Lykidis A, Baburina I, Jackowski S
Department of Biochemistry, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA.
J Biol Chem. 1999 Sep 17;274(38):26992-7001. doi: 10.1074/jbc.274.38.26992.
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.
磷酸胆碱胞苷转移酶是磷脂酰胆碱生物合成的主要调节因子。单一异构体CCTα已被广泛研究，最近又鉴定出第二种异构体CCTβ。我们鉴定并表征了第三个cDNA，即CCTβ2，它在羧基末端与CCTβ1不同，预计是CCTβ基因的剪接变体。与CCTα一样，CCTβ2在体内高度磷酸化，而CCTβ1则不然。在所检测的人体组织中，CCTβ1和CCTβ2的mRNA表达存在差异，而CCTα的表达则更为均匀。使用异构体特异性抗体，CCTβ1和CCTβ2均定位于细胞的内质网，而CCTα除了与内质网结合外还存在于细胞核中。CCTβ2蛋白在体外具有酶活性，并且能够弥补CHO58细胞中温度敏感型胞苷转移酶的缺陷，就像CCTα和CCTβ1在非允许条件下支持细胞增殖一样。过表达实验并未揭示这三种催化相同生化反应的异构体具有离散的生理功能；然而，不同的细胞定位和组织特异性分布表明，CCTβ1和CCTβ2可能发挥与普遍表达的CCTα不同的作用。
