J Biol Chem, Vol. 274, Issue 40, 28405-28412, October 1, 1999

Intracellular Localization of Human Cytidine Deaminase
IDENTIFICATION OF A FUNCTIONAL NUCLEAR LOCALIZATION SIGNAL

Angelika Somasekaram, Adam Jarmuz, Alan How, James Scott^¶, and Naveenan Navaratnam

From the  MRC Molecular Medicine Group, Clinical Science Centre and ^¶ Division of National Heart and Lung Institute, Imperial College School of Medicine, Hammersmith Hospital, Du Cane Road, London W12 ONN, United Kingdom

The cytidine deaminases belong to the family of multisubunit enzymes that catalyze the hydrolytic deamination of their substrate to a corresponding uracil product. They play a major role in pyrimidine nucleoside and nucleotide salvage. The intracellular distribution of cytidine deaminase and related enzymes has previously been considered to be cytosolic. Here we show that human cytidine deaminase (HCDA) is present in the nucleus. A highly specific, affinity purified polyclonal antibody against HCDA was used to analyze the intracellular localization of native HCDA in a variety of mammalian cells by in situ immunochemistry. Native HCDA was found to be present in the nucleus as well as the cytoplasm in several cell types. Indirect immunofluorescence microscopy indicated a predominantly nuclear localization of FLAG-tagged HCDA overexpressed in these cells. We have identified an amino-terminal bipartite nuclear localization signal that is both necessary and sufficient to direct HCDA and a non-nuclear reporter protein to the nucleus. We also show HCDA binding to the nuclear import receptor, importin . Similar putative bipartite nuclear localization sequences are found in other cytidine/deoxycytidylate deaminases. The results presented here suggest that the pyrimidine nucleotide salvage pathway may operate in the nucleus. This localization may have implications in the regulation of nucleoside and nucleotide metabolism and nucleic acid biosynthesis.