Golgi stress response and organelle zones.

Organelles have been studied traditionally as single units, but a novel concept is now emerging: each organelle has distinct functional zones that regulate specific functions. The Golgi apparatus seems to have various zones, including zones for: glycosylphosphatidylinositol-anchored proteins; proteoglycan, mucin and lipid glycosylation; transport of cholesterol and ceramides; protein degradation (Golgi membrane-associated degradation); and signalling for apoptosis. The capacity for these specific functions and the size of the corresponding zones appear to be tightly regulated by the Golgi stress response to accommodate cellular demands. For instance, the proteoglycan and mucin zones seem to be separately augmented during the differentiation of chondrocytes and goblet cells, respectively. The mammalian Golgi stress response consists of several response pathways. The TFE3 pathway regulates the general function of the Golgi, such as structural maintenance, N-glycosylation and vesicular transport, whereas the proteoglycan pathway increases the expression of glycosylation enzymes for proteoglycans. The CREB3 and HSP47 pathways regulate pro- and anti-apoptotic functions, respectively. These observations indicate that the Golgi is a dynamic organelle, the capacity of which is upregulated according to cellular needs.