CTP

phosphocholine cytidylyltransferase is a major regulator of phosphatidylcholine biosynthesis. A single isoform, CCTalpha, has been studied extensively and a second isoform, CCTbeta, was recently identified. We identify and characterize a third cDNA, CCTbeta2, that differs from CCTbeta1 at the carboxyl-terminal end and is predicted to arise as a splice variant of the CCTbeta gene. Like CCTalpha, CCTbeta2 is heavily phosphorylated in vivo, in contrast to CCTbeta1. CCTbeta1 and CCTbeta2 mRNAs were differentially expressed by the human tissues examined, whereas CCTalpha was more uniformly represented. Using isoform-specific antibodies, both CCTbeta1 and CCTbeta2 localized to the endoplasmic reticulum of cells, in contrast to CCTalpha which resided in the nucleus in addition to associating with the endoplasmic reticulum. CCTbeta2 protein has enzymatic activity in vitro and was able to complement the temperature-sensitive cytidylyltransferase defect in CHO58 cells, just as CCTalpha and CCTbeta1 supporting proliferation at the nonpermissive conditions. Overexpression experiments did not reveal discrete physiological functions for the three isoforms that catalyze the same biochemical reaction; however, the differential cellular localization and tissue-specific distribution suggest that CCTbeta1 and CCTbeta2 may play a role that is distinct from ubiquitously expressed CCTalpha.