The Golgi apparatus.

What is it?Golgi apparatus is a stack of smooth membrane sacks (cisternae), dis-like and often slightly curved.

What are it’s basic functions? Modification of proteins and lipid by glycosylation Activation of peptides by proteolysis or phosphorylation Production of vesicles, with different destinations, into

which macromolecules are stores Synthesis of glycosamionglycans and mucin Selection of enzymes to be delivered to lysosomes Organelle deputed to the sorting and shipping of the

proteins out of the cell or to different sites into the cell

What is the function of each part of the system?The Smooth and Rough Endoplasmatic Reticulum do protein and lipid synthesis. The cis portion of the Golgi does the phosphorylation of proteins, medial does addition of sugar residues, trans does protein proteolysis, lipid and protein sorting. The secretion is done by exocytosis, through special cell vesicles (lysosomes) or incorporation of new cell membrane.

What is the anatomy?

Cis face: closer to the endoplasmic reticulum, the first face to be crossed by the substancesTrans face: further from the endoplasmic reticulum, the second face to be crossed by the substances

Cis, medial and trans cisternae are morphologically and biochemically different. Expansion and number of the Golgi apparatus cisternae depends on the cell activity, more than one Golgi apparatus may be present in the cell.

What is the destination of proteins synthetized in the RER and then passed to the Golgi apparatus?

1) They come back to the RER2) They remain in the Golgi3) They enter the lysosomes4) They undergo exocytosis

1) Proteins found in the lumen of the Golgi that need to be transported to the RER have a sequence of AA called “kdel” in other to be recognized by a kdel receptor. K – lysine, D- aspartic acid, E– glutamic acid, L–leucine

2) Proteins that continue in the Golgi lack the signal peptide kdel, but may present other specific signals.

3) Proteins to be delivered to lysosomes have a specific marker, the 6-phosphate mannose.

4) Exocytosis or secretion: The content of vesicles leaving the Golgi apparatus may undergo 2 secretion pathways: constitutive secretion and regulated secretion.

Constitutive secretion: Vesicles coated by coating proteins (cops)

Not dependent on specific stimulation The secretion products are produce and immediately

secreted Is a continuous process

Regulated secretion: Vesicles are coated by clatherin-like proteins; secretion is

under control of specific stimulation (ex: release of hormones, neutrotrasmitters, enzymes)

Proteins to be secreted are stored into vesicles that accumulate into the cell (maturation of vesicles)

Vesicles accumulate close to the plasma membrane and fuse with the membrane after specific extracellular stimulation.

During mitosis the Golgi apparatus is delivered to the two daughter cells (check slide for image)

Vesicle trafficking: an overview.

Why are some vesicles coated? Coating of proteins favours the bending of the membrane

during the formation of the vesicle Coating allows the selection of the components that have

to be inserted and transported into the vesicle

What is the coating made of?Coated vesicles are surrounded by proteins, the coating proteins (COPs)

Vesicles coating by COP II move from the RER to the Golgi apparatus (anterograde transport, towards the outside)

Vesicles coated by COP I move from the Golgi to the RER (retrograde transport, toward inside)

Vesicles coated by clathrin move from the Golgi to the plasma membrane or the endosome.

How are vesicles directed towards of a specific cell compartment?The transported vesicles must be specifically recognized the destination compartment in order to deliver their content into the corret place. the transmembrane protein v-snare present on the vesicle is specifically recognized by sequences t-snare on the target membrane.

Endocytosis, pinocytosis and phagocytosis.

EndocytosisThe vesicle originates from the plasma membrane and internalized extracellular material.

Types of mediated endocytosis:

Receptor-mediated endocytosis Cargo receptors have the role to recognize and to bind the

molecules to be transported. Clathrin molecules aggregate forming a basket-shaped

network on the cytoplasmic side of cell membrane thus allowing the stabilization of the molecular binding

Dynamin molecules bind GTP and aggregate forming a ring aroung the origin of each pit when it is deeply invaginated.

Clathrin-mediated endocytosis1. Extracellular ligands are bound by cell-surface receptors

and associate with adaptor proteins, which, in turn, bind to clathrin. Clathrin forms a cage-like curved pit at the plasma

membrane. As the coat grows, the pit deepens until it pinches off as a closed vesicle.

2. Following internalization, the clathrin coat diassembles.3. The vesicle fuses with other previously internalized

membranes to form early endosomes.4. In the acidic early endosome, ligands are released from

their receptors.5. Receptors are sorted away from soluble ligand and fuse

back with the plasma membrane.6. Internal vesicles will ultimately be delivered to lysosomes.

Pinocytosis

Is the non specific introduction into the cell of very small liquid droplets picked up form the extracellular environment by the formation of pinocytosis vesicles (micropinocytosis and micropinocytosis, visible at TEM)

Phagocytosis

Is the process by which cells engulgh solid matter such as bacteria, cells, cells fragments

Specialized cells, called phagocytes (macrophages, granulocytes) play it. They derive from blood and are able to destroy bacteria, viruses, dead or dmaged cells.

Sequence:1) Recognition (direct or indirect) and absorption of the

particle to be ingested2) Internalization of the particle, digestion in lysosomes.

LysosomesContain more than fifty lytic enzymes that break down waste materials. Proteins of the lysosomes are synthesized in the Rough Endoplasmic Reticulum and tagged with mannose-6-phosphate to differentiate them from other enzymes. Lysosomes receive endocytosed proteins and lipids that need to be digested, and also endogenous proteins/molecules that need to be degraded. The hydrolases inside the lysosomes are inactive if released in the cytosol, because of the cytosol’s Ph (7,2_. The acidicity inside the lysosome (Ph 5,0) is sustained by a proton pump in it’s membrane. It has heterogeneous morphology and it’s dimensions are 0,2 – 0,5 μm.

It’s functions are: Main site of cellular digestion Digestion of materials introduced in the cell Destroy microorganisms or damaged cells that have been

phagocytosed Number is highly variable Abundant in cells with intense phagocytosis

Lysosomes can also be a site for melanin storage produced by skin melanocytes from the tyrosine aminoacid, in that case they are called melanosomes.

Clinical drop: lysosomal disorders are a group of several genetic diseases that originate from an abnormal accumulation of substances inside lysosomes, they are due to defects in lysosomal enzymes required for the metabolism of lipids, glycoproteins or mucopolyssacharides.

PeroxisomesWhat are it’s characteristics?

They are a heterogeneous group of membrane-bound organelles containg several enzymes acting in diferente metabolic activities. Key feature of peroxisomes is the presence of the enzyme CATALASE, representing 40% of total enzymes.

Size: 0.6-0.7 um Shape: ranging from spherical oval to rod-shape Number: few units to hundreds/thousands

What are it’s functions?

Contain enzymes involved in fatty acids and aminoacid oxidation: they acitivty produces h2o2

Catalase transforms h2o2 in water and molecular oxygen, avoiding the diffusion of toxic H²O² in the cytoplasm.

Does degradation of fatty acids and aminoacid oxidation. Invactivation of H²O² by catalase. Detoxification of dangerous substances Removal of free radicals and reactive oxygen species, such as superoxide radical,

hydroxyl radicals that may damage the cell dna and proteins.

Peroximes and CNS.

Oligodendrocytes have a fundamental role in the support of axonal integrity. Oligodendrocyte perixomes are importante guardiands of axonal vitality. Damage of perixomes leads to axonal loss and demyelination.

Biogenisis of peroxisomes.

Microvesicles containing peroxisome proteins in their membrane originate from the RER, then they fuse to form the peroxisomes.