Behavior of a transitional tubulovesicular compartment at the cis side of the Golgi apparatus in in vivo fusion studies of mammalian cells.

We have investigated the behavior in in vivo cell fusion experiments of a transitional compartment lying between the endoplasmic reticulum and Golgi apparatus to determine if the compartment, as recognized by the antibody G1/93, might congregate in a similar manner to Golgi apparatus [W. C. Ho et al. (1990) Eur. J. Cell Biol. 52, 315-327]. The distributions of the transitional tubulovesicular compartment, endoplasmic reticulum, and Golgi apparatus in HeLa cells were assessed by immunofluorescent staining using mouse monoclonal antibody G1/93, mouse monoclonal antibody HP 24, and rabbit anti-galactosyltransferase, respectively. In agreement with previous results [W. C. Ho et al. (1990) Eur. J. Cell Biol. 52, 315-327], the Golgi apparatus was observed to congregate gradually over a 3- to 6-h period, forming a large, extended, central Golgi complex in uv-inactivated Sindbis virus-fused HeLa cells. Concomitant with this was a marked congregation of the transitional tubulovesicular compartment. Congregation of the tubulovesicular compartment was not affected by cycloheximide. The endoplasmic reticulum retained its web-like distribution throughout the syncytoplasm and rimmed the nuclear periphery. Treatment of HeLa cells with nocodazole prior to fusion followed by incubation of the syncytia in drug-containing media blocked congregation of the G1/93-positive compartment. With this long-term nocodazole treatment, Golgi apparatus was dispersed into scattered Golgi elements and the G1/93 distribution was endoplasmic reticulum-like. These results suggest that the transitional tubulovesicular compartment recognized by G1/93 is normally structured on microtubules and microtubule organizing centers and may be considered to be a subcompartment of a greater, perinuclear, Golgi complex.