Investigating the secretory pathway of the baculovirus-insect cell system using a secretory green fluorescent protein

The secretory pathway is important in actively transporting proteins into the extracellular environment of eucaryotic cells. In this study a green fluorescent protein (GFP) mutant engineered to contain a secretion signal was used as a model protein in order to visualize the secretion process inside insect cells. Fluorescent microscopy indicated that significant amounts of secreted green fluorescent protein (sGFP) accumulated in High-Five, Trichoplusia ni, cells following infection with a baculovirus vector containing the gene under the polyhedrin promoter. Laser scanning confocal microscopy was used to reconstruct whole cell images of the infected High-Five cells at multiple days postinfection. While the protein was widely distributed at 2 days postinfection, certain intracellular regions appeared to contain higher or lower concentrations of the sGFP. A layer by layer examination indicated pockets in which sGFP was absent, and these appear to be vesicles that have recently released the sGFP or are not yet accumulating sGFP. By 3 days postinfection, the sGFP in some cells was concentrated in a number of widely dispersed globules, which may represent the vesicle remnants of a deteriorating secretory pathway. In contrast, nonsecreted GFP was more uniformly distributed in the cells than sGFP and did not accumulate in vesicles. In addition to GFP, the lectins wheat germ agglutinin (WGA) and concanavalin A (ConA), which have affinities for sugar residues, were used to examine the secretory pathway. The WGA, which is a Golgi marker, was distributed around the nucleus prior to infection but then was found to be polarized in one region of the cell following the baculovirus infection. The expansion of other cellular compartments following the baculovirus infection may have caused a change in intracellular distribution of the Golgi. While some of the sGFP was found to colocalize with the WGA label, much of the sGFP was outside this Golgi region. In contrast, ConA labeling, which was not as specific as WGA, was found throughout the cell both before and after infection similar to the sGFP distribution. These studies demonstrate that confocal visualization of fluorescent proteins can be used as an in vivo tool for examining secretory processing in insect cells.