CTP-PHOSPHOETHANOLAMINE CYTIDYLYLTRANSFERASE

CTP:phosphoethanolamine cytidylyltransferase (ET) catalyzes the conver sion of phosphoethanolamine into CDP-ethanolamine. Immunogold electron microscopy studies have demonstrated that, in hepatocytes, ET is loca lized predominantly in areas of the cytoplasm that are rich in rough e ndoplasmic reticulum (RER). Within these areas the enzyme shows a bimo dal distribution between the cisternae of the RER and the cytosolic sp ace. Studies on the substrate specificity of ET have shown that it can utilize both CTP and dCTP as substrates, but not other trinucleotides . In addition, the enzyme shows a very pronounced specificity for phos phoethanolamine. Under most conditions ET contributes significantly to the overall regulation of the CDP-ethanolamine pathway. Reversible bi nding of the enzyme to the endoplasmic reticulum could potentially pla y a key-role in metabolic channeling of phosphatidylethanolamine synth esis. ET has been purified from rat liver. Convincing evidence has bee n provided that ET and CTP:phosphocholine cytidylyltransferase (CT), t he analogous enzyme in the CDP-choline pathway, are separate activitie s that reside on different proteins. The gene coding for yeast ET has been cloned. The deduced amino acid sequence contained a region in the N-terminal half with significant similarities to the conserved cataly tic domain of both yeast and rat CT. The human cDNA for ET was also cl oned recently. The predicted amino acid sequence of human ET shows a h igh degree of similarity (36% identity) to that of yeast ET, but the h uman protein is longer than the yeast protein, especially at the C-ter minal region. Interestingly, both yeast and human ET have a large repe titive sequence in their N-terminal and C-terminal half. (C) 1997 Else vier Science B.V.