Golgi apparatus polarization

Biosynthetic and secretory cargo leaving the ER is packaged in COPII-coated vesicles for delivery to the Golgi complex 146 The Golgi apparatus is a highly polarized structure consisting of a series of flattened cisternae, usually located near the nucleus and the centrosome 146... 

How the Golgi apparatus maintains its polarized structure while molecules move from one compartment to another is still a matter of debate. Two models were originally proposed based on different experimental evidence (Fig. 9-5) the vesicular transport model and the cisternal maturation model. A third model known as the dual transport model combines elements from both vesicular transport and cisternal maturation models and can better explain intra-Golgi transport. 

The most important membranes in animal cells are the plasma membrane, the inner and outer nuclear membranes, the membranes of the endoplasmic reticulum (ER) and the Golgi apparatus, and the inner and outer mitochondrial membranes. Lysosomes, peroxisomes, and various vesicles are also separated from the cytoplasm by membranes. In plants, additional membranes are seen in the plastids and vacuoles. All membranes show polarity—e., there is a difference in the composition of the inner layer (facing toward the cytoplasm) and the outer layer (facing away from it). 

Sulfation (or sulfonation) is catalyzed by sulfo-transferases (STs)/ which metabolize phenolS/ hydrox-ylamineS/ or alcohols to sulfate esters as shown in Scheme 11.32/ converting somewhat polar to very polar functionalities that are fully ionized at neutral pH. Like glucuronidatioii/ there are multiple ST subfamilies (more than 10 in humans). One subfamily is cytosolic and associated with drug metabolism and the other is membrane-bound/ localized in the Golgi apparatus/ and associated with sulfation of glycoproteinS/ proteinS/ and glycosaminoglycans (41). The STs are widely distributed in human tissues. Five cytosolic ST isoforms have been identified and characterized in human tissue four catalyze sulfation of phenolS/ one the sulfation of hydroxy steroids. 

Fig. 2.9 Liver cell and sinusoidal cells with organelles and polarized membrane compartments hepatocytes (H), sinusoids (S), Disse s space (D), erythrocytes (ER), endothelial cells (E), Kupffer cells (K), Ito cells (I), microvilli (MV), canahculus (BC), nucleolus (N), tight junctions (tj), cell nucleus (CN), mitochondria (M), smooth endoplasmic reticulum (SER), rough endoplasmic reticulum (RER), Golgi apparatus (GA), lysosomes (L), peroxisomes (P), ribosomes (R), microfilaments (ME) (modified from L. Cossel) (s. figs. 2.16-2.18)...

Type III coronins are different from other coronins in that they consist of two coronins fused in tandem but lacking coiled-coiled domains. In humans, they are represented by coronin 7. Type III coronins from Caenorhabditis (POD-1) sxiA Drosophila (Dpodl) have been shown to be involved with actin but seem participate in different processes. The embryonic-lethal phenotype of POD-1 mutants suggests that it is required for polarized membrane trafficking necessary for the establishment of anterior-posterior polarity in the embryo. Consistent with this phenotype, coronin 7 is associated with the Golgi apparatus and has been impheated in vesicle trafficking ... 

Figure 3. Membrane topology of the CMP-sialic acid transporter. Using an epitope insertion approach, ten transmembrane domains have been identified in the CMP-sialic acid transporter. Positions in the primary sequence where the HA-epitope was inserted are indicated by triangles. All constructs summarised in this figure were correctly transported to the Golgi apparatus. The orientation of the epitope in relation to the Golgi membrane was analyzed by indirect immunofluorescence. Filled triangles (HAM, HA7, HA6) indicate insertion mutants that were unable to translocate CMP-sialic acid. Transport active constructs are marked with open triangles. Polar and charged residues are indicated by filled circles and +, -, respectively.

Microtubules, cyhndrical tubes composed of tubulin subunits, are present in all nucleated cells and the platelets in blood (Fig. 10.26). They are responsible for the positioning of organelles in the cell cytoplasm and the movement of vesicles, including phagocytic vesicles, exocytotic vesicles, and the transport vesicles between the ER, Golgi, and endosomes (see Fig. 10.24). They also form the spindle apparatus for cell division. The microtnbnle network (the minus end) begins in the nucleus at the centriole and extends outward to the plasma membrane (usually the plus end). Microtubule-associated proteins (MAPs) attach microtubules to other cellular components, and can determine cell shape and polarity. 

---