How are lysosomes and peroxisomes produced?

What are lysosomes?

Lysosomes are the cells' garbage disposal system. They degrade the products of ingestion, such as the bacterium that has been taken in by phagocytosis seen in the above cartoon. After the bacterium is enclosed in a vacuole, vesicles containing lysosomal enzymes (sometimes called primary lysosomes) fuse with it. The pH becomes more acidic and this activates the enzymes. The vacuole thus becomes a secondary lysosome and degrades the bacterium.

Lysosomes also degrade worn out organelles such as mitochondria. In this cartoon, a section of rough endoplasmic reticulum wraps itself around a mitochondrion and forms a vacuole. Then, vesicles carrying lysosomal enzymes fuse with the vesicle and the vacuole becomes an active secondary lysosome.

A third function for lysosomes is to handle the products of receptor-mediated endocytosis such as the receptor, ligand and associated membrane. In this case, the early coalescence of vesicles bringing in the receptor and ligand produces an endosome. Then, the introduction of lysosomal enzymes and the lower pH causes release, and degradation of the contents. This can be used for recycling of the receptor and other membrane components. See the Web page on Receptor mediated endocytosis for more information.

Lysosomes carry hydrolases that degade nucleotides, proteins, lipids, phospholipids, and also remove carbohydrate, sulfate, or phosphate groups from molecules. The hydrolases are active at an acid pH which is fortunate because if they leak out of the lysosome, they are not likely to do damage (at pH 7.2) unless the cell has become acidic. A Hydrogen ion ATPase is found in the membrane of lysosomes to acidify the environment.

Lysosomal morphology varies with the state of the cell and its degree of degradative activity. Lysosomes have pieces of membranes, vacuoles, granules and parts of mitochondia inside. Phagolysosomes may have parts of bacteria or the cell it has injested. This electron micrograph shows typical secondary lysosomes. They have been detected by cytochemical labeling for acid phosphatase. This is a good marker for lysosomes. Recall that it is also used as a marker for the Trans Golgi Cisternae.

How and Where are lysosomal enzymes produced? Introduction to the Ribosome-Endoplasmic Reticulum Unit

Lysosomal enzymes are made with polyribosomes and initially sequestered in the rough  endoplasmic reticulum.  The left hand view of this cartoon shows the free polyribosomes connected by the mRNA. They are arranged in rosettes and these can be seen in the cytoplasm in conventional electron micrographs. The right hand view shows the arrangement of polyribosomes on the rough endoplasmic reticulum. Note that the growing polypeptide chain (which projects down from the large subunit) is inserted through the membrane and into the cisterna of the rough endoplasmic reticulum.  It contains an initial "signal peptide" that allows it to be recognized by the rough endoplasmic reticulum.  Signal peptide binds to a receptor and helps anchor the polyribosomes on the surface of the rough endoplasmie reticulum. 
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This electron micrograph shows a high magnification of a longitudinal section through the rough endoplasmic reticulum. The electron dense ribosomes are on its outside surface. Inside the sac (cisterna) is flocculent material, the newly synthesized proteins. The details of ribosomal structure cannot be appreciated in this micrograph. They look like small irregular balls on the outside of the membrane. Note that the sacs of rough endoplasmic reticulum are bridged by a junction.

How do lysosomal proteins translocate into the lumen of  the rough endoplasmic reticulum?

This simplified cartoon shows that this is the first part of the protein produced.  After the signal sequence is completed, protein synthesis is further inhibited.  This is to allow the interaction of the signal sequence with a complex on the rough endoplasmic reticulum.  In the above cartoon, note that the signal peptide is allowed to enter and essentially guide the protein into the lumen of the rough endoplasmic reticulum.  Once the signal sequence is detected, protein synthesis resumes and the rest of the protein is inserted in the lumen.  Note that a signal peptidase near the inner surface of the membrane works to cleave the signal sequence from the growing peptide.

The complex is actually more complicated than the above. The cartoon to the left shows a view of the signal sequence binding and interaction

Note that the signal sequence is recognized by a Recognition Particle, or SRP.   This is then bound to a receptor.  This complex guides the protein through a channel like region.  It also consists of a docking site for the ribosome.

Once they are inside, lysosomal enzymes are processed like other proteins.  Mannose  and other types of sugars are attached.  Mannose will serve as a basic site for the sorting signal.

How do lysosomal proteins move to the Golgi Complex?Bannykh, S and Balch WE  Membrane Dynamics at the Endoplasmic Reticulum-Golgi Interface. J Cell Biol 138: 1-4 (1997)

Small vesicles bud from ER and immediately enter the tubular vesicular complex zone.  This is like a train station that organizes them to enter the Golgi complex.

Area I shows budding from ER that is arranged facing a central zone at one end of the Golgi complex.  These buds become vesicles and are coated with COPII protein coats.  

Area II  is the  vesicular-tubular cluster. The vesicle then lose their  COPII coat and  merge with vesicles carrying all soluble and membrane proteins to the Golgi complex.  This is a mixture of vesicles including secretory proteins and lysosomal enzymes. 

Area III designates the entire complex which is unique in the cytoplasm.  It is termed the 'export complex' and contains unique proteins that suggest it is specialized for information flow to and from ER and the Golgi complex. Again, it is like a train station, although you stay on the same train (in the same vesicle) all the way to the Golgi complex.

This drawing shows an actual interface between the ER and the Golgi complex.  The "Export complex" is seen at the top of the drawing.  Note that the vesicle are moving to contribute to the cis-Golgi network of vesicles and cisternae.

The movement of these special transport vesicles is an energy requiring process. If one blocks production of ATP, the transport will not happen. This drawing shows how the rough endoplasmic reticulum forms vesicles (without ribosomes attached) that carry the newly synthesized proteins to the Golgi complex. 

The inside of the vesicle becomes continuous with the inside of the Golgi cisternae, so that protein groups pointing towards the inside, could eventually be directed to face the outside of the cell. 

Carbohydrate groups are attached and any subunits may be joined in these cisternae. The protein is then passed to the final region of the Golgi called the "trans face". There it is placed in vacuoles that bud from this region of the Golgi complex. These may be a certain size or density, characteristic of the cell itself. The vacuoles continue to condense the proteins and the final mature secretory granule is then moved to the membrane for secretion

How does the Golgi Complex sort lysosomal enzymes?

The Golgi complex sorts the lysosomal enzyme in the Trans region. It is received from the rough endoplasmic reticulum (RER in this cartoon) in the cis region. The sorting depends on that mannose attached to the lysosomal enzymes.

It has the following steps:

• First, a phosphate radical is added to the mannose (Phosphorylation) at the 6th position.
• This mannose -6 phosphate becomes a sorting signal.
• Then, a specific receptor in the Golgi complex recognizes the mannose-6-phosphate and binds the lysosomal enzyme.
• This membrane bound receptor then sequesters the enzyme away from the other types of proteins.
• Sequestration is in the trans-Golgi region by molecules called Adaptin

• One end of adaptin binds to a signal sequence on the receptor molecule, the other end binds to "clathrin"

How do you make a lysosome?

• the trans Golgi region begins to "bud"
• a "cage" or "coat" made of clathrin forms around the bud (to strengthen it) to strengthen the bud.
• Adaptin binds to the receptor and the clathrin to keep the receptor from moving out of the budding region (anchors it to the membrane region).
• Vesicle with the lysosomal enzyme moves away to  fuse with a developing lysosome (such as the vacuoles seen in the previous figure). The vesicle loses its clathrin coat.
• The vesicles fuse with the developing lysosome (called a phagosome, endosome, or autosome; see previous figure) and the receptor with its attached lysosomal enzymes becomes part of the lysosomal membrane.
•  The developing lysosome contains a hydrogen ion pump on its surface. The pump works to acidify the environment inside the lysosome. This removes the phosphate and dissociates the enzymes from the receptor. The receptor is then recycled back to the Golgi complex.  The lysosomes are available to do their digestive action.

To summarize:  you start with a vacuole containing ingested material, or material to be destroyed. Vesicles from the Golgi complex containing lysosomal enzymes move to the vacuole and fuse with it. The hydrogen pump acidifies the interior and the enzymes are both released and activated.  The material is then digested and destroyed inside the lysosome.

Lysosomes can actually be detected by pH indicator dyes. This photograph shows dyes that indicate different pH's with different colors. The red lysosomes (pH 5.0) are probably typical lysosomes. The blue and green lysosomes are probably endosomes. This change can be detected if you link a ligand to fluorescein. Fluorescein will not fluoresce at pH's lower than 6.0. Therefore, one can follow entry of the receptor-ligand complex and then see the fluorescence disappear as the endosome containing the complex is acidified.
 

What is the difference between a lysosome and an "endosome?

An endosome is formed from the process called "endocytosis" and frequently this is a regulated process that is mediated by signalling molecules (like hormones or growth factors) binding to specific receptors in the plasma membrane.  The process is called "receptor mediated endocytosis". See the above figure for a cartoon of the process.

Early endosomes do not appear to receive lysosomal enzymes.  They do  have lowered pH (5.9-6) and this can release the receptor and ligand.  The receptor may be recycled to the surface by vesicles that bud from the endosome and then target the plasma membrane. After these recycling vesicles fuse with the plasma membrane, the receptor is returned to the cell surface for further binding and activity. 

The early endosome may convert to a late endosome.  Then, a late endosome can be converted to a lysosome as the pH is lowered and enzymes are added.  The cartoon below shows that small vesicles communicate with the late endosome and lysosome bringing the enzymes bound to the receptor to these bodies. 

The digestive properties of the endosomes may not be as extreme as those of the lysosomes.  The endosomes need to preserve receptor and ligand chemistry, however the lysosomes serve to destroy everything.

Cholesterol uptake and metabolism:  A clinical example of how this needs to work (and what can go wrong).

The above example shows receptors for LDL (which binds cholesterol).  Recall that LDL is considered the "bad" cholesterol because its role is to bring cholesterol into the cell so it can be used.  If it is high in the blood stream, this means that cholesterol is not being taken up, or is too high.  Sometimes this can be due to a defect in the LDL receptor.  In A, above, note that the LDL receptor + cholesterol move to clathrin coated pits thanks to Adaptin which binds the receptor and the clathrin.  Then, they move from early to late endosomes. Normally, vesicles then take the cholesterol to the Golgi complex or it is released for use by the cell.

In B, however the LDL receptor has lost its adaptin binding site.  This is a genetic mutation and causes hypercholesterolinemia.  The LDL receptor can not be sequestered in the clathrin coated pit and can not be taken up. 

In another disease, called, Nieman Pick Type C, the LDL receptor is fine and cholesterol is taken up and manages to get all the way to the late endosome. However, it can not get out of the endosome and thus cholesterol can not get out and be used by the cell.  The result is expansion of the endosome.  It is believed to be caused by a mutation in the Neiman Pick C 1 protein that is involved in transport of the cholesterol to where it is needed.

Why peroxisomes are not like lysosomes.

Peroxisomes are organelles that contain oxidative enzymes, such as D-amino acid oxidase, ureate oxidase, and catalase. They may resemble a lysosome, however, they are not formed in the Golgi complex. Peroxisomes are distinguished by a crystalline structure inside a sac which also contains amorphous gray material. They are self replicating, like the mitochondria. Components accumulate at a given site and they can be assembled into a peroxisome. They may look like storage granules, however, they are not formed in the same way as storage granules.  They also enlarge and bud to produce new peroxisomes.

Peroxisomes function to rid the body of toxic substances like hydrogen peroxide, or other metabolites. They are a major site of oxygen utilization and are numerous in the liver where toxic byproducts are going to accumulate.

• Learn more about how proteins are handled in the rough endoplasmic reticulum
• Study how proteins get to the Golgi Complex
• Go to the Cell Biology Web page

visitors since 8/10/01
URL Address: Lysosomes and Peroxisomes
Gwen V. Childs, Ph.D.
childsgwenv@uams.edu
© text copyright 1996 Gwen V. Childs, Ph.D.