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The Golgi apparatus—the organelle with the funny name—serves as a sort of post office for the cell, receiving newly synthesized proteins from the endoplasmic reticulum, appending post-translational modifications and then sending the modified proteins off to the correct cellular destination. For the last several decades there have been two models—the vesicular transport model and the cisternal maturation model—to describe how secretory proteins move from the early (cis) Golgi cisternal compartment to the late (trans) cisternae.

According to the vesicular transport model, Golgi cisternae are predicted to be stable, unchanging compartments, whereas they are thought to be transient, evolving structures according to the cisternal maturation model. Though biochemical evidence provided support for both theories (or in some cases, a hybrid of the two), visual confirmation by high-resolution live-cell imaging was missing. Now, as described in two papers published simultaneously in Nature, technical developments in confocal microscopy allowed the independent research groups of Benjamin Glick at the University of Chicago and Akihiko Nakano at RIKEN Discovery Research Institute in Japan to obtain live-cell images of fluorescently labeled Golgi cisternae (Losev et al., 2006 and Matsuura-Tokita et al., 2006).

Both Glick and Nakano chose to work with yeast cells, in which the Golgi cisternae are not stacked, allowing easier visualization of individual cisternae. The largest technical hurdle was to develop advanced imaging techniques; both groups developed custom-built three-dimensional confocal video microscopy systems allowing multicolor live-cell imaging at unprecedented resolution. “We can now clearly observe the tubular network structure of yeast Golgi cisternae, which we never imagined to see without the use of electron microscopy,” says Nakano. Both groups used GFP to label a cis Golgi resident protein and a red fluorescent protein variant to label a trans Golgi protein (or vice versa). They hypothesized that if proteins moving through the Golgi are transported by vesicles, then the green fluorescence marking an individual cis cisterna should be retained indefinitely. If cisternae mature from a cis state to a trans state, however, then the fluorescence should change from green to red as cisternae acquire new biochemical properties.

Both groups clearly observed that individual green cis cisternae changed color to red over a matter of minutes (Fig. 1). “The color change clearly ruled out the classic simple vesicular transport model,” explains Nakano. Glick was also surprised that the visual evidence for cisternal maturation was so clear-cut. “We had been concerned that Golgi cisternae might undergo frequent fusion and fission, and that these events would complicate the interpretation,” he says.

Figure 1: Video microscopy frames showing that the resident Golgi protein composition of individual cisternae changes over time, in support of the maturation model (Losev et al., 2006).
figure 1

Green, cis cisternae; red, trans cisternae. Scale bar, 2 μm. Reprinted with permission from Nature Publishing Group.

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The success Glick and Nakano have had in answering a long-standing question in cell biology is also an excellent example of how working together rather than racing to compete for the same goal can provide fruitful results. “These papers will be scrutinized closely, so it was important to take the time to do the experiments right,” says Glick. “The fact that our two groups independently reached similar conclusions is reassuring and should also make the story more persuasive to others in the field.”