Golgi structure and function
The Golgi complex plays a central role in processing and sorting of biosynthetic cargo in all eukaryotic cells. In mammals, the Golgi complex consists of sets of flattened cisternal membranes arranged in stacks with associated tubules and vesicles, which are usually collected at the microtubule organizing center (MTOC) in a ribbon structure (Klumperman, 2011). This structure is not essential for the known functions of the Golgi, and may suggest additional functions. Golgi structure is also quite dynamic; the organelle is disassembled at mitosis and then reassembled (Wang and Seemann, 2011). The organelle can also accommodate cargo of different shapes and sizes (Machamer, 2013). We previously hypothesized that mammalian Golgi organization may have evolved in part to sense and transduce specific stress signals to the nucleus (Hicks and Machamer, 2005).
Golgi structure in mammalian cells is maintained by the cytoskeleton, and GRASPs and golgins (Figure 1A). GRASP65 and GRASP55 form homo- or hetero-oligomers and mediate stacking and can contribute to the Golgi ribbon structure (Ramirez and Lowe, 2009; Xiang and Wang, 2010). The golgin family comprises a group of peripheral Golgi membrane proteins with long coiled coil domains. Some golgins are vesicle tethers, some function in Golgi stack structure, and others may be involved in trafficking of specific cargo molecules (Munro, 2011). Disassembly of the Golgi in mitosis or apoptosis results from reversible phosphorylation of GRASPs and golgins or irreversible cleavage, respectively.
Figure 1  Golgi structure in life, stress and death. (A) Golgi morphology in a typical mammalian cell, with the key structural players shown in the inset. For simplicity, individual golgins and GRASPs are not indicated. (B) Golgi stress due to cargo load or size, ionic imbalance, infection with intracellular pathogens, or perturbation of glycosylation or the cytoskeleton results in structural alterations that can signal to the nucleus to help repair the stress. (1) Dephosphorylation of TFE3 and tranlocation to the nucleus results in transcription of genes with a GASE, including some glycosyltransferases and trafficking components. (2) Activation of local caspase-2 cleaves select golgins, and fragments enter the nucleus to perform an unknown function. (3) Phosphorylation of GRASPs and golgins or their cleavage can result in a more complete disassembly of the Golgi, although the consequences for signaling to the nucleus are unknown. (C) With irreparable stress, the Golgi is completely disassembled as the cell undergoes apoptosis.