Golgi Apparatus: Structure, Function, and Cellular Roles

The Golgi Apparatus: Structure and Function

The Golgi Apparatus (GA) is found in all eukaryotic cells, essential in the transfer and packaging of products from the Endoplasmic Reticulum (ER), and involved in the synthesis of glycoproteins and mucopolysaccharides.

Distribution and Morphology

Its distribution depends on microtubules and motor proteins in secreting cells. In protein-secreting cells, the GA is located between the nucleus and the apical pole. In ganglion nerve cells, it is located around the nucleus with expansions to the cell surface. In Leydig cells, it is an ovoid mass located at one pole of the nucleus. In hepatocytes, it is distributed throughout the cytoplasm.

Its form differs from one cell to another and also within the same cell, depending on the functional cycle. The size varies depending on the type and activity of the cell to which it belongs. The GA is bulky in gland or nerve cells, small in muscle cells, highly developed in overactive cells, and undeveloped in resting or hypoactive cells.

Structural Components: Dictyosomes and Cisternae

The GA is composed of units called dictyosomes, which are made up of approximately 4-8 stacked cisternae. Among them, there is a constant band of 200 Å of hyaloplasm.

Vesicle Populations and Golgi Regions

Near the dictyosomes, three populations of vesicles are found, categorized by their regions:

1. Cis Regions: The cisternae found most proximal to the nucleus.
2. Medial Regions: Occupied by a variable number of cisternae, depending on the nature of cell activity and the intensity of their secretory activity.
3. Trans Regions: Occupied by a concave cisterna whose membrane is thicker than that of the cis cisterna.

Types of Vesicles from the Trans Face

From this trans face, two types of more voluminous vesicles emerge:

• Clathrin Vesicles: These are secretory granules involved in controlled secretion. Transport vesicles are targeted to endosomes or lysosomes.
• Coatomer Vesicles: These carry bulk products and are involved in constitutive secretion.

Membrane Composition and Thickness

The membranes of proximal transition vesicles and cis cisternae have a thickness of 60 Å. The membranes of secretory vesicles and trans cisternae have a thickness of 75 Å. There is a progressive increase in membrane thickness from transition vesicles to secretory vesicles. The molecular composition of the GA membrane undergoes a gradual transition between the ER and the plasma membrane.

A. Lipids

Lipids constitute 35% of the membrane. The cis layer has a lipid content of 30%. The cisternae of the trans side contain 40%.

B. Proteins

The proportion of membrane proteins also varies. On the cis face, proteins constitute 70% of the membrane, and on the trans face, this percentage decreases to 60%. Most proteins are enzymes that justify the Golgi's mission. A transport chain consists of cytochrome b₅ reductase. Involved in the elongation of fatty acids, these enzymes are found in the trans cisternae.

Functions of the Golgi Apparatus

Glycosylation

Glycosylation (primarily in the medial region) is the main modification proteins undergo during their journey through the Golgi Apparatus.

Sulfation

Sulfation occurs in the trans cisterna, utilizing a cytosolic donor molecule as a source of sulfur.

Phosphorylation

Phosphorylation takes place in the cis region. Phosphotransferase phosphorylates mannose into mannose 6-phosphate (M6P) in N-glycosylated proteins.