To find out the differences between trans Golgi and the cell-surface beta (1-4) galactosyltransferase (GT) enzyme whose synthesis are differentially regulated we continued this year the studies on the primary structure of GT in relation to its gene structure. The 1st exon of GT gene codes for the first 141 residues of the 401 residue long precursor GT protein. 72-residue-NH2-terminal peptide of the 141 residue domain, coded by the 1st Exon, contains the membrane anchoring domain and is cleaved to generate the soluble and secreted form of GT. The 1st exon is separated from the second exon by about 12Kb intervening sequence. The amino-terminal end sequence of the low molecular form of secreted and soluble form of the protein, which is enzymatically active, corresponds to the nucleotide sequence of the second exon. We also show that a) there are tissue specific variations in the size of 3'-noncoding sequence of GT mRNAs which are generated by utilizing other potential polyadenylation signals than the canonical sequence. In the lactating mammary gland of various species there is, besides the major 4.2Kb mRNA, a 2.2Kb mRNA which is shorter at the 3'-noncoding region. In addition there are multiple poly(A)+RNAs which are devoid of the 3'-end sequence coding for the COOH-terminal end of the enzyme. Our results suggest the possibility that the surface and Golgi GT might be generated by alternate splicing or from second copy of gene or both.
