Distribution of CTP:Phosphocholine Cytidylyltransferase (CCT) Isoforms

Abstract

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