3.06: Golgi Apparatus
- Page ID
- 4245
The Golgi apparatus is a cell structure mainly devoted to processing the proteins synthesized in the endoplasmic reticulum (ER). Some of these will eventually end up as integral membrane proteins embedded in the plasma membrane. Other proteins moving through the Golgi will end up in lysosomes or be secreted by exocytosis (e.g., digestive enzymes). The major processing activity is glycosylation: the adding of sugar molecules to form glycoproteins. In some cells, e.g., mucus-secreting cells in epithelia, the amount of carbohydrate so far exceeds that of the protein that the product is called a mucopolysaccharide (also known as a proteoglycan). In plant cells, the Golgi secretes the cell plate and cell wall.
Small peptides, e.g., some hormones and neurotransmitters, are typically too small to be synthesized directly by ribosomes. Instead, the ribosomes on the ER synthesize a large precursor protein that is later cut up into small peptide fragments as it traverses the Golgi. For example, proopiomelanocortin (POMC) is a polypeptide of 241 amino acids from which is cut ACTH, alpha and beta MSH, beta- endorphin, and others. POMC is cleaved to give rise to multiple peptide hormones.
- α-MSH produced by neurons in the arcuate nucleus has important roles in the regulation of appetite and sexual behavior, while α-MSH secreted from the intermediate lobe of the pituitary regulates the production of melanin.
- ACTH is a peptide hormone that regulates the secretion of glucocorticoids from the adrenal cortex.
- β-Endorphin and [Met]enkephalin are endogenous opioid peptides with widespread actions in the brain.
A hierarchical diagram showing how the precursor protein POMC is cut into smaller peptide hormones. At the top, a wide box labeled 'POMC' spans the full width. Dashed vertical lines drop from POMC to three second-level boxes, left to right: '??-MSH' (shaded gray), 'ACTH' (shaded gray), and '??-lipotropin' (unshaded); the dashed lines align with the segment of POMC that yields each product. ??-MSH is a terminal product. From ACTH, dashed lines lead down to two third-level terminal boxes: '??-MSH' (shaded gray) and 'CLIP' (unshaded). From ??-lipotropin, dashed lines lead to two third-level terminal boxes: '??-lipotropin' (unshaded) and '??-endorphin' (unshaded). From ??-lipotropin, dashed lines lead down to a fourth-level terminal box: '??-MSH' (shaded gray). The gray shading distinguishes the melanocortin peptides (??-MSH, ACTH, ??-MSH, ??-MSH) from the other cleavage products (??-lipotropin, CLIP, ??-lipotropin, ??-endorphin), which are unshaded. Each dashed connector indicates that the lower peptide is cleaved from the corresponding portion of the peptide above it.The Golgi consists of a stack of membrane-bounded cisternae located between the endoplasmic reticulum and the cell surface (Figure
Two red rectangles, one near the top and one near the bottom, enclose stacks of flattened, curved membrane compartments. Small rounded vesicles surround their edges. Additional membranes and organelles fill the grainy cytoplasm. There is no scale bar or other label in the image.The Outbound Path (Membrane Fission)
Two mechanisms appear to participate in the migration of proteins from the endoplasmic reticulum through the Golgi apparatus.
- Mechanism 1: Transition vesicles pinch off from the surface of the endoplasmic reticulum carrying integral membrane proteins, soluble proteins awaiting processing, and processing enzymes. Pinching off requires that the vesicle be coated with COPII (Coat Protein II). The transition vesicles move toward the cis Golgi on microtubules. As they do so, their COPII coat is removed and they may fuse together forming larger vesicles; these fuse with the cis Golgi. Sugars are added to proteins in small packets so many glycoproteins have to undergo a large number of sequential steps of glycosylation, each requiring its own enzymes. These steps take place as shuttle vesicles carry the proteins from cis to medial to the trans Golgi compartments. At the outer face of the trans Golgi, vesicles pinch off and carry their completed products to their various destinations.
- Mechanism 2: In addition to the pinching off and fusing of shuttle vesicles, the cisternae of the Golgi actually migrate themselves, that is, the cis Golgi gradually migrates up the stack becoming a medial and finally a trans Golgi (Figure
Diagram of vesicular protein transport through the Golgi apparatus. At the bottom, the endoplasmic reticulum (ER), drawn as an orange cisterna studded with ribosomes (black dots), contains processing proteins (pink squiggles) and proteins being processed (red bars and green dots). Green COPII-coated transition vesicles bud from the ER and move along microtubules (labeled at left) to the cis Golgi. Proteins then move upward through the stack via shuttle vesicles, indicated by red upward arrows between the cis, medial, and trans Golgi cisternae. Each cisterna contains red bars and green dots (proteins being processed) plus processing proteins. On the right side of the stack, purple COPI-coated vesicles carrying pink squiggle processing proteins move downward in retrograde transport, shown by downward arrows. At the top, three vesicles bud from the trans Golgi and travel upward (black arrows) to the plasma membrane, storage vesicles, or lysosomes. Legend: pink squiggle = processing proteins; red bar & green dot = proteins being processed; green circle = COP II; purple circle = COP I. The Inbound Path (Membrane Fusion)
The movement of cisternal contents through the stack means that essential processing enzymes are also moving away from their proper site of action. Using a variety of signals, the Golgi separates the products from the processing enzymes that made them and returns the enzymes back to the endoplasmic reticulum. This transport is also done by pinching off vesicles, but the inbound vesicles are coated with COPI (coat protein I). A vesicle recognize its correct target by involving pairs of complementary integral membrane proteins:
- v-SNAREs = "vesicle SNAREs" — on the vesicle surface
- t-SNAREs = "target SNAREs" — on the surface of the target membrane
v-SNAREs and t-SNAREs bind specifically to each other thanks to the complementary structure of their surface domains. Binding is followed by fusion of the two membranes (Figure
A circular vesicle on the left touches the boundary of a receiving compartment on the right. Labels point to v-SNARE at the vesicle membrane and t-SNARE at the target membrane. Red and blue segments meet at the contact point, illustrating the protein interaction that brings the membranes together.The Golgi breaks up and disappears at the onset of mitosis. By telophase of mitosis, the Golgi reappears. How it is recreated is still uncertain.