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J. Biol. Chem., Vol. 281, Issue 43, 32852-32860, October 27, 2006

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Modulation of GalT1 and SialT1 Sub-Golgi Localization by SialT2 Expression Reveals an Organellar Level of Glycolipid Synthesis Control^*

From the Centro de Investigaciones en Química Biológica de Córdoba, CIQUIBIC (Universidad Nacional de Córdoba-Consejo Nacional de Investigaciones Científicas y Técnicas (UNC-CONICET)), Departamento de Química Biológica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Ciudad Universitaria, 5000 Córdoba, Argentina

Ganglioside glycosyltransferases organize as multienzyme complexes that localize in different sub-Golgi compartments. Here we studied whether in CHO-K1 cells lacking CMP-NeuAc: GM3 sialyltransferase (SialT2), the sub-Golgi localization of UDP-Gal:glucosylceramide -1,4-galactosyltransferase (GalT1) and CMP-NeuAc:lactosylceramide sialyltransferase (SialT1) complex is affected when SialT2, another member of this complex, is coexpressed. GalT1 and SialT1 sub-Golgi localization was determined by studying the effect of brefeldin A (BFA) and monensin on the synthesis of glycolipids and on the sub-Golgi localization of GalT1_1-52-CFP (cyan fluorescent protein) and SialT1_1-54-YFP (yellow fluorescent protein) chimeras by single cell fluorescence microscopy and by isopycnic subfractionation. We found that BFA, and also monensin, impair the synthesis of glycolipids beyond GM3 ganglioside in wild type (WT) cells but beyond GlcCer in SialT2⁺ cells. Although BFA redistributed GalT1-CFP and SialT1-YFP to the endoplasmic reticulum in WT cells, a fraction of these chimeras remained associated with a distal Golgi compartment, enriched in trans Golgi network, and recycling endosome markers in SialT2⁺ cells. In BFA-treated cells, the percentage of GalT1-CFP and SialT1-YFP associated with Golgi-like membrane fractions separated by isopycnic subfractionation was higher in SialT2⁺ cells than in WT cells. These effects were reverted by knocking down the expression of SialT2 with specific siRNA. Results indicate that sub-Golgi localization of glycosyltransferase complexes may change according to the relative levels of the expression of participating enzymes and reveal a capacity of the organelle to adapt the topology of the glycolipid synthesis machinery to functional states of the cell.

Received for publication, June 16, 2006 , and in revised form, August 23, 2006.

^* This work was supported in part by grants from the Howard Hughes Medical Institute (to H. J. F. M.), Secretaría de Ciencia y Tecnología, Universidad Nacional de Córdoba (to J. L. D. and H. J. F. M.), CONICET (to J. L. D. and H. J. F. M.), Fundación Antorchas (to J. L. D.), and Agencia Nacional de Promoción Científica y Tecnológica (to J. L. D. and H. J. F. M.). 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 Recipients of CONICET Fellowships.

² Present address: Laboratory of Cell Biology, NHLBI, National Institutes of Health, Bethesda, MD 20892.

³ Career Investigators of CONICET.

⁴ To whom correspondence should be addressed. Tel.: 54-351-4334168/4334171; Fax: 54-351-4334074; E-mail: maccioni{at}dqb.fcq.unc.edu.ar.

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