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J. Biol. Chem., Vol. 281, Issue 41, 31106-31118, October 13, 2006

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The CMP-sialic Acid Transporter Is Localized in the Medial-Trans Golgi and Possesses Two Specific Endoplasmic Reticulum Export Motifs in Its Carboxyl-terminal Cytoplasmic Tail^*

Weihan Zhao, Tung-Ling L. Chen, Barbara M. Vertel, and Karen J. Colley¹

From the Department of Biochemistry and Molecular Genetics, University of Illinois, College of Medicine, Chicago, Illinois 60607 and the Department of Cell Biology and Anatomy, Rosalind Franklin University of Medicine and Science, North Chicago, Illinois 60064

The addition of sialic acid to glycoproteins and glycolipids requires Golgi sialyltransferases to have access to their glycoconjugate substrates and nucleotide sugar donor, CMP-sialic acid. CMP-sialic acid is transported into the lumen of the Golgi complex through the CMP-sialic acid transporter, an antiporter that also functions to transport CMP into the cytosol. We localized the transporter using immunofluorescence and deconvolution microscopy to test the prediction that it is broadly distributed across the Golgi stack to serve the many sialyltransferases involved in glycoconjugate sialylation. The transporter co-localized with ST6GalI in the medial and trans Golgi, showed partial overlap with a medial Golgi marker and little overlap with early Golgi or trans Golgi network markers. Endoplasmic reticulum-retained forms of sialyltransferases did not redistribute the transporter from the Golgi to the endoplasmic reticulum, suggesting that transporter-sialyltransferase complexes are not involved in transporter localization. Next we evaluated the role of the transporter's N- and C-terminal cytoplasmic tails in its trafficking and localization. The N-tail was not required for either endoplasmic reticulum export or Golgi localization. The C-tail was required for endoplasmic reticulum export and contained di-Ile and terminal Val motifs at its very C terminus that function as independent endoplasmic reticulum export signals. Deletion of the last four amino acids of the C-tail (IIGV) eliminated these export signals and prevented endoplasmic reticulum export of the transporter. This form of the transporter supplied limited amounts of CMP-sialic acid to Golgi sialyltransferases but was unable to completely rescue the transporter defect of Lec2 Chinese hamster ovary cells.

Received for publication, June 9, 2006 , and in revised form, July 17, 2006.

^* This work was supported by National Institutes of Health Grant RO1 GM48134 (to K. J. C.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Dept. of Biochemistry and Molecular Genetics, University of Illinois, College of Medicine, 900 S. Ashland Ave., M/C 669, Chicago, IL 60607. Tel.: 312-996-7756; Fax: 312-413-0353; E-mail: karenc{at}uic.edu.

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