Architecture of the mammalian Golgi

Abstract

Since its first visualization in 1898, the Golgi has been a topic of intense morphological research. A typical mammalian Golgi consists of a pile of stapled cisternae, the Golgi stack, which is a key station for modification of newly synthesized proteins and lipids. Distinct stacks are interconnected by tubules to form the Golgi ribbon. At the entrance site of the Golgi, the cis-Golgi, vesicular tubular clusters (VTCs) form the intermediate between the endoplasmic reticulum and the Golgi stack. At the exit site of the Golgi, the trans-Golgi, the trans-Golgi network (TGN) is the major site of sorting proteins to distinct cellular locations. Golgi functioning can only be understood in light of its complex architecture, as was revealed by a range of distinct electron microscopy (EM) approaches. In this article, a general concept of mammalian Golgi architecture, including VTCs and the TGN, is described.

Figure 1.

Schematic representation of the Golgi and associated membrane networks. (A) Overview of the Golgi stack in relation to ERES, central (c) and peripheral (p) VTCs and TGN and the occurrence of COPII, COPI, and clathrin coats. Membranes are drawn as if cross-sectioned in side view. Because of space limitations the COPI vesicles are indicated at only one side of the Golgi stack and the noncompact zone is only drawn at the cis-cisterna. CV = condensing vacuole. (B) Tangential view of two cisternae (C1 and C2) from adjacent Golgi stacks that are bridged by the tubular network of the noncompact zone.

Figure 2.

Golgi region of a human dendritic (D1) cell. Cells were prepared by high-pressure-freezing fixation and freeze-substitution for examination by EM. Multiple Golgi stacks (G) are surrounded by numerous vesicles on which the characteristic dense, regular, coatomer protein complex-I (COPI) coat is clearly visible (arrows). Similar coats are also seen on the rims of Golgi cisternae (arrows); these COP coats are markedly different from the more spiky and wider clathrin coat that is abundant at the trans-side of the Golgi (arrowheads). The asterisk indicates a tangential section of a Golgi cisterna. M = mitochondrion, E = endosomes. Bar, 200 nm. This figure was modified from Rabouille and Klumperman (2005) and reprinted with permission from Nature Review Molecular Cell Biology © 2005.

Figure 3.

Interface between ER exit sites, VTCs and the Golgi stack. Human hepatoma HepG2 cells were fixed by a mixture of formaldehyde and glutaraldehyde and prepared for ultrathin cryosectioning and immunoEM. (A) Immunogold labeling for the VTC marker ERGIC53 (10 nm gold) shows a budding profile (arrow) on an ER cisterna. The ERES faces a characteristic tubulovesicular VTC. (B) Double immunogold labeling for COPII (10 nm gold) and the ER-Golgi SNARE hsec22b (15 nm gold) outlines ERES located in the central Golgi region. Note that the membrane buds emerging from the ER (arrows) are labeled for COPII. G = Golgi stack. Bars, 200 nm. Figure 3A is modified from Hay et al. (1998) and reprinted with permission from Journal of Cell Biology © 1998. Figure 3B is modified from Klumperman et al. (1998) and reprinted with permission from Journal of Cell Science © 1998.

Figure 4.

Interface between the Golgi stack, VTCs and TGN. Human hepatoma HepG2 cells were fixed using a mixture of formaldehyde and glutaraldehyde and prepared for ultrathin cryosectioning and immunoEM. (A) Immunogold labeling for the VTC marker ERGIC53 (10 nm gold) illustrates the boundary between VTCs and the Golgi stack (G). ERGIC53 label is restricted to the VTC membranes and cis-most cisterna of the Golgi stack. The TGN is devoid of ERGIC-53 label. The arrowhead points to a clathrin-coated TGN membrane. (B) Immunogold labeling for the TGN markers TGN46 (10 nm gold) and cation-independent mannose 6-phosphate receptor (CI-MPR; 15 nm gold) outlines the cisternal and tubular membranes of the TGN. Arrowheads point to clathrin-coated TGN membranes. N = nucleus, P = plasma membrane. Bars, 200 nm. Figure 4A is modified from Klumperman et al. (1998) and reprinted with permission from Journal of Cell Science © 1998.

Figure 5.

Replica’s of isolated Golgi cisternae showing tubular interconnections and coated budding profiles. Images were prepared from a Golgi enriched membrane fraction of Chinese hamster ovary cells (CHO). (A) A bud protruding from a nonfenestrated edge of the central part of a cisterna. (B) A coated bud protruding from the side of a tubular loop (arrow). (C) A fully formed coated vesicle with fibrous attachments to the cisterna. (D) A short tubule with a coated bud at the tip. Arrows point to tubules that seem to bend back and fuse with the edge of the same cisterna that they emerge from, thus forming an irregular fenestration. Although not formally identified in these preparations, current evidence indicates that the coats shown here are of the COPI type. Bar, 0.5 µm. This figure was taken from Weidman et al. (1993) and reprinted with permission from Cell © 1993.