Jos Jansen

Chapter 1 12 trans-Golgi Network). How the new glycoproteins traffic between the stacks remains unclear and several theories exist. The most widely accepted version is the Golgi maturation model.(18) In this model, the Golgi stacks gain a specific set of enzymes, or ‘mature’, as they progress from the cis-part to the TGN. A consequence of this model is that enzymes, such as glycosyltransferases needed for glycosylation, are present in gradients. These gradients are maintained by coatomer protein complex (COP)I vesicles that provide retrograde transport between Golgi stacks and between the Golgi and the ER/ERGIC.(17) In the TGN new proteins are budded off in vesicles destined for intercellular or extracellular locations. This topic will not be part of this thesis. Figure 2. Overview of the secretory pathway. The nascent protein is formed in the ER, the purple tunnel complex in the lower left corner. Thereafter, the Golgi (the green stacks) further finetunes the protein before packing it in endosomes. These endosomes traffic to the cell membrane for endocytosis and release of the protein in the extracellular matrix. Derived from De Matteis and Luini, NEJM, 2011 (19)