The

Endomembrane

System and

Peroxisomes

Chapter 12

Becker’s The World of Cell

Endoplasmic Reticulum

Especially prevalent within most

eukaryotic cells is the endomembrane

system, an elaborate array of membrane

bounded organelles derived from the

endoplasmic reticulum (ER). The ER itself is

a network of sacs, tubules, and vesicles

surrounded by a single membrane that

separates the ER lumen from the

surrounding cytosol.

The rough ER has ribosomes that synthesize proteins destined for the plasma membrane, for secretion, or for various organelles of the endomembrane system— the nuclear envelope, Golgi complex, endosomes, and lysosomes. Both the rough and smooth ER synthesize lipids for cellular membranes. The smooth ER is also the site of drug detoxification, carbohydrate metabolism, calcium storage, and steroid biosynthesis in some cells.

The Golgi Complex

The Golgi complex plays an important

role in the glycosylation of proteins and in

the sorting of proteins for transport to

other organelles, for transport to the

plasma membrane, or for secretion.

Transition vesicles that bud from the ER fuse with the cis- Golgi network (CGN), delivering lipids and proteins to the Golgi complex. Proteins then move through the Golgi cisternae toward the trans-Golgi network (TGN).

Before proteins leave the ER in transition vesicles, they undergo the first few steps of protein modification. ER-specific proteins catalyze core glycosylation and folding of polypeptides, elimination of misfolded proteins, and the assembly of multimeric proteins.

Roles of the ER and Golgi Complex

in Protein Glycosylation

During their journey through the Golgi complex, proteins are further modified as oligosaccharide side chains are trimmed or further glycosylated in the Golgi lumen.

Some ER-specific proteins have a retention signal that prevents them from leaving the ER as vesicles move from the ER to the CGN. Other ER proteins have a retrieval tag that allows them to return to the ER in vesicles that return from the CGN.

Roles of the ER and Golgi Complex in Protein Trafficking

Numerous transport vesicles bud from the TGN Golgi network and carry the processed proteins to their final destinations. The tag that identifies a particular protein and its destination may be a short amino acid sequence, an oligosaccharide side chain, or some other structural feature.

Hydrolytic enzymes destined for the lysosome are phosphorylated on a mannose residue. Vesicles containing specific receptors for this mannose phosphate group carry these enzymes to the lysosome. During acidification in the lysosome, the enzymes are released from the receptors, which are then recycled back to the TGN.

Exocytosis and Endocytosis

Exocytosis adds lipids and proteins to the plasma membrane when secretory granules release their contents to the extracellular medium by fusing with the plasma membrane. This addition of material to the plasma membrane is balanced by endocytosis, which removes lipids and proteins from the plasma membrane as extracellular material is internalized in vesicles.

Phagocytosis is a type of endocytosis involving the ingestion of extracellular particles through invagination of the plasma membrane. Receptor-mediated endocytosis depends on highly specific binding of ligands to corresponding receptors on the cell surface. In both cases, after sorting of receptors and other necessary proteins back to the plasma membrane, ingested material is sent to lysosomes for digestion or to other locations for reuse.

Coated Vesicles in Cellular

Transport Processes ■ Transport vesicles carry material throughout the endomembrane system. Coat proteins—which include clathrin, COPI, COPII, and caveolin—participate in the sorting of molecules fated for different destinations as well as in the formation of vesicles.

The specific coat proteins covering a vesicle indicate its origin and help determine its destination within the cell. Clathrin coated vesicles deliver material from the TGN or plasma membrane to endosomes. COPII-coated vesicles carry materials from the ER to the Golgi, while COPI-coated vesicles transport material from the Golgi back to the ER.

Once a transport vesicle nears its destination, it is recognized and bound by tethering proteins attached to the target membrane. At this point the v-SNAREs in the transport vesicle membrane and the t-SNAREs in the target membrane interact physically, helping to promote membrane fusion.

Lysosomes and Cellular Digestion

Extracellular material obtained from phagocytosis or receptor-mediated endocytosis is sorted in early endosomes, which mature to form late endosomes and lysosomes as they fuse with vesicles containing inactive hydrolytic enzymes packaged in the TGN.

The late endosomal membrane contains ATP-dependent proton pumps that lower the pH of the endosomal lumen and help to transform the late endosome into a lysosome. Then, latent acid hydrolases capable of degrading most biological molecules become active due to the low pH.

Lysosomes also function in autophagy, the turnover and recycling of cellular structures that are damaged or no longer needed. In some cells, extracellular material is digested by enzymes that lysosomes discharge out of the cell by exocytosis.

The Plant Vacuole: A

Multifunctional Organelle

The plant vacuole is an acidic compartment resembling the animal lysosome. In addition to having hydrolytic enzymes for digestion of macromolecules, it helps the plant cell maintain positive turgor pressure and serves as a storage compartment for a variety of plant metabolites.

Peroxisomes Peroxisomes, which are not part of the

endomembrane system, appear to increase in number by the division of preexisting organelles rather than by the coalescence of vesicles from the ER or Golgi. However, there is currently a debate concerning the existence of protoperoxisomes, vesicles that some researchers believe bud off from the ER and develop into new peroxisomes.

Some peroxisomal membrane lipids are synthesized by peroxisomal enzymes, while others are carried from the ER by phospholipid exchange proteins. Most peroxisomal proteins are synthesized by cytosolic ribosomes and are imported posttranslationally. Others are believed to travel via a proposed subdomain of the ER known as the peroxisomal ER (pER).

The defining enzyme of a peroxisome is catalase, an enzyme that degrades the toxic hydrogen peroxide that is generated by various oxidases in the peroxisome. Animal cell peroxisomes are important for detoxification of harmful substances, oxidation of fatty acids, and metabolism of nitrogen-containing compounds. In plants, peroxisomes play distinctive roles in the conversion of stored lipids into carbohydrate by glyoxysomes and in photorespiration by leaf peroxisomes.