DS28-6, a temperature-sensitive mutant of Chinese hamster ovary cells, expresses key phenotypic changes associated with brefeldin A treatment.

The temperature-sensitive Chinese hamster ovary (CHO) cell mutant DS28-6 has been previously shown to be pleiotropically defective in protein secretion. We have examined the mutant cells to determine the intracellular site of the block in secretion. By transmission electron microscopy a time-dependent disassembly of the Golgi apparatus was found under nonpermissive temperature, which resulted in the loss of the cisternal stack. Complete reorganization of the Golgi apparatus occurred after shift to permissive temperature. Under nonpermissive temperature, a microtubule- and energy-dependent redistribution of Golgi mannosidase II and galactosyltransferase into a pattern characteristic of the endoplasmic reticulum (ER) was observed. Inhibition of protein synthesis by cycloheximide had no influence on Golgi mannosidase II redistribution. Evidence for Golgi apparatus-associated processing of oligosaccharides in the ER was obtained by lectin-gold cytochemistry revealing the presence of the galactose (beta 1----4)N-acetylglucosamine sequence and sialic acid residues. Furthermore, 7-nitrobenz-2-oxa-1,3-diazol-4-yl-tagged ceramide, a lipidic trans-Golgi apparatus marker in CHO cells, exhibited an energy-dependent redistribution into the ER. These effects were fully reversible upon shift to permissive temperature. Thus, mutant DS28-6 cells exhibit key features of the brefeldin A phenotype, which suggests that the observed brefeldin A effects result from interference with a normally occurring cellular process.