Enzymatic basis for blood groups in man

A historical overview of advances in molecular genetic/genomic studies of the ABO blood group system

In 1990, 90 years after the discovery of ABO blood groups by Karl Landsteiner, my research team at the Molecular Biology Laboratory of the now-defunct Biomembrane Institute elucidated the molecular genetic basis of the ABO polymorphism. Henrik Clausen, Head of the Immunology Laboratory, initiated the project by isolating human group A transferase (AT), whose partial amino acid sequence was key to its success. Sen-itiroh Hakomori, the Scientific Director, provided all the institutional support. The characterization started from the 3 major alleles (A1, B, and O), and proceeded to the alleles of A2, A3, Ax and B3 subgroups and also to the cis-AB and B(A) alleles, which specify the expression of A and B antigens by single alleles. In addition to the identification of allele-specific single nucleotide polymorphism (SNP) variations, we also experimentally demonstrated their functional significance in glycosyltransferase activity and sugar specificity of the encoded proteins. Other scientists interested in blood group genes later characterized more than 250 ABO alleles. However, recent developments in next-generation sequencing have enabled the sequencing of millions of human genomes, transitioning from the era of genetics to the era of genomics. As a result, numerous SNP variations have been identified in the coding and noncoding regions of the ABO gene, making ABO one of the most studied loci for human polymorphism. As a tribute to Dr. Hakomori's scientific legacy, a historical overview in molecular genetic/genomic studies of the human ABO gene polymorphism is presented, with an emphasis on early discoveries made at his institute.

Regulation of glycoprotein biosynthesis by formation of specific glycosyltransferase complexes

Eukaryotic surfaces simultaneously express a complex array of protein-linked oligosaccharides and, during differentiation and on neoplastic transformation, these arrays change. The oligosaccharides linked to asparagine residues of proteins are a complex family of structures that are derived from a common precursor oligosaccharide by a network of competing biosynthetic pathways. The glycosylation reactions that make up these biosynthetic pathways often share common intermediates, requiring the competition between rival glycosyltransferases to be regulated at the supramolecular level. Investigation of two sequential glycosyltransferases that together add N-acetyllactosamine to glycoproteins has revealed their ability to form specific complexes. The functional consequences of complex formation were assessed by investigating the coupled reaction carried out by these sequential enzymes. The membrane enzymes are readily adsorbed by preformed liposomes, and their ability to interact after liposome adsorption has allowed direct investigation of the action carried out by these sequential enzymes when they are coadsorbed to the same liposome or adsorbed to separate liposomes shows the preferential use of endogenously generated intermediate over exogenously added glycoproteins. This facilitated passage of the intermediate glycosylated glycoprotein within the complex results in a tight coupling of the sequential enzymes. The formation of such complexes by sequential glycosyltransferases is proposed as a mechanism for controlling the competition between potentially rival glycosylation sequences during glycoprotein synthesis. The formation of different specific complexes under different conditions provides a flexible mechanism for regulating the synthesis of cell surface glycoproteins during development.