Temperature-sensitive Glycosaminoglycan Biosynthesis in a Chinese Hamster Ovary Cell Mutant Containing a Point Mutation in Glucuronyltransferase I.

In previous studies, we reported the isolation and characterization of a Chinese hamster ovary cell mutant (pgsG) defective in glucuronyltransferase I (GlcATI). This enzyme adds the terminal GlcA residue in the core protein-linkage tetrasaccharide (GlcAβl,3Galβl,3Galβ1, 4Xylβ-O-) on which glycosaminoglycan assembly occurs (Bai, X. M., Wei, G., Sinha, A., and Esko, J. D. (1999) J. Biol. Chem. 274, 13017-13024; Wei, G., Bai, X. M., Sarkar, A. K., and Esko, J. D. (1999) J. Biol. Chem. 274, 7857-7864). Here we show that incorporation of 35SO4 into glycosaminoglycans in the mutant is temperature-sensitive, with greater synthesis occurring at 33 °C compared with 37 °C. Wild-type cells show the opposite thermal dependence. Rabbit antiserum to hamster GlcATI failed to detect cross-reactive material in pgsG cells by immunofluorescence and Western blotting. Furthermore, expression of chimeric proteins composed of mutant GIcATI fused to IgG binding domain of protein A or to green fluorescent protein did not yield the proteins at the expected mass. The green fluorescent proteintagged version appeared as a truncated protein, and immunofluorescence showed large perinuclear bodies at 30 °C. At 37 °C, the fusion protein was not readily detectable. Sequencing cDNAs from mutant and wildtype cells revealed a single base transition (G331A) in the open reading frame in pgsG cells, which resulted in a Val-111 → Met substitution. These data suggest that pgsG cells contain a labile form of GlcATI that causes conditional expression of glycosaminoglycans dependent on temperature. [ABSTRACT FROM AUTHOR]