PRIMARY STRUCTURE AND TOPOLOGY OF BETA 1-4 GALATOSYLTRANSFERASE

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Studies on the primary structure of beta 1-4 galactosyltransferase (1-4 galtransferase) continued in this year. To deduce the complete amino acid sequence of 1-4 galtransferase, genomic clones were isolated. NH2-terminal end sequence derived from the cDNA and genomic clones showed that the 1-4 galtransferase is synthesized as a larger precursor. The soluble and secreted form is missing approximately 72-residue-NHz-terminal peptide which contains the membrane anchoring domain as well as the potential protease susceptible sequence - Arg-x-x-Arg-Arg-Leu-. The milk enzyme can be generated from the larger precursor by proteolytic cleavage between the NHz-signal-anchor and the catalytic domain. The topology of the Golgi and cell surface 1-4 galtransferase can be predicted to be similar to a class of membrane proteins, like the transferrin receptor, the asialoglycoprotein receptor, the beta- galactoside alpha-2, 6-sialyltransferase, the influenza virus sialidase etc., which have an NH2-terminal "stem" structure extending from the luminal side of the transmembrane bilayer to the cytoplasmic side. The bulk of the protein structure, which carries the main function, faces the lumen side.