This week, the image of the week highlights the Golgi apparatus. This week's contribution is brought to us by Peter Thul, a postdoctoral researcher who works on the Subcellular protein atlas, and specializes in understanding secretory pathway.

The Golgi apparatus was discovered by the Italian physician and scientist Camillo Golgi, who discovered the fine membraneous structure in 1898 (Mazzarello P., Garbarino C., & Calligaro A. 2009). Since then it has frequently drawn the attention of researcher because of its prominent role in the secretory pathway of cells.

The secretory pathway describes the route that proteins take to get to the outside of the cell. The Golgi apparatus functions as the hub on this route: all proteins that enter the Golgi apparatus are modified, packed and sorted to the correct exit site. From there, proteins leave the Golgi apparatus in small transport vesicles toward the plasma membrane, where they are secreted to the extracellular space (Farquhar M.G. & Palade G.E. 1998).

Not only do secreted proteins pass through the Golgi apparatus, but proteins on the cell surface also have to take this pathway. Cell surface proteins often have characteristic sugars on them, and these sugars are bound by Golgi-resident proteins (a process called glycosylation). The importance of this is illustrated by the blood group system: depending on which version of a Golgi protein you have, a different sugar appears on the surface of red blood cells that determines your blood type (Molecular Cell Biology. 4th edition)!!!

This system of cellular transport does not just determine who what blood type you can receive in the hospital, but also plays a major role in your health. Mutations in the glycosylation pathway have been shown to cause a number of neuromuscular diseases such as Duchenne's muscular distrophy (Freeze H.H. Ng B.G. 2011).

Figure 1 shows the antibody staining of the protein Golgin A2, (also known as GM130), in A-431 epidermal carcinoma cells. Golgin A2 belongs to a family of proteins that is required to maintain the distinct structure and positioning of the human Golgi apparatus: it is located close to the nucleus and consists of interconnected stacks of flattened membrane sacks, which give rise to the look of a twisted ribbon (Witkos T.M. & Martin Lowe M. 2016).

It is interesting to note that this structure is only typical for the Golgi in higher animals such as mammals! In other organisms, such as yeast or plants the Golgi is spread out in small units throughout the whole cell. Why the Golgi apparatus has this typical architecture in humans is still discussed, however it might be connected to the regulation of the cell cycle (paper: Jen-Hsuan W. & Seemann J. 2010). So, even after more than 100 years of research, the Golgi apparatus still has its secrets!

We would like to thank all the members of the Subcellular Human Protein Atlas who generate these images and especially to Peter Thul for contributing this article about the Golgi apparatus.