IDENTIFICATION OF AN 11-RESIDUE PORTION OF CTP-PHOSPHOCHOLINE CYTIDYLYLTRANSFERASE THAT IS REQUIRED FOR ENZYME-MEMBRANE INTERACTIONS

CTP-phosphocholine cytidylyltransferase (CT) is a key regulatory enzym e in the biosynthesis of phosphatidylcholine (PC) in many cells. Enzym e-membrane interactions appear to play an important role in CT activat ion. A putative membrane-binding domain appears to be located between residues 236 and 293 from the N-terminus. To map the membrane-binding domain more precisely, glutathione S-transferase fusion proteins were prepared that contained deletions of various domains in this putative lipid-binding region. The fusion proteins were assessed for their bind ing of [H-3]PC/oleic acid vesicles. Fusion proteins encompassing resid ues 267-277 bound to PC/oleic acid vesicles, whereas fragments lacking this region exhibited no specific binding to the lipid vesicles. The membrane-binding characteristics of the CT fusion proteins were also e xamined using intact lung microsomes. Only fragments encompassing resi dues 267-277 competed with full-length I-125-labelled CT, expressed in recombinant Sf9 insect cells, for microsomal membrane binding. To inv estigate the role of this region in PC biosynthesis, A549 and L2 cells were transfected with cDNA for CT mutants under the control of a gluc ocorticoid-inducible long terminal repeat (LTR) promoter. Induction of CT mutants containing residues 267-277 in transfectants resulted in r educed PC synthesis. The decrease in PC synthesis was accompanied by a shift in endogenous CT activity from the particulate to the soluble f raction. Expression of CT mutants lacking this region in A549 and L2 c ells did not affect PC formation and subcellular distribution of CT ac tivity. These results suggest that the CT region located between resid ues 267 and 277 from the N-terminus is required for the interaction of CT with membranes.