A major step during the cell cycle is the partitioning of different organelles between daughter cells during the division process. Organelles with endosymbiotic origins such as mitochondria and chloroplasts cannot form de novo and are replicated by division. The Golgi apparatus however, with a close structural and functional relationship to the ER, displays different mechanisms of inheritance (52).
Different Models of Golgi Biogenesis
Two ways by which Golgi stacks could multiply are discussed in the literature: either by de novo formation from the ER or through fission of an existing stack (53). In animal cells, mitosis leads to a complete breakdown of Golgi stacks during prophase and remnant mitotic vesicular Golgi clusters may be formed (54); however, the relationship of these small clusters with the ER, and the importance of the ER in the organization of Golgi remnants and in the reconstruction of the Golgi, is a hotly debated topic (55,56). It has been reported that such Golgi clusters formed in telophase are segregated in pairs between daughter cells and fuse just before completion of cytokinesis (57). Alternatively, Golgi stacks may form de novo either from ERES, as in the yeast P. pastoris(58–61), or from mitotic vesicular clusters (56,62,63). A model for Golgi disassembly and reassembly during mitosis in mammalian cells has been proposed in which sequential inactivation of Sar1 and Arf1 leads to disruption of ERES and redistribution of Golgi enzymes to the ER, whereas sequential activation of those two proteins initiates Golgi reformation (55). In Toxoplasma gondii, an intracellular protozoan parasite, the Golgi apparatus is a single copy organelle that grows by lateral extension and undergoes medial fission during cell division (64). Studies on other protozoan parasites like Trypanosoma brucei have shown de novo synthesis, suggesting that both models of Golgi biogenesis can exist in protists (65–67).
Golgi Biogenesis in Plants and Algae
It is well documented in plant cells that Golgi bodies can reform on washout of Brefeldin A (BFA) from treated material (68), indicating that the ER has the capacity to generate Golgi de novo. Hanton et al. (17) have shown, using Sec24 as a marker, that de novo export site formation can be cargo induced, indicating that perhaps Golgi bodies can form in response to cargo production if export sites and Golgi stacks do behave as a single unit. During mitosis and cytokinesis in plants, Golgi bodies and membranes do not disaggregate as in mammalian cells. Whether secretion per se stops is not known, but from late anaphase onwards, the Golgi apparatus is highly active in producing new cell wall membrane and polysaccharide for the phragmoplast region (69,70). Data on Golgi inheritance in higher plant cells are contradictory. Golgi stacks were reported to double during metaphase in onion root meristems (71), while duplication was claimed to occur during cytokinesis in synchronized cultures of Catharanthus roseus(72). More recently, a tomographic analysis of Arabidopsis shoot meristem cells demonstrated a doubling of the number of Golgi stacks in G2 just prior to mitosis (73). Cells with high secretory activity such as pollen tubes and root hairs seem to produce large numbers of new Golgi stacks depending on their task and growth status, and this is not related to the cell cycle or division (74).
Recent studies in the single-celled alga Chlamydomonas noctigama, which has non-motile Golgi stacks around the nucleus (75), and in BY-2 cells with mobile Golgi (21) have shown that de novo Golgi biogenesis and Golgi fission can take place within the same system (Figure 2). Experiments were based on a complete deconstruction of Golgi stacks with BFA and reformation after BFA washout. Initially, in both systems, vesicle clustering was a first indication of Golgi reformation. After the first fusion events, mini-Golgi stacks were formed, starting at 200 nm diameter with up to five cisternae (Figure 2A–C,H,I). An increase in ERES number on the tER accompanied the early reformation events in C. noctigama. Mini-Golgi stacks displayed a very early cis-to-trans polarity, and in BY-2 cells, this could also be observed in Golgi stacks with a 250 nm diameter. In both studies, there was no clear indication that COPII-coated vesicles or membrane took part in early stages of biogenesis. Although budding sites on the ER were observed (Figure 2A), they did not label with antibodies to the Sec13 component of the COPII coat. From immunogold labelling, it was however shown that COPI proteins may play a role in the early membrane fusion events forming initial cisternae.
Figure 2. Golgi biogenesis and fission in tobacco BY-2 cells (A–G) and Chlamydomonas noctigama (H–M) in BFA washout experiments. An increased number of ER-budding sides (A) and a vesicular cluster (B) are the first steps of Golgi recovery in BY-2 cells 15 min after washing out BFA. These vesicle clusters tend to fuse (C) and form mini-Golgi stacks (D, size around 250 nm) within the first hour of recovery. Some of these mini-stacks show very early cis (c)–trans (t) polarity. Mini-stacks often appear in groups (E). After maturing and forming double-sized larger Golgi stacks, the majority divide in a cis-to-trans direction about 180 min after BFA washout (F–G) arrowheads point to intercisternal filaments. Golgi biogenesis in C. noctigama starts with the formation of vesicular tubular clusters (H) and continues as described for BY2 cells: formation of mini-Golgi stacks (I), lateral growth (J), double stacks (K), division (L) and the appearance of normal-sized stacks 3 h after BFA washout (M). D, F and G are taken from Langhans et al. (21). Copyright American Society of Plant Biologists. (A–B) and (H–J) bars = 100 nm and (C–G) and (K–M) bars = 200 nm. H, K, L and M are taken from Hummel et al. (75). Copyright German Botanical society.
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After stack formation, Golgi cisternae increase in size. The growth seems to be related to an increased number of budding sites on the ER in C. noctigama(75), and there appeared to be an increased formation of budding profiles on the ER in BY-2 cells with mobile Golgi stacks. In both Chlamydomonas and BY-2 cells, reforming Golgi stacks continued to grow to double the size of those in control cells and then divided vertically in a cis-to-trans direction (21,75). There is however no indication as to what triggers the overgrowth of the stacks or induced their division, but we have to hypothesize on the existence of molecular regulators of Golgi stack size. Could this be a putative role for some of the matrix proteins?
In mammalian cells, Golgi matrix proteins, mainly GM130 and p115, have been implicated in Golgi biogenesis (76), and as discussed earlier, homologues of Golgi matrix proteins have been described for plant cells (45,47), although a GM130 homologue does not exist. However, the p115 homologue is most likely situated towards the cis-Golgi and is a good candidate for a tether involved in early Golgi biogenesis. In Figure 3, we propose a sequence of events that may be involved in the birth of an individual Golgi stack from the ER. First, an exit site differentiates on the ER surface through interplay of Sec16, Sec12 and Sar1 (Figure 3A). This may also involve cis-Golgi matrix or tethering factors. A COPII-coated bud forms from the ER membrane and is tethered to the ER through the proto-Golgi matrix (Figure 3B). The bud or buds grow to form a tubulovesicular complex, which contains COPI buds, vesicles and SNARES, and is surrounded by a matrix (Figure 3C). Whether this is fed by direct membrane connections to the ER or by vesicles is still to be ascertained but quickly differentiates into a mini proto-Golgi stack with structural characteristics of both cis- and trans-faces, including clathrin-coated buds (Figure 3D). At some stage, membrane-bound Golgi enzymes are transferred into this structure from the ER and are anchored in the correct cisternae as the stack continues to mature. This whole complex is most likely motile with the ER surface (see subsequently).
Figure 3. Proposed model for the early stages of the biogenesis of a Golgi stack from the ER. A) Differentiation of exit sites on the ER. B) Formation of a tethered COPII bud at the exit site. C) Formation of a tubulovesicular complex with associated COPI. D) Differentiation of a small proto-Golgi stack. It is not known if mysosins are associated with the ER or Golgi.
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