Distribution of CTP:phosphocholine cytidylyltransferase (CCT) isoforms -: Identification of a new CCTβ splice variant

CTP:phosphocholine cytidylyltransferase is a major regulator of phosphatidylcholine biosynthesis. A single isoform, CCT alpha, has been studied extensively and a second isoform, CCT beta, was recently identified. We identify and characterize a third cDNA, CCT beta 2, that differs from CCT beta 1 at the carboxyl-terminal end and is predicted to arise as a splice variant of the CCT beta gene. Like CCT alpha, CCT beta 2 is heavily phosphorylated in vivo, in contrast to CCT beta 1. CCT beta 1 and CCT beta 2 mRNAs were differentially expressed by the human tissues examined, whereas CCT alpha was more uniformly represented. Using isoform-specific antibodies, both CCT beta 1 and CCT beta 2 localized to the endoplasmic reticulum of cells, in contrast to CCT alpha which resided in the nucleus in addition to associating with the endoplasmic reticulum. CCT beta 2 protein has enzymatic activity in vitro and was able to complement the temperature-sensitive cytidylyltransferase defect in CHO58 cells, just as CCT alpha and CCT beta 1 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 CCT beta 1 and CCT beta 2 may play a role that is distinct from ubiquitously expressed CCT alpha.