Cell surface expression of 4β‐galactosyltransferase accompanies rat parotid gland acinar cell transition to growth

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Abstract
Rat parotid gland acinar cells stimulated to divide by a chronic regimen of isoproterenol demonstrate a dramatic increase in the synthesis of the glycosyltransferase 4β‐galactosyltransferase. A plasma membrane localization for much of the increase in 4β‐galactosyltransferase was determined by density gradient membrane fractionation. Golgi‐enriched fractions showed no increase in specific activity, while plasma membrane activity increased 40‐fold. This selective increase at the cell surface was confirmed by immunofluorescence of intact, nonpermeabilized cells from treated glands, using a monospecific antibody prepared against the purified bovine milk transferase. In detergent‐permeabilized cells staining of nontreated cells was seen only as groups of perinuclear vesicles, presumed to be Golgi apparatus. In isoproterenol‐treated and permcabilized cells both presumptive Golgi and cell surface staining was apparent. Enzyme assays performed on intact cells established that the enzyme's active site was oriented to the exterior of the cells. The transferase could be detected as early as 3 hr after the primary challenge with isoproterenol. Pretrcatment of rats with cycloheximide prevented its appearance.