS1, providing another molecular mechanism of FORS1 appearance in cancer of ordinary FORS1-negative individuals.
Highlights
Human histo-blood group A transferase (AT) catalyzes the biosynthesis of oligosaccharide A antigen important in blood transfusion and cell/tissue/organ transplantation
The methionine residue was rarely present at that position in the GBGT1 genes it was coincidentally found in mangrove killifish
In our previous studies utilizing COS1(B3GALNT1) cells as DNA transfection recipients, we showed that the Met69Ser/Thr or LeuGlyGly266-268GlyGlyAla substitution of the human AT, or the deletion of exon 3 or 4 of the human AT mRNA resulted in the generation of proteins with the Forssman glycolipid synthase (FS) activity[16,24]
Summary
Human histo-blood group A transferase (AT) catalyzes the biosynthesis of oligosaccharide A antigen important in blood transfusion and cell/tissue/organ transplantation This enzyme may synthesize Forssman antigen (FORS1) of the FORS blood group system when exon 3 or 4 of the AT mRNA is deleted and/or the LeuGlyGly tripeptide at codons 266–268 of AT is replaced by GlyGlyAla. The Met69Ser/Thr substitutions confer weak Forssman glycolipid synthase (FS) activity. Using the DNA transfection assays of African green monkey kidney COS1 cells expressing Gb4 as recipients, we showed that the substitution of the Ser to Gly or the Arg to glutamine (Gln), corresponding amino acids of the FSs in Forssman antigen-positive mice and some other species, rendered the human protein with weak FS activity whereas the reversion of both conferred strong FS activity. The Arg269Gln reversion was later found to be responsible for the activation of GBGT1 gene in rare FORS1 + individuals[3]
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